reproductive system lab assignment

27.2 Anatomy and Physiology of the Female Reproductive System

Learning objectives.

By the end of this section, you will be able to:

• Describe the structure and function of the organs of the female reproductive system
• List the steps of oogenesis
• Describe the hormonal changes that occur during the ovarian and menstrual cycles
• Trace the path of an oocyte from ovary to fertilization

The female reproductive system functions to produce gametes and reproductive hormones, just like the male reproductive system; however, it also has the additional task of supporting the developing fetus and delivering it to the outside world. Unlike its male counterpart, the female reproductive system is located primarily inside the pelvic cavity ( Figure 27.9 ). Recall that the ovaries are the female gonads. The gamete they produce is called an oocyte . We’ll discuss the production of oocytes in detail shortly. First, let’s look at some of the structures of the female reproductive system.

External Female Genitals

The external female reproductive structures are referred to collectively as the vulva ( Figure 27.10 ). The mons pubis is a pad of fat that is located at the anterior, over the pubic bone. After puberty, it becomes covered in pubic hair. The labia majora (labia = “lips”; majora = “larger”) are folds of hair-covered skin that begin just posterior to the mons pubis. The thinner and more pigmented labia minora (labia = “lips”; minora = “smaller”) extend medial to the labia majora. Although they naturally vary in shape and size from woman to woman, the labia minora serve to protect the female urethra and the entrance to the female reproductive tract.

The superior, anterior portions of the labia minora come together to encircle the clitoris (or glans clitoris), an organ that originates from the same cells as the glans penis and has abundant nerves that make it important in sexual sensation and orgasm. The hymen is a thin membrane that sometimes partially covers the entrance to the vagina. An intact hymen cannot be used as an indication of “virginity”; even at birth, this is only a partial membrane, as menstrual fluid and other secretions must be able to exit the body, regardless of penile–vaginal intercourse. The vaginal opening is located between the opening of the urethra and the anus. It is flanked by outlets to the Bartholin’s glands (or greater vestibular glands).

The vagina , shown at the bottom of Figure 27.9 and Figure 27.10 , is a muscular canal (approximately 10 cm long) that serves as the entrance to the reproductive tract. It also serves as the exit from the uterus during menses and childbirth. The outer walls of the anterior and posterior vagina are formed into longitudinal columns, or ridges, and the superior portion of the vagina—called the fornix—meets the protruding uterine cervix. The walls of the vagina are lined with an outer, fibrous adventitia; a middle layer of smooth muscle; and an inner mucous membrane with transverse folds called rugae . Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. The thin, perforated hymen can partially surround the opening to the vaginal orifice. The hymen can be ruptured with strenuous physical exercise, penile–vaginal intercourse, and childbirth. The Bartholin’s glands and the lesser vestibular glands (located near the clitoris) secrete mucus, which keeps the vestibular area moist.

The vagina is home to a normal population of microorganisms that help to protect against infection by pathogenic bacteria, yeast, or other organisms that can enter the vagina. In a healthy woman, the most predominant type of vaginal bacteria is from the genus Lactobacillus . This family of beneficial bacterial flora secretes lactic acid, and thus protects the vagina by maintaining an acidic pH (below 4.5). Potential pathogens are less likely to survive in these acidic conditions. Lactic acid, in combination with other vaginal secretions, makes the vagina a self-cleansing organ. However, douching—or washing out the vagina with fluid—can disrupt the normal balance of healthy microorganisms, and actually increase a woman’s risk for infections and irritation. Indeed, the American College of Obstetricians and Gynecologists recommend that women do not douche, and that they allow the vagina to maintain its normal healthy population of protective microbial flora.

The ovaries are the female gonads (see Figure 27.9 ). Paired ovals, they are each about 2 to 3 cm in length, about the size of an almond. The ovaries are located within the pelvic cavity, and are supported by the mesovarium, an extension of the peritoneum that connects the ovaries to the broad ligament . Extending from the mesovarium itself is the suspensory ligament that contains the ovarian blood and lymph vessels. Finally, the ovary itself is attached to the uterus via the ovarian ligament.

The ovary comprises an outer covering of cuboidal epithelium called the ovarian surface epithelium that is superficial to a dense connective tissue covering called the tunica albuginea. Beneath the tunica albuginea is the cortex, or outer portion, of the organ. The cortex is composed of a tissue framework called the ovarian stroma that forms the bulk of the adult ovary. Oocytes develop within the outer layer of this stroma, each surrounded by supporting cells. This grouping of an oocyte and its supporting cells is called a follicle . The growth and development of ovarian follicles will be described shortly. Beneath the cortex lies the inner ovarian medulla, the site of blood vessels, lymph vessels, and the nerves of the ovary. You will learn more about the overall anatomy of the female reproductive system at the end of this section.

The Ovarian Cycle

The ovarian cycle is a set of predictable changes in a female’s oocytes and ovarian follicles. During a woman’s reproductive years, it is a roughly 28-day cycle that can be correlated with, but is not the same as, the menstrual cycle (discussed shortly). The cycle includes two interrelated processes: oogenesis (the production of female gametes) and folliculogenesis (the growth and development of ovarian follicles).

Gametogenesis in females is called oogenesis . The process begins with the ovarian stem cells, or oogonia ( Figure 27.11 ). Oogonia are formed during fetal development, and divide via mitosis, much like spermatogonia in the testis. Unlike spermatogonia, however, oogonia form primary oocytes in the fetal ovary prior to birth. These primary oocytes are then arrested in this stage of meiosis I, only to resume it years later, beginning at puberty and continuing until the woman is near menopause (the cessation of a woman’s reproductive functions). The number of primary oocytes present in the ovaries declines from one to two million in an infant, to approximately 400,000 at puberty, to zero by the end of menopause.

The initiation of ovulation —the release of an oocyte from the ovary—marks the transition from puberty into reproductive maturity for women. From then on, throughout a woman’s reproductive years, ovulation occurs approximately once every 28 days. Just prior to ovulation, a surge of luteinizing hormone triggers the resumption of meiosis in a primary oocyte. This initiates the transition from primary to secondary oocyte. However, as you can see in Figure 27.11 , this cell division does not result in two identical cells. Instead, the cytoplasm is divided unequally, and one daughter cell is much larger than the other. This larger cell, the secondary oocyte, eventually leaves the ovary during ovulation. The smaller cell, called the first polar body , may or may not complete meiosis and produce second polar bodies; in either case, it eventually disintegrates. Therefore, even though oogenesis produces up to four cells, only one survives.

How does the diploid secondary oocyte become an ovum —the haploid female gamete? Meiosis of a secondary oocyte is completed only if a sperm succeeds in penetrating its barriers. Meiosis II then resumes, producing one haploid ovum that, at the instant of fertilization by a (haploid) sperm, becomes the first diploid cell of the new offspring (a zygote). Thus, the ovum can be thought of as a brief, transitional, haploid stage between the diploid oocyte and diploid zygote.

The larger amount of cytoplasm contained in the female gamete is used to supply the developing zygote with nutrients during the period between fertilization and implantation into the uterus. Interestingly, sperm contribute only DNA at fertilization —not cytoplasm. Therefore, the cytoplasm and all of the cytoplasmic organelles in the developing embryo are of maternal origin. This includes mitochondria, which contain their own DNA. Scientific research in the 1980s determined that mitochondrial DNA was maternally inherited, meaning that you can trace your mitochondrial DNA directly to your mother, her mother, and so on back through your female ancestors.

Everyday Connection

Mapping human history with mitochondrial dna.

When we talk about human DNA, we’re usually referring to nuclear DNA; that is, the DNA coiled into chromosomal bundles in the nucleus of our cells. We inherit half of our nuclear DNA from our father, and half from our mother. However, mitochondrial DNA (mtDNA) comes only from the mitochondria in the cytoplasm of the fat ovum we inherit from our mother. She received her mtDNA from her mother, who got it from her mother, and so on. Each of our cells contains approximately 1700 mitochondria, with each mitochondrion packed with mtDNA containing approximately 37 genes.

Mutations (changes) in mtDNA occur spontaneously in a somewhat organized pattern at regular intervals in human history. By analyzing these mutational relationships, researchers have been able to determine that we can all trace our ancestry back to one woman who lived in Africa about 200,000 years ago. Scientists have given this woman the biblical name Eve, although she is not, of course, the first Homo sapiens female. More precisely, she is our most recent common ancestor through matrilineal descent.

This doesn’t mean that everyone’s mtDNA today looks exactly like that of our ancestral Eve. Because of the spontaneous mutations in mtDNA that have occurred over the centuries, researchers can map different “branches” off of the “main trunk” of our mtDNA family tree. Your mtDNA might have a pattern of mutations that aligns more closely with one branch, and your neighbor’s may align with another branch. Still, all branches eventually lead back to Eve.

But what happened to the mtDNA of all of the other Homo sapiens females who were living at the time of Eve? Researchers explain that, over the centuries, their female descendants died childless or with only male children, and thus, their maternal line—and its mtDNA—ended.

Folliculogenesis

Again, ovarian follicles are oocytes and their supporting cells. They grow and develop in a process called folliculogenesis , which typically leads to ovulation of one follicle approximately every 28 days, along with death to multiple other follicles. The death of ovarian follicles is called atresia, and can occur at any point during follicular development. Recall that, a female infant at birth will have one to two million oocytes within her ovarian follicles, and that this number declines throughout life until menopause, when no follicles remain. As you’ll see next, follicles progress from primordial, to primary, to secondary and tertiary stages prior to ovulation—with the oocyte inside the follicle remaining as a primary oocyte until right before ovulation.

Folliculogenesis begins with follicles in a resting state. These small primordial follicles are present in newborn females and are the prevailing follicle type in the adult ovary ( Figure 27.12 ). Primordial follicles have only a single flat layer of support cells, called granulosa cells , that surround the oocyte, and they can stay in this resting state for years—some until right before menopause.

After puberty, a few primordial follicles will respond to a recruitment signal each day, and will join a pool of immature growing follicles called primary follicles . Primary follicles start with a single layer of granulosa cells, but the granulosa cells then become active and transition from a flat or squamous shape to a rounded, cuboidal shape as they increase in size and proliferate. As the granulosa cells divide, the follicles—now called secondary follicles (see Figure 27.12 )—increase in diameter, adding a new outer layer of connective tissue, blood vessels, and theca cells —cells that work with the granulosa cells to produce estrogens.

Within the growing secondary follicle, the primary oocyte now secretes a thin acellular membrane called the zona pellucida that will play a critical role in fertilization. A thick fluid, called follicular fluid, that has formed between the granulosa cells also begins to collect into one large pool, or antrum . Follicles in which the antrum has become large and fully formed are considered tertiary follicles (or antral follicles). Several follicles reach the tertiary stage at the same time, and most of these will undergo atresia. The one that does not die will continue to grow and develop until ovulation, when it will expel its secondary oocyte surrounded by several layers of granulosa cells from the ovary. Keep in mind that most follicles don’t make it to this point. In fact, roughly 99 percent of the follicles in the ovary will undergo atresia, which can occur at any stage of folliculogenesis.

Hormonal Control of the Ovarian Cycle

The process of development that we have just described, from primordial follicle to early tertiary follicle, takes approximately two months in humans. The final stages of development of a small cohort of tertiary follicles, ending with ovulation of a secondary oocyte, occur over a course of approximately 28 days. These changes are regulated by many of the same hormones that regulate the male reproductive system, including GnRH, LH, and FSH.

As in men, the hypothalamus produces GnRH, a hormone that signals the anterior pituitary gland to produce the gonadotropins FSH and LH ( Figure 27.13 ). These gonadotropins leave the pituitary and travel through the bloodstream to the ovaries, where they bind to receptors on the granulosa and theca cells of the follicles. FSH stimulates the follicles to grow (hence its name of follicle-stimulating hormone), and the five or six tertiary follicles expand in diameter. The release of LH also stimulates the granulosa and theca cells of the follicles to produce the sex steroid hormone estradiol, a type of estrogen. This phase of the ovarian cycle, when the tertiary follicles are growing and secreting estrogen, is known as the follicular phase.

The more granulosa and theca cells a follicle has (that is, the larger and more developed it is), the more estrogen it will produce in response to LH stimulation. As a result of these large follicles producing large amounts of estrogen, systemic plasma estrogen concentrations increase. Following a classic negative feedback loop, the high concentrations of estrogen will stimulate the hypothalamus and pituitary to reduce the production of GnRH, LH, and FSH. Because the large tertiary follicles require FSH to grow and survive at this point, this decline in FSH caused by negative feedback leads most of them to die (atresia). Typically only one follicle, now called the dominant follicle, will survive this reduction in FSH, and this follicle will be the one that releases an oocyte. Scientists have studied many factors that lead to a particular follicle becoming dominant: size, the number of granulosa cells, and the number of FSH receptors on those granulosa cells all contribute to a follicle becoming the one surviving dominant follicle.

When only the one dominant follicle remains in the ovary, it again begins to secrete estrogen. It produces more estrogen than all of the developing follicles did together before the negative feedback occurred. It produces so much estrogen that the normal negative feedback doesn’t occur. Instead, these extremely high concentrations of systemic plasma estrogen trigger a regulatory switch in the anterior pituitary that responds by secreting large amounts of LH and FSH into the bloodstream (see Figure 27.13 ). The positive feedback loop by which more estrogen triggers release of more LH and FSH only occurs at this point in the cycle.

It is this large burst of LH (called the LH surge) that leads to ovulation of the dominant follicle. The LH surge induces many changes in the dominant follicle, including stimulating the resumption of meiosis of the primary oocyte to a secondary oocyte. As noted earlier, the polar body that results from unequal cell division simply degrades. The LH surge also triggers proteases (enzymes that cleave proteins) to break down structural proteins in the ovary wall on the surface of the bulging dominant follicle. This degradation of the wall, combined with pressure from the large, fluid-filled antrum, results in the expulsion of the oocyte surrounded by granulosa cells into the peritoneal cavity. This release is ovulation.

In the next section, you will follow the ovulated oocyte as it travels toward the uterus, but there is one more important event that occurs in the ovarian cycle. The surge of LH also stimulates a change in the granulosa and theca cells that remain in the follicle after the oocyte has been ovulated. This change is called luteinization (recall that the full name of LH is luteinizing hormone), and it transforms the collapsed follicle into a new endocrine structure called the corpus luteum , a term meaning “yellowish body” (see Figure 27.12 ). Instead of estrogen, the luteinized granulosa and theca cells of the corpus luteum begin to produce large amounts of the sex steroid hormone progesterone, a hormone that is critical for the establishment and maintenance of pregnancy. Progesterone triggers negative feedback at the hypothalamus and pituitary, which keeps GnRH, LH, and FSH secretions low, so no new dominant follicles develop at this time.

The post-ovulatory phase of progesterone secretion is known as the luteal phase of the ovarian cycle. If pregnancy does not occur within 10 to 12 days, the corpus luteum will stop secreting progesterone and degrade into the corpus albicans , a nonfunctional “whitish body” that will disintegrate in the ovary over a period of several months. During this time of reduced progesterone secretion, FSH and LH are once again stimulated, and the follicular phase begins again with a new cohort of early tertiary follicles beginning to grow and secrete estrogen.

The Uterine Tubes

The uterine tubes (also called fallopian tubes or oviducts) serve as the conduit of the oocyte from the ovary to the uterus ( Figure 27.14 ). Each of the two uterine tubes is close to, but not directly connected to, the ovary and divided into sections. The isthmus is the narrow medial end of each uterine tube that is connected to the uterus. The wide distal infundibulum flares out with slender, finger-like projections called fimbriae . The middle region of the tube, called the ampulla , is where fertilization often occurs. The uterine tubes also have three layers: an outer serosa, a middle smooth muscle layer, and an inner mucosal layer. In addition to its mucus-secreting cells, the inner mucosa contains ciliated cells that beat in the direction of the uterus, producing a current that will be critical to move the oocyte.

Following ovulation, the secondary oocyte surrounded by a few granulosa cells is released into the peritoneal cavity. The nearby uterine tube, either left or right, receives the oocyte. Unlike sperm, oocytes lack flagella, and therefore cannot move on their own. So how do they travel into the uterine tube and toward the uterus? High concentrations of estrogen that occur around the time of ovulation induce contractions of the smooth muscle along the length of the uterine tube. These contractions occur every 4 to 8 seconds, and the result is a coordinated movement that sweeps the surface of the ovary and the pelvic cavity. Current flowing toward the uterus is generated by coordinated beating of the cilia that line the outside and lumen of the length of the uterine tube. These cilia beat more strongly in response to the high estrogen concentrations that occur around the time of ovulation. As a result of these mechanisms, the oocyte–granulosa cell complex is pulled into the interior of the tube. Once inside, the muscular contractions and beating cilia move the oocyte slowly toward the uterus. When fertilization does occur, sperm typically meet the egg while it is still moving through the ampulla.

Interactive Link

Watch this video to observe ovulation and its initiation in response to the release of FSH and LH from the pituitary gland. What specialized structures help guide the oocyte from the ovary into the uterine tube?

If the oocyte is successfully fertilized, the resulting zygote will begin to divide into two cells, then four, and so on, as it makes its way through the uterine tube and into the uterus. There, it will implant and continue to grow. If the egg is not fertilized, it will simply degrade—either in the uterine tube or in the uterus, where it may be shed with the next menstrual period.

The open-ended structure of the uterine tubes can have significant health consequences if bacteria or other contagions enter through the vagina and move through the uterus, into the tubes, and then into the pelvic cavity. If this is left unchecked, a bacterial infection (sepsis) could quickly become life-threatening. The spread of an infection in this manner is of special concern when unskilled practitioners perform abortions in non-sterile conditions. Sepsis is also associated with sexually transmitted bacterial infections, especially gonorrhea and chlamydia. These increase a woman’s risk for pelvic inflammatory disease (PID), infection of the uterine tubes or other reproductive organs. Even when resolved, PID can leave scar tissue in the tubes, leading to infertility.

Watch this series of videos to look at the movement of the oocyte through the ovary. The cilia in the uterine tube promote movement of the oocyte. What would likely occur if the cilia were paralyzed at the time of ovulation?

The Uterus and Cervix

The uterus is the muscular organ that nourishes and supports the growing embryo (see Figure 27.14 ). Its average size is approximately 5 cm wide by 7 cm long (approximately 2 in by 3 in) when a female is not pregnant. It has three sections. The portion of the uterus superior to the opening of the uterine tubes is called the fundus . The middle section of the uterus is called the body of uterus (or corpus). The cervix is the narrow inferior portion of the uterus that projects into the vagina. The cervix produces mucus secretions that become thin and stringy under the influence of high systemic plasma estrogen concentrations, and these secretions can facilitate sperm movement through the reproductive tract.

Several ligaments maintain the position of the uterus within the abdominopelvic cavity. The broad ligament is a fold of peritoneum that serves as a primary support for the uterus, extending laterally from both sides of the uterus and attaching it to the pelvic wall. The round ligament attaches to the uterus near the uterine tubes, and extends to the labia majora. Finally, the uterosacral ligament stabilizes the uterus posteriorly by its connection from the cervix to the pelvic wall.

The wall of the uterus is made up of three layers. The most superficial layer is the serous membrane, or perimetrium , which consists of epithelial tissue that covers the exterior portion of the uterus. The middle layer, or myometrium , is a thick layer of smooth muscle responsible for uterine contractions. Most of the uterus is myometrial tissue, and the muscle fibers run horizontally, vertically, and diagonally, allowing the powerful contractions that occur during labor and the less powerful contractions (or cramps) that help to expel menstrual blood during a woman’s period. Anteriorly directed myometrial contractions also occur near the time of ovulation, and are thought to possibly facilitate the transport of sperm through the female reproductive tract.

The innermost layer of the uterus is called the endometrium . The endometrium contains a connective tissue lining, the lamina propria, which is covered by epithelial tissue that lines the lumen. Structurally, the endometrium consists of two layers: the stratum basalis and the stratum functionalis (the basal and functional layers). The stratum basalis layer is part of the lamina propria and is adjacent to the myometrium; this layer does not shed during menses. In contrast, the thicker stratum functionalis layer contains the glandular portion of the lamina propria and the endothelial tissue that lines the uterine lumen. It is the stratum functionalis that grows and thickens in response to increased levels of estrogen and progesterone. In the luteal phase of the menstrual cycle, special branches off of the uterine artery called spiral arteries supply the thickened stratum functionalis. This inner functional layer provides the proper site of implantation for the fertilized egg, and—should fertilization not occur—it is only the stratum functionalis layer of the endometrium that sheds during menstruation.

Recall that during the follicular phase of the ovarian cycle, the tertiary follicles are growing and secreting estrogen. At the same time, the stratum functionalis of the endometrium is thickening to prepare for a potential implantation. The post-ovulatory increase in progesterone, which characterizes the luteal phase, is key for maintaining a thick stratum functionalis. As long as a functional corpus luteum is present in the ovary, the endometrial lining is prepared for implantation. Indeed, if an embryo implants, signals are sent to the corpus luteum to continue secreting progesterone to maintain the endometrium, and thus maintain the pregnancy. If an embryo does not implant, no signal is sent to the corpus luteum and it degrades, ceasing progesterone production and ending the luteal phase. Without progesterone, the endometrium thins and, under the influence of prostaglandins, the spiral arteries of the endometrium constrict and rupture, preventing oxygenated blood from reaching the endometrial tissue. As a result, endometrial tissue dies and blood, pieces of the endometrial tissue, and white blood cells are shed through the vagina during menstruation, or the menses . The first menses after puberty, called menarche , can occur either before or after the first ovulation.

The Menstrual Cycle

Now that we have discussed the maturation of the cohort of tertiary follicles in the ovary, the build-up and then shedding of the endometrial lining in the uterus, and the function of the uterine tubes and vagina, we can put everything together to talk about the three phases of the menstrual cycle —the series of changes in which the uterine lining is shed, rebuilds, and prepares for implantation.

The timing of the menstrual cycle starts with the first day of menses, referred to as day one of a woman’s period. Cycle length is determined by counting the days between the onset of bleeding in two subsequent cycles. Because the average length of a woman’s menstrual cycle is 28 days, this is the time period used to identify the timing of events in the cycle. However, the length of the menstrual cycle varies among women, and even in the same woman from one cycle to the next, typically from 21 to 32 days.

Just as the hormones produced by the granulosa and theca cells of the ovary “drive” the follicular and luteal phases of the ovarian cycle, they also control the three distinct phases of the menstrual cycle. These are the menses phase, the proliferative phase, and the secretory phase.

Menses Phase

The menses phase of the menstrual cycle is the phase during which the lining is shed; that is, the days that the woman menstruates. Although it averages approximately five days, the menses phase can last from 2 to 7 days, or longer. As shown in Figure 27.15 , the menses phase occurs during the early days of the follicular phase of the ovarian cycle, when progesterone, FSH, and LH levels are low. Recall that progesterone concentrations decline as a result of the degradation of the corpus luteum, marking the end of the luteal phase. This decline in progesterone triggers the shedding of the stratum functionalis of the endometrium.

Proliferative Phase

Once menstrual flow ceases, the endometrium begins to proliferate again, marking the beginning of the proliferative phase of the menstrual cycle (see Figure 27.15 ). It occurs when the granulosa and theca cells of the tertiary follicles begin to produce increased amounts of estrogen. These rising estrogen concentrations stimulate the endometrial lining to rebuild.

Recall that the high estrogen concentrations will eventually lead to a decrease in FSH as a result of negative feedback, resulting in atresia of all but one of the developing tertiary follicles. The switch to positive feedback—which occurs with the elevated estrogen production from the dominant follicle—then stimulates the LH surge that will trigger ovulation. In a typical 28-day menstrual cycle, ovulation occurs on day 14. Ovulation marks the end of the proliferative phase as well as the end of the follicular phase.

Secretory Phase

In addition to prompting the LH surge, high estrogen levels increase the uterine tube contractions that facilitate the pick-up and transfer of the ovulated oocyte. High estrogen levels also slightly decrease the acidity of the vagina, making it more hospitable to sperm. In the ovary, the luteinization of the granulosa cells of the collapsed follicle forms the progesterone-producing corpus luteum, marking the beginning of the luteal phase of the ovarian cycle. In the uterus, progesterone from the corpus luteum begins the secretory phase of the menstrual cycle, in which the endometrial lining prepares for implantation (see Figure 27.15 ). Over the next 10 to 12 days, the endometrial glands secrete a fluid rich in glycogen. If fertilization has occurred, this fluid will nourish the ball of cells now developing from the zygote. At the same time, the spiral arteries develop to provide blood to the thickened stratum functionalis.

If no pregnancy occurs within approximately 10 to 12 days, the corpus luteum will degrade into the corpus albicans. Levels of both estrogen and progesterone will fall, and the endometrium will grow thinner. Prostaglandins will be secreted that cause constriction of the spiral arteries, reducing oxygen supply. The endometrial tissue will die, resulting in menses—or the first day of the next cycle.

Disorders of the...

Female reproductive system.

Research over many years has confirmed that cervical cancer is most often caused by a sexually transmitted infection with human papillomavirus (HPV). There are over 100 related viruses in the HPV family, and the characteristics of each strain determine the outcome of the infection. In all cases, the virus enters body cells and uses its own genetic material to take over the host cell’s metabolic machinery and produce more virus particles.

HPV infections are common in both men and women. Indeed, a recent study determined that 42.5 percent of females had HPV at the time of testing. These women ranged in age from 14 to 59 years and differed in race, ethnicity, and number of sexual partners. Of note, the prevalence of HPV infection was 53.8 percent among women aged 20 to 24 years, the age group with the highest infection rate.

HPV strains are classified as high or low risk according to their potential to cause cancer. Though most HPV infections do not cause disease, the disruption of normal cellular functions in the low-risk forms of HPV can cause the male or female human host to develop genital warts. Often, the body is able to clear an HPV infection by normal immune responses within 2 years. However, the more serious, high-risk infection by certain types of HPV can result in cancer of the cervix ( Figure 27.16 ). Infection with either of the cancer-causing variants HPV 16 or HPV 18 has been linked to more than 70 percent of all cervical cancer diagnoses. Although even these high-risk HPV strains can be cleared from the body over time, infections persist in some individuals. If this happens, the HPV infection can influence the cells of the cervix to develop precancerous changes.

Risk factors for cervical cancer include having unprotected sex; having multiple sexual partners; a first sexual experience at a younger age, when the cells of the cervix are not fully mature; failure to receive the HPV vaccine; a compromised immune system; and smoking. The risk of developing cervical cancer is doubled with cigarette smoking.

When the high-risk types of HPV enter a cell, two viral proteins are used to neutralize proteins that the host cells use as checkpoints in the cell cycle. The best studied of these proteins is p53. In a normal cell, p53 detects DNA damage in the cell’s genome and either halts the progression of the cell cycle—allowing time for DNA repair to occur—or initiates apoptosis. Both of these processes prevent the accumulation of mutations in a cell’s genome. High-risk HPV can neutralize p53, keeping the cell in a state in which fast growth is possible and impairing apoptosis, allowing mutations to accumulate in the cellular DNA.

The prevalence of cervical cancer in the United States is very low because of regular screening exams called pap smears. Pap smears sample cells of the cervix, allowing the detection of abnormal cells. If pre-cancerous cells are detected, there are several highly effective techniques that are currently in use to remove them before they pose a danger. However, women in developing countries often do not have access to regular pap smears. As a result, these women account for as many as 80 percent of the cases of cervical cancer worldwide.

In 2006, the first vaccine against the high-risk types of HPV was approved. There are now two HPV vaccines available: Gardasil ® and Cervarix ® . Whereas these vaccines were initially only targeted for women, because HPV is sexually transmitted, both men and women require vaccination for this approach to achieve its maximum efficacy. A recent study suggests that the HPV vaccine has cut the rates of HPV infection by the four targeted strains at least in half. Unfortunately, the high cost of manufacturing the vaccine is currently limiting access to many women worldwide.

The Breasts

Whereas the breasts are located far from the other female reproductive organs, they are considered accessory organs of the female reproductive system. The function of the breasts is to supply milk to an infant in a process called lactation. The external features of the breast include a nipple surrounded by a pigmented areola ( Figure 27.17 ), whose coloration may deepen during pregnancy. The areola is typically circular and can vary in size from 25 to 100 mm in diameter. The areolar region is characterized by small, raised areolar glands that secrete lubricating fluid during lactation to protect the nipple from chafing. When a baby nurses, or draws milk from the breast, the entire areolar region is taken into the mouth.

Breast milk is produced by the mammary glands , which are modified sweat glands. The milk itself exits the breast through the nipple via 15 to 20 lactiferous ducts that open on the surface of the nipple. These lactiferous ducts each extend to a lactiferous sinus that connects to a glandular lobe within the breast itself that contains groups of milk-secreting cells in clusters called alveoli (see Figure 27.17 ). The clusters can change in size depending on the amount of milk in the alveolar lumen. Once milk is made in the alveoli, stimulated myoepithelial cells that surround the alveoli contract to push the milk to the lactiferous sinuses. From here, the baby can draw milk through the lactiferous ducts by suckling. The lobes themselves are surrounded by fat tissue, which determines the size of the breast; breast size differs between individuals and does not affect the amount of milk produced. Supporting the breasts are multiple bands of connective tissue called suspensory ligaments that connect the breast tissue to the dermis of the overlying skin.

During the normal hormonal fluctuations in the menstrual cycle, breast tissue responds to changing levels of estrogen and progesterone, which can lead to swelling and breast tenderness in some individuals, especially during the secretory phase. If pregnancy occurs, the increase in hormones leads to further development of the mammary tissue and enlargement of the breasts.

Hormonal Birth Control

Birth control pills take advantage of the negative feedback system that regulates the ovarian and menstrual cycles to stop ovulation and prevent pregnancy. Typically they work by providing a constant level of both estrogen and progesterone, which negatively feeds back onto the hypothalamus and pituitary, thus preventing the release of FSH and LH. Without FSH, the follicles do not mature, and without the LH surge, ovulation does not occur. Although the estrogen in birth control pills does stimulate some thickening of the endometrial wall, it is reduced compared with a normal cycle and is less likely to support implantation.

Some birth control pills contain 21 active pills containing hormones, and 7 inactive pills (placebos). The decline in hormones during the week that the woman takes the placebo pills triggers menses, although it is typically lighter than a normal menstrual flow because of the reduced endometrial thickening. Newer types of birth control pills have been developed that deliver low-dose estrogens and progesterone for the entire cycle (these are meant to be taken 365 days a year), and menses never occurs. While some women prefer to have the proof of a lack of pregnancy that a monthly period provides, menstruation every 28 days is not required for health reasons, and there are no reported adverse effects of not having a menstrual period in an otherwise healthy individual.

Because birth control pills function by providing constant estrogen and progesterone levels and disrupting negative feedback, skipping even just one or two pills at certain points of the cycle (or even being several hours late taking the pill) can lead to an increase in FSH and LH and result in ovulation. It is important, therefore, that the woman follow the directions on the birth control pill package to successfully prevent pregnancy.

Aging and the...

Female fertility (the ability to conceive) peaks when women are in their twenties, and is slowly reduced until a women reaches 35 years of age. After that time, fertility declines more rapidly, until it ends completely at the end of menopause. Menopause is the cessation of the menstrual cycle that occurs as a result of the loss of ovarian follicles and the hormones that they produce. A woman is considered to have completed menopause if she has not menstruated in a full year. After that point, she is considered postmenopausal. The average age for this change is consistent worldwide at between 50 and 52 years of age, but it can normally occur in a woman’s forties, or later in her fifties. Poor health, including smoking, can lead to earlier loss of fertility and earlier menopause.

As a woman reaches the age of menopause, depletion of the number of viable follicles in the ovaries due to atresia affects the hormonal regulation of the menstrual cycle. During the years leading up to menopause, there is a decrease in the levels of the hormone inhibin, which normally participates in a negative feedback loop to the pituitary to control the production of FSH. The menopausal decrease in inhibin leads to an increase in FSH. The presence of FSH stimulates more follicles to grow and secrete estrogen. Because small, secondary follicles also respond to increases in FSH levels, larger numbers of follicles are stimulated to grow; however, most undergo atresia and die. Eventually, this process leads to the depletion of all follicles in the ovaries, and the production of estrogen falls off dramatically. It is primarily the lack of estrogens that leads to the symptoms of menopause.

The earliest changes occur during the menopausal transition, often referred to as peri-menopause, when a women’s cycle becomes irregular but does not stop entirely. Although the levels of estrogen are still nearly the same as before the transition, the level of progesterone produced by the corpus luteum is reduced. This decline in progesterone can lead to abnormal growth, or hyperplasia, of the endometrium. This condition is a concern because it increases the risk of developing endometrial cancer. Two harmless conditions that can develop during the transition are uterine fibroids, which are benign masses of cells, and irregular bleeding. As estrogen levels change, other symptoms that occur are hot flashes and night sweats, trouble sleeping, vaginal dryness, mood swings, difficulty focusing, and thinning of hair on the head along with the growth of more hair on the face. Depending on the individual, these symptoms can be entirely absent, moderate, or severe.

After menopause, lower amounts of estrogens can lead to other changes. Cardiovascular disease becomes as prevalent in women as in men, possibly because estrogens reduce the amount of cholesterol in the blood vessels. When estrogen is lacking, many women find that they suddenly have problems with high cholesterol and the cardiovascular issues that accompany it. Osteoporosis is another problem because bone density decreases rapidly in the first years after menopause. The reduction in bone density leads to a higher incidence of fractures.

Hormone therapy (HT), which employs medication (synthetic estrogens and progestins) to increase estrogen and progestin levels, can alleviate some of the symptoms of menopause. In 2002, the Women’s Health Initiative began a study to observe women for the long-term outcomes of hormone replacement therapy over 8.5 years. However, the study was prematurely terminated after 5.2 years because of evidence of a higher than normal risk of breast cancer in patients taking estrogen-only HT. The potential positive effects on cardiovascular disease were also not realized in the estrogen-only patients. The results of other hormone replacement studies over the last 50 years, including a 2012 study that followed over 1,000 menopausal women for 10 years, have shown cardiovascular benefits from estrogen and no increased risk for cancer. Some researchers believe that the age group tested in the 2002 trial may have been too old to benefit from the therapy, thus skewing the results. In the meantime, intense debate and study of the benefits and risks of replacement therapy is ongoing. Current guidelines approve HT for the reduction of hot flashes or flushes, but this treatment is generally only considered when women first start showing signs of menopausal changes, is used in the lowest dose possible for the shortest time possible (5 years or less), and it is suggested that women on HT have regular pelvic and breast exams.

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• Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix
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27 Chapter 27 The Reproductive System

By Rajeev Chandra

Motivation. 

Washington, D.C. has among the highest rates of sexually transmitted diseases (STDs) and unintended pregnancy in the United States. Increasing everyone’s reproductive health knowledge may help address these reproductive health issues. This analysis assessed whether high-risk pregnant African American women in Washington, D.C. who participated in an intervention to reduce behavioral and psychosocial risks had greater reproductive health knowledge than women receiving usual care.

Learning Objectives

Upon completion of the work in this chapter students should be able to:

• Describe female reproductive organ histology and anatomy
• Describe the gross and microscopic anatomy of the male reproductive organs
• Relate the structure of sperm to its function

Background.

Overview of the Female Reproductive System

The female reproductive system functions to produce gametes and reproductive hormones, just like the male reproductive system; however, it also has the additional task of supporting a developing fetus and delivering it to the outside world. Unlike its male counterpart, the female reproductive system is located primarily inside the pelvic cavity (Figure 27.2). Recall that the ovaries are the female gonads and the gamete that is produced is called an oocyte .

The ovaries are the female gonads (Figure 27.2 and Figure 27.3). Paired ovals, they are each about 2 to 3 cm in length, about the size of an almond. The ovaries are located within the pelvic cavity, and are supported by the mesovarium, an extension of the peritoneum that connects the ovaries to the broad ligament . Extending from the mesovarium itself is the suspensory ligament that contains the ovarian blood and lymphatic vessels. Finally, the ovary itself is attached to the uterus via the ovarian ligament .

The ovary consists of multiple layers of tissue. The outer-most covering of cuboidal epithelium called the ovarian surface epithelium sits just superficial to a dense connective tissue layer, known as the tunica albuginea . Beneath the tunica albuginea is the cortex , or outer portion, of the organ itself. The cortex is composed of a tissue framework called the ovarian stroma that forms the bulk of the adult ovary. Oocytes develop within the outer layer of this stroma, each surrounded by supporting cells. This grouping of an oocyte and its supporting cells is called a follicle (Figure 27.3). The growth and development of ovarian follicles will be described shortly. Beneath the cortex lies the inner ovarian medulla , where the majority of blood vessels, lymphatic vessels, and the nerves of the ovary are localized to.

The Ovarian Cycle and Oogenesis

The ovarian cycle is a set of predictable changes in a female’s oocytes and ovarian follicles. During a woman’s reproductive years, it is a roughly 28-day cycle that can be correlated with, but is not the same as, the menstrual cycle . The cycle includes two interrelated processes: oogenesis (the production of female gametes) and folliculogenesis (the growth and development of ovarian follicles).

Gametogenesis in females is called oogenesis . The process begins with ovarian stem cells, or oogonia (pleural: oogonium ) (Figure 27.4). Oogonia are formed during fetal development, and divide via mitosis , much like spermatogonia in the testis. Unlike spermatogonia, however, oogonia form primary oocytes in the fetal ovary prior to birth. These primary oocytes are then arrested in prophase of meiosis I, only to resume it years later, beginning at puberty and continuing until the woman is near menopause (the cessation of a woman’s reproductive functions). The number of primary oocytes present in the ovaries declines from one to two million in an infant, to approximately 400,000 at puberty, to zero by the end of menopause.

The unequal cell division of oogenesis produces one to three polar bodies that later degrade, as well as a single haploid ovum, which is produced only if there is penetration of the secondary oocyte by a sperm cell.

The initiation of ovulation , the release of an oocyte from the ovary, marks the transition from puberty into reproductive maturity for women. From the onset of ovulation and throughout a woman’s reproductive years, ovulation occurs approximately once every 28 days. Just prior to ovulation, a surge of luteinizing hormone triggers the resumption of meiosis in a primary oocyte. This initiates the transition from primary to secondary oocyte . However, as you can see in Figure 27.4, this cell division does not result in two identical cells. Instead, the cytoplasm is divided unequally, and one daughter cell is much larger than the other. This larger cell, the secondary oocyte , eventually leaves the ovary during ovulation. The smaller cell, called the first polar body , may or may not complete meiosis and produce second polar bodies; in either case, it eventually disintegrates. Therefore, even though oogenesis produces up to four cells, only one survives.

A question still remains though: How does the diploid secondary oocyte become an ovum —the haploid female gamete? Meiosis of a secondary oocyte is completed only if a sperm succeeds in penetrating its barriers . If union of a secondary oocyte and a sperm is successful, only then will meiosis II resume. This fusion will produce one haploid ovum that, at the moment of fertilization by a (haploid) sperm, becomes the first diploid cell of the new offspring (a zygote ). Thus, the ovum can be thought of as a brief, transitional, haploid stage between the diploid oocyte and diploid zygote.

The larger amount of cytoplasm contained in the female gamete is used to supply the developing zygote with nutrients during the period between fertilization and implantation into the uterus. Interestingly, sperm contribute only DNA at fertilization —not cytoplasm. Therefore, the cytoplasm and all of the cytoplasmic organelles in the developing embryo are of maternal origin.

Folliculogenesis

Remember, ovarian follicles are oocytes and their supporting cells. They grow and develop in a process called folliculogenesis , which typically leads to ovulation of one follicle approximately every 28 days, along with death to multiple other follicles. The death of ovarian follicles is called atresia , and can occur at any point during follicular development. Recall that, a female infant at birth will have one to two million oocytes within her ovarian follicles, and that this number declines throughout life until menopause, when no follicles remain. As you’ll see next, follicles progress from primordial , to primary , to secondary and finally tertiary stages prior to ovulation—with the oocyte inside the follicle remaining as a primary oocyte until right before ovulation.

Folliculogenesis begins with follicles in a resting state. These small primordial follicles are present in newborn females and are the prevailing follicle type in the adult ovary (Figure 27.5). Primordial follicles have only a single flat layer of supporting cells, called granulosa cells , that surround the primary oocyte, and they can stay in this resting state for years—some until right before menopause.

After puberty, a few primordial follicles will respond to a recruitment signal each day, and will join a pool of immature growing follicles called primary follicles . Primary follicles start with a single layer of granulosa cells, but the granulosa cells then become active and transition from a flat or squamous shape to a rounded, cuboidal shape as they increase in size and proliferate. As the granulosa cells divide, the follicles—now called secondary follicles (Figure 27.5)—increase in diameter, adding a new outer layer of connective tissue, blood vessels, and theca cells —cells that work with the granulosa cells to produce estrogens. Within the growing secondary follicle, the primary oocyte now secretes a thin acellular membrane called the zona pellucida that will play a critical role in fertilization. A thick fluid, called follicular fluid, that has formed between the granulosa cells also begins to collect into one large pool, the antrum . Follicles in which the antrum has become large and fully formed are considered tertiary follicles (or antral follicles ). Several follicles reach the tertiary stage at the same time, and most of these will undergo atresia. The one that does not die will continue to grow and develop until ovulation, when it will expel its secondary oocyte surrounded by several layers of granulosa cells from the ovary. Keep in mind that most follicles don’t make it to this point. In fact, roughly 99 percent of the follicles in the ovary will undergo atresia, which can occur at any stage of folliculogenesis.

Hormonal Control of the Ovarian Cycle

The process of development that we have just described, from primordial follicle to early tertiary follicle, takes approximately two months in humans. The final stages of development of a small cohort of tertiary follicles, ending with ovulation of a secondary oocyte, occur over a course of approximately 28 days. These changes are regulated by many of the same hormones that regulate the male reproductive system, including GnRH , LH , and FSH .

As in men, the hypothalamus produces GnRH, a hormone that signals the anterior pituitary gland to produce the gonadotropins FSH and LH (Figure 27.6). These gonadotropins leave the pituitary and travel through the bloodstream to the ovaries, where they bind to receptors on the granulosa and theca cells of the follicles. FSH stimulates the follicles to grow (hence its name of follicle-stimulating hormone), and the five or six tertiary follicles expand in diameter. The release of LH also stimulates the granulosa and theca cells of the follicles to produce the sex steroid hormone estradiol , a type of estrogen .

This phase of the ovarian cycle, when the tertiary follicles are growing and secreting estrogen, is known as the follicular phase . The more granulosa and theca cells a follicle has (that is, the larger and more developed it is), the more estrogen it will produce in response to LH stimulation. As a result of these large follicles producing large amounts of estrogen, systemic plasma estrogen concentrations increase. Following a classic negative feedback loop, the high concentrations of estrogen will stimulate the hypothalamus and pituitary to reduce the production of GnRH, LH, and FSH. Because the large tertiary follicles require FSH to grow and survive at this point, this decline in FSH caused by negative feedback leads most of them to die (atresia) (Figure 27.6, step 1). Typically, only one follicle, now called the dominant follicle , will survive this reduction in FSH, and this follicle will be the one that releases an oocyte. Scientists have studied many factors that lead to a particular follicle becoming dominant: size, the number of granulosa cells, and the number of FSH receptors on those granulosa cells all contribute to a follicle becoming the one surviving dominant follicle.

When only the dominant follicle remains in the ovary, it again begins to secrete estrogen. It produces more estrogen than all of the developing follicles did together before the negative feedback occurred. It produces so much estrogen that the normal negative feedback doesn’t occur. Instead, these extremely high concentrations of systemic plasma estrogen trigger a regulatory switch in the anterior pituitary that responds by secreting large amounts of LH and FSH into the bloodstream (Figure 27.6, step 2).

The positive feedback loop by which more estrogen triggers release of more LH and FSH only occurs at this point in the cycle. It is this large burst of LH (called the LH surge) that leads to ovulation of the dominant follicle. The LH surge induces many changes in the dominant follicle, including stimulating the resumption of meiosis of the primary oocyte to a secondary oocyte. As noted earlier, the polar body that results from unequal cell division simply degrades. The LH surge also triggers proteases (enzymes that cleave proteins) to break down structural proteins in the ovary wall on the surface of the bulging dominant follicle. This degradation of the wall, combined with pressure from the large, fluid-filled antrum, results in the expulsion of the oocyte surrounded by granulosa cells into the peritoneal cavity. This release is ovulation .

There is one more important event that occurs in the ovarian cycle. The surge of LH also stimulates a change in the granulosa and theca cells that remain in the follicle after the oocyte has been ovulated. This change is called luteinization and it transforms the collapsed follicle into a new endocrine structure called the corpus luteum , a term meaning “yellowish body” (Figure 27.5). Instead of estrogen, the luteinized granulosa and theca cells of the corpus luteum begin to produce large amounts of the sex steroid hormone progesterone , a hormone that is critical for the establishment and maintenance of pregnancy. Progesterone triggers negative feedback at the hypothalamus and pituitary, which keeps GnRH, LH, and FSH secretions low, so no new dominant follicles develop at this time. This post-ovulatory phase of progesterone secretion is known as the luteal phase of the ovarian cycle (Figure 27.6, step 3). If pregnancy does not occur within 10 to 12 days, the corpus luteum will stop secreting progesterone and degrade into the corpus albicans , a nonfunctional “whitish body” that will degenerate in the ovary over a period of several months. During this time of reduced progesterone secretion, FSH and LH are once again stimulated, and the follicular phase begins again with a new cohort of early tertiary follicles beginning to grow and secrete estrogen.

Uterine (Fallopian) Tubes

The uterine tubes (also called fallopian tubes or oviducts ) serve as the conduit of the oocyte from the ovary to the uterus (Figure 27.3). Each of the two uterine tubes is close to, but not directly connected to, the ovary and each is divided into sections. The isthmus is the narrow medial end of each uterine tube that is connected to the uterus. The wide distal infundibulum flares out with slender, finger-like projections called fimbriae . The middle region of the tube, called the ampulla , is where fertilization often occurs. The uterine tubes also have three layers of tissue: an outer serosa, a middle smooth muscle layer, and an inner mucosal layer. In addition to its mucus-secreting cells, the inner mucosa contains ciliated cells that beat in the direction of the uterus, producing a current that will be critical to move the oocyte.

Following ovulation, the secondary oocyte surrounded by a few granulosa cells is released into the peritoneal cavity. The nearby uterine tube, either left or right, receives the oocyte. Unlike sperm, oocytes lack flagella, and therefore cannot move on their own. So how do they travel into the uterine tube and toward the uterus? High concentrations of estrogen that occur around the time of ovulation induce contractions of the smooth muscle along the length of the uterine tube. These contractions occur every 4 to 8 seconds, and the result is a coordinated movement that sweeps the surface of the ovary and the pelvic cavity. As a result of these mechanisms, the oocyte–granulosa cell complex is pulled into the interior of the tube. Once inside, the muscular contractions and beating cilia move the oocyte slowly toward the uterus. When fertilization does occur, sperm typically meet the egg while it is still moving through the ampulla.

The uterus is the muscular organ that nourishes and supports the growing embryo (Figure 27.3). Its average size is approximately 5 cm wide by 7 cm long when a female is not pregnant. It has three sections: the portion of the uterus superior to the opening of the uterine tubes is called the fundus , the middle section of the uterus is called the body or corpus , and the cervix is the narrow inferior portion of the uterus that projects into the vagina.

The wall of the uterus is made up of three layers (Figure 27.3 and Figure 27.7). The most superficial layer is the serous membrane, or perimetrium , which consists of epithelial tissue that covers the exterior portion of the uterus. The middle layer, or myometrium , is a thick layer of smooth muscle responsible for uterine contractions. Most of the uterus is myometrial tissue, and the muscle fibers run horizontally, vertically, and diagonally, allowing the powerful contractions that occur during labor and the less powerful contractions (or cramps) that help to expel menstrual blood during a woman’s period.

The innermost layer of the uterus is called the endometrium . Structurally, the endometrium consists of two layers: the stratum basalis ( basal layer ) and the stratum functionalis ( functional layer ). The stratum basalis layer lies adjacent to the myometrium; this layer does not shed during menses. In contrast, the thicker stratum functionalis layer contains the glandular endothelial tissues that line the uterine lumen. It is the stratum functionalis that grows and thickens in response to increased levels of estrogen and progesterone. In the luteal phase of the menstrual cycle, special branches off of the uterine artery called spiral arteries supply the thickened stratum functionalis (Figure 27.7). This inner functional layer provides the proper site of implantation for a fertilized egg, and—should fertilization not occur—it is only the functional layer of the endometrium that sheds during menstruation.

Figure 27.7 Layers and arterial vasculature of the uterus. The wall of the uterus consists of 3 layers: the outer perimetrium (not shown), the myometrium, and the endometrium. Credit: Mikael Häggström, Wikimedia Commons, license Public Domain.

Several ligaments maintain the position of the uterus within the abdominopelvic cavity (Figure 27.3). The broad ligament is a fold of peritoneum that serves as a primary support for the uterus, extending laterally from both sides of the uterus and attaching it to the pelvic wall. The round ligament attaches to the uterus near the uterine tubes, and extends to the labia majora.

The vagina , shown at the bottom of Figure 27.2 and in Figure 27.8, is a muscular canal (approximately 10 cm long) that serves as the entrance to the reproductive tract. It also serves as the exit from the uterus during menses and childbirth. The outer walls of the anterior and posterior vagina are formed into longitudinal columns, or ridges , and the superior portion of the vagina—called the fornix —meets the protruding uterine cervix. The walls of the vagina are lined with an outer, fibrous adventitia, a middle layer of smooth muscle, and an inner mucous membrane with transverse folds called rugae . Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. A thin, perforated hymen can partially surround the opening to the vaginal orifice ( opening ).

External Genitalia

The external female reproductive structures are referred to collectively as the vulva (Figures 27.2 and 27.9) and they include the structures that will be discussed next. The mons pubis is a pad of fat that is located anteriorly, over the pubic bone. After puberty, it becomes covered in pubic hair. The labia majora (labia = “lips”; majora = “larger”) are folds of hair-covered skin that begin just posterior to the mons pubis. The thinner and more pigmented labia minora (labia = “lips”; minora = “smaller”) extend medial to the labia majora and the space between labia minora is known as the vestibule . Although they naturally vary in shape and size from woman to woman, the labia minora serve to protect the female urethra and the entrance to the female reproductive tract.

The superior, anterior portions of the labia minora come together to encircle the clitoris (or glans clitoris ), an organ that originates from the same cells as the glans penis, and has abundant nerves that make it important in sexual sensation and orgasm. The hymen is a thin membrane that sometimes partially covers the entrance to the vagina. The vaginal opening, also known as the vaginal orifice , is located between the opening of the urethra and the anus. It is flanked by outlets to the Bartholin’s glands (or greater vestibular glands) .

Whereas the breasts are located far from the other female reproductive organs, they are considered accessory organs of the female reproductive system. The function of the breasts is to supply milk to an infant in a process called lactation . The external features of the breast include a nipple surrounded by a pigmented areola (Figure 27.10), whose coloration may deepen during pregnancy. The areolar region is characterized by small, raised areolar glands that secrete lubricating fluid during lactation to protect the nipple from chafing.

Internally, breast milk is produced by the mammary glands , which are modified sweat glands. The milk itself exits the breast through the nipple via 15 to 20 lactiferous ducts that open on the surface of the nipple. These lactiferous ducts each extend to a lactiferous sinus that connects to a glandular lobe ( lobule ) within the breast itself that contains groups of milksecreting cells in clusters called alveoli (Figure 27.10). Once milk is made in the alveoli, stimulated myoepithelial cells that surround the alveoli contract to push the milk to the lactiferous sinuses. From here, a baby can draw milk through the lactiferous ducts by suckling. The lobules themselves are surrounded by fat tissue, which determines the size of the breast; breast size differs between individuals and does not affect the amount of milk produced. Supporting the breasts are multiple bands of connective tissue called suspensory ligaments that connect the breast tissue to the dermis of the overlying skin.

During the normal hormonal fluctuations in the menstrual cycle, breast tissue responds to changing levels of estrogen and progesterone, which can lead to swelling and breast tenderness in some individuals, especially during the secretory phase. If pregnancy occurs, the increase in hormones leads to further development of the mammary tissue and enlargement of the breasts.

Overview of the Male Reproductive System

The function of the male reproductive system is to produce male gametes , known as sperm , to transfer these to the female reproductive tract, and to secrete the hormones that support male reproductive physiology. The paired gonads , or gamete-producing structures, are the testes (singular, testis ) and they are a crucial component of the male’s reproductive system. While the testes produce both sperm and androgens , several accessory organs and ducts aid in the process of sperm maturation and transport of the sperm and other seminal components to the penis , which delivers sperm to the female reproductive tract.

The structures of the male reproductive system include the testes , the epididymis , and the penis , as well as the ducts and glands that produce and carry semen (Figure 27.11).

Scrotum and Testes

The testes (singular, testis ) are located in a skin-covered, highly pigmented, muscular sack called the scrotum . The scrotum extends from the body behind the penis (Figure 27.11). This location is important to sperm production, which occurs within the testes. The scrotum helps to regulate the temperature of the testes and maintains it around 35 degrees Celsius (95 degrees Fahrenheit). Temperature control is accomplished by the smooth muscles of the scrotum moving the testes either closer to or further away from the abdomen, dependent upon the ambient temperature. This regulatory action is accomplished by the cremaster muscle in the abdomen and the dartos fascia (muscular tissue under the skin) within the scrotum.

The dartos muscle makes up the subcutaneous muscle layer of the scrotum (Figure 27.12). It continues internally to make up the scrotal septum, a wall that divides the scrotum into two compartments, each housing one testis. Descending from the internal oblique muscle of the abdominal wall are the two cremaster muscles , which cover each testis like a muscular net. By contracting simultaneously, the dartos and cremaster muscles can elevate the testes in cold weather (or water), moving the testes closer to the body and decreasing the surface area of the scrotum to retain heat. Alternatively, as the environmental temperature increases, the scrotum relaxes, moving the testes farther from the body core and increasing scrotal surface area, which promotes heat loss. Externally, the scrotum has a raised medial thickening on the surface called the raphe (Figure 27.12).

The testes are the male gonads —that is, the male reproductive organs. They produce both sperm and androgens , such as testosterone , and are active throughout the reproductive lifespan of the male.

Paired ovals, the testes are each approximately 4 to 5 cm in length and are housed within the scrotum (Figures 27.11 and 27.12). They are surrounded by two distinct layers of protective connective tissue (Figure 27.13). The outer tunica vaginalis is a double-layered serous membrane. Beneath the tunica vaginalis is the tunica albuginea , a tough, white, dense connective tissue layer covering the testis itself. Not only does the tunica albuginea cover the outside of the testis, it also invaginates to form septa that divide the testis into 300 to 400 structures called seminal vesicle lobules (or just lobules ). Within each lobule, sperm develop in tube-like structures known as the seminiferous tubules .

Inside the seminiferous tubules are six different cell types. These include supporting cells called sustentacular ( Sertoli ) cells , hormone producing interstitial ( Leydig ) cells , as well as five types of developing sperm cells called germ cells . Germ cell development progresses from the basement membrane—at the perimeter of the tubule—toward the lumen. Let’s look more closely at these cell types.

The least mature germ cells, the spermatogonia (singular; spermatogonium ), line the basement membrane just inside the tubule. Spermatogonia are the stem cells of the testis, meaning that they are still able to differentiate into a variety of different cell types throughout adulthood. Spermatogonia initially divide to produce primary and then secondary spermatocytes , then spermatids , which will finally produce mature sperm . The process that begins with spermatogonia and concludes with the production of sperm is called spermatogenesis , which will be discussed next.

Spermiogenesis and the Structure of a Mature Sperm

Sperm are smaller than most cells in the body; in fact, the volume of a sperm cell is 85,000 times less than that of the female gamete. Approximately 100 to 300 million sperm are produced each day, whereas women typically ovulate only one oocyte per month. As is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head , mid-piece , and tail region (Figure 27.15).

The head of the sperm contains the extremely compact haploid nucleus with very little cytoplasm. These qualities contribute to the overall small size of the sperm (the head is only 5 μ m long). A structure called the acrosome covers most of the head of the sperm cell as a “cap” that is filled with lysosomal enzymes important for preparing sperm to participate in fertilization. Tightly packed mitochondria fill the mid-piece of the sperm. ATP produced by these mitochondria will power the flagellum , which extends from the neck and the mid-piece through the tail of the sperm, enabling it to move the entire sperm cell.

To fertilize an egg, sperm must be moved from the seminiferous tubules in the testes, through the epididymis, and—later during ejaculation—along the length of the penis and out into the female reproductive tract.

From the lumen of the seminiferous tubules, immotile sperm are surrounded by testicular fluid and moved to the epididymis (plural; epididymides ), a coiled tube attached to the testis where newly formed sperm continue to mature ( Figure 4 ). Though the epididymis does not take up much room in its tightly coiled state, it would be approximately 6 m (20 feet) long if straightened. It takes an average of 12 days for sperm to move through the coils of the epididymis, with the shortest recorded transit time in humans being one day. Sperm enter the head of the epididymis and are moved along predominantly by the contraction of smooth muscles lining the epididymal tubes. As they are moved along the length of the epididymis, a region known as the body , the sperm further mature and acquire the ability to move under their own power. Once inside the female reproductive tract, they will use this ability to move independently toward the unfertilized egg. The more mature sperm are then stored in the tail of the epididymis (the final section) until ejaculation occurs.

Spermatic cord

During ejaculation, sperm exit the tail of the epididymis and are pushed by smooth muscle contraction to the ductus deferens (also called the vas deferens ). The vas deferens is a thick, muscular tube that is bundled together inside the scrotum with connective tissue, blood vessels, and nerves, forming a structure known as the spermatic cord (see Figure 27.11 and Figure 27.12). Since the ductus deferens is physically accessible within the scrotum, surgical sterilization to interrupt sperm delivery can be performed by cutting and sealing a small section of the ductus (vas) deferens. This procedure is called a vasectomy , and it is an effective form of male birth control.

As sperm pass through the ampulla (enlarged region) of the ductus deferens at ejaculation, they mix with fluid from the associated seminal vesicles (Figure 27.11 and Figure 27.16). The paired seminal vesicles are glands that contribute approximately 60% of the semen volume. Seminal vesicle fluid contains large amounts of fructose, which is used by the sperm mitochondria to generate ATP to allow movement through the female reproductive tract. The fluid, now containing both sperm and seminal vesicle secretions, next moves into the associated ejaculatory duct , a short structure formed from the ampulla of the ductus deferens and the duct of the seminal vesicle. The paired ejaculatory ducts transport the seminal fluid into the next structure, the prostate gland.

Prostate Gland

As shown in Figure 27.16, the centrally located prostate gland sits anterior to the rectum at the base of the bladder surrounding the prostatic urethra (the portion of the urethra that runs within the prostate). About the size of a walnut, the prostate is formed of both muscular and glandular tissues. It excretes an alkaline, milky fluid into the passing seminal fluid—now called semen .

The External Genitalia

The penis is the male organ of copulation (sexual intercourse). It is flaccid for non-sexual actions, such as urination , and turgid and rod-like with sexual arousal. When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract.

The shaft of the penis surrounds the urethra ( Figure 27.17 ). Internally, the shaft is composed of three column-like chambers of erectile tissue that span the length of the shaft. Each of the two larger lateral chambers is the corpus cavernosum (plural; corpora cavernosa ). Together, these make up the bulk of the penis. The corpus spongiosum , which can be felt as a raised ridge on the erect penis, is a smaller chamber that surrounds the spongy , or penile , urethra .

The end of the penis, called the glans penis , has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation (Figure 27.11). The skin from the shaft extends down over the glans and forms a collar called the prepuce or foreskin (Figure 27.11 and Figure 27.17). The foreskin also contains a dense concentration of nerve endings, and both lubricate and protect the sensitive skin of the glans penis. A surgical procedure called circumcision, often performed for religious or social reasons, removes the prepuce, typically within days of birth.

Both sexual arousal and REM sleep (during which dreaming occurs) can induce an erection. Penile erections are the result of engorgement of the tissues because more arterial blood flows into the penile tissues than is leaving through the veins. To initiate this process during sexual arousal, nitric oxide (NO) is released from nerve endings near these blood vessels within the corpora cavernosa and spongiosum. Release of the NO activates a pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the penis and induces the endothelial cells in the penile arterial walls to also secrete NO and perpetuate the vasodilation. This rapid increase in blood volume fills the erectile chambers, and the increased pressure of the filled chambers compresses the thin-walled penile venules, preventing venous drainage of the penis. The result of this increased blood flow to the penis and reduced blood return from the penis is erection (Figure 27.17).

Hormones of the Male Reproductive System

Testosterone , an androgen, is a steroid hormone produced by Leydig cells . The alternate term for Leydig cells , interstitial cells , reflects their location between the seminiferous tubules in the testes. In male embryos, testosterone is secreted by Leydig cells by the seventh week of development, with peak concentrations reached in the second trimester. This early release of testosterone results in the anatomical differentiation of the male sexual organs. In childhood, testosterone concentrations are low, though they increase during puberty, activating characteristic physical changes and initiating spermatogenesis.

The continued presence of testosterone is necessary to keep the male reproductive system working properly, and Leydig cells produce approximately 6-7 mg of testosterone per day. Maintaining these normal concentrations of testosterone promotes spermatogenesis, whereas low levels of testosterone can lead to infertility. The regulation of testosterone concentrations throughout the body is critical for male reproductive function, requiring an intricate interplay between the endocrine system and the reproductive system. The relationship between these two systems is shown in Figure 27.18.

Together, the hypothalamus and pituitary gland regulate the production of testosterone and the cells that assist in spermatogenesis. Initially, gonadotropin-releasing hormone ( GnRH ) from the hypothalamus activates the anterior pituitary to produce luteinizing hormone ( LH ) and follicle stimulating hormone ( FSH ), which in turn stimulate Leydig cells and Sertoli cells, respectively. The system also establishes a negative feedback loop because the end products of the pathway, testosterone and inhibin, interact with the activity of GnRH to inhibit their own production (Figure 27.18, steps 2 and 3).

The regulation of Leydig cell production of testosterone begins outside of the testes. The hypothalamus and the pituitary gland in the brain integrate external and internal signals to control testosterone synthesis and secretion. Pulsatile release of GnRH from the hypothalamus stimulates the endocrine release of hormones from the pituitary gland. Binding of GnRH to its receptors on the anterior pituitary gland stimulates release of the two gonadotropins: LH and FSH. These two hormones are critical for reproductive function in both men and women. In men, FSH binds predominantly to the Sertoli cells within the seminiferous tubules to promote spermatogenesis. FSH also stimulates the Sertoli cells to produce hormones called inhibins, which function to inhibit FSH release from the pituitary, thus reducing testosterone secretion. In men, LH binds to receptors on Leydig cells in the testes and upregulates the production of testosterone. As previously noted, a negative feedback loop predominantly controls the synthesis and secretion of both of these hormones and testosterone.

In addition to intra-testicular secretion, testosterone is also released into the systemic circulation and plays an important role in muscle development, bone growth, the development of secondary sex characteristics, and maintaining libido (sex drive) in both males and females. In females, the ovaries secrete small amounts of testosterone, although most is converted to estradiol. A small amount of testosterone is also secreted by the adrenal glands in both sexes.

Pre-Laboratory Questions

1.What are the female gonads called?

2.Where does fertilization of the egg by the sperm typically occur?

uterine tube

3.The vulva includes the ________.

lactiferous duct, rugae, and hymen

lactiferous duct, endometrium, and bulbourethral glands

mons pubis, endometrium, and hymen

mons pubis, labia majora, and Bartholin’s glands

4.From what structure does the corpus luteum originate?

uterine corpus

dominant follicle

fallopian tube

corpus albicans

5.What are male gametes called?

testosterone

6.Which hypothalamic hormone contributes to the regulation of the male reproductive system?

luteinizing hormone

gonadotropin-releasing hormone

follicle-stimulating hormone

7.Spermatogenesis takes place in the ________.

prostate gland

glans penis

seminiferous tubules

ejaculatory duct

8.What is the function of the epididymis?

sperm maturation and storage

produces the bulk of seminal fluid

provides nitric oxide needed for erections

spermatogenesis

Exercise 1 Overview of the female reproductive system

Exercise 2 microanatomy of the ovaries, exercise 3 microanatomy of the uterus, exercise 4 anatomy of the breast, exercise 5 overview of the male reproductive system, exercise 6 gross anatomy of the testes, exercise 7 microanatomy of the testes, exercise 8 histology of sperm, exercise 9 external genitalia.

Required Materials

• Torso models
• Female Reproductive System Poster
• Female Pelvis Models
• Human Uterus and Ovary Pathology Model
• Post-it notes
• Labeling tape
• Look at the charts and models of the female reproductive system for a general orientation. Locate the following structures. Use the post-it notes or labeling tape to label each structure on the models. Take pictures and insert these below. Alternatively, you can sketch and label.
• Uterine (fallopian) tube
• Vaginal canal
• Labia minora (singular, labium minus )
• Labia majora (singular, labium majus )

• Compound microscope
• Microscope lens paper
• Microscope lens cleaner
• Microscope immersion oil
• Slide of Human Ovary

1.Obtain a prepared slide or a histological section of the ovary. If using a microscope, observe the sample on low power.

2.Using the slide or provided image, locate the medulla , the highly vascularized tissue in the middle of each ovary. Once identified, look for circular structures within this region. These circles are ovarian follicles . Locate the primordial follicles in your preparation. These follicles contain primary oocytes , while more mature follicles will have secondary oocytes . Sketch and label these structures below:

3.Now observe your slide under high magnification. Using your prepared sample or image, locate the primary , secondary , and tertiary follicles . Some follicles may contain oocytes. Primary follicles will have a single layer of follicular cells surrounding an oocyte; secondary follicles will have multiple layers of follicular cells surrounding an oocyte; tertiary follicles contain significant amounts of fluid in the region known as the antrum . Using your sample and Figure 27.5, try to identify a mature ovarian ( Graafian ) follicle ; these will be the largest follicles present. Sketch and label your observations below:

• Slide of Human Uterus

1.Obtain a prepared slide or a histological section of the uterus. If using a microscope, observe the sample on low power. Using the slide or provided image, locate the three layers of the uterus. Sketch and label these in the space below:

2.Now, examine the endometrium under higher power magnification. You should be able to identify two layers, including the functional layer and the basal layer . The functional layer will be the more superficial layer that is shed during menstruation , while the basal layer is deeper and will be retained. Using the same preparation or image, locate the myometrium. This layer will sit just deep to the endometrial tissue and it can be distinguished by the presence of smooth muscle. Draw an example of what you see below and label your drawing:

• Breast Cross Section Model (pathologies)
• Torso model
• Observe the external anatomy of the breast on the provided models. The major external features of the breast include the pigmented areola , the protruding nipple , the body of the breast, and the axillary tail . Identify each of these structures in the provided materials.
• Use the provided models and charts to locate the internal structures of the breast. Much of the breast is composed of adipose tissue and embedded mammary glands . These glands are responsible for producing milk in lactating females. Identify the mammary gland and the following associated structures on the figure. Each gland consists of clusters of 15-20 lobes . Each lobe contains groups of milk-secreting cells in clusters called alveoli . These clusters can change in size depending on the amount of milk in the alveolar lumen. In nursing females, the mammary glands increase in size and lead to lactiferous ducts , which collect and direct milk to the lactiferous sinuses . Together, the ducts and sinuses collect and direct milk to exit the breast through the nipple.
• Using post-it notes or labeling tape, label these structures on the models. Take pictures and insert these in the space below. Alternatively you can sketch and label:

• Male Reproductive System Poster
• Male Pelvis Models
• Male Pelvis with Testicular Pathology Model
• Scrotal sac (scrotum)
• Ductus (vas) deferens
• Seminal vesicle
• Prostate gland
• Bulbourethral gland

• Examine charts and a model of the testes. The testes are paired organs, sitting outside of the body.
• Using the models identify the tunica albuginea , a tough connective sheath that surrounds the testes. Locate the invaginations of the membrane, where it invaginates to form many lobules within each testis. Located superficial to the tunica, is the scrotal sac ( scrotum ), this structure keeps the testes on the exterior of the body, where temperature tends to be cooler and more supportive to spermatozoa ( sperm ) production. Locate the dartos muscle , a component of the scrotal sac. When the testes are cold, the muscle contracts, tightening the sac and bringing the testes closer to the body. The opposite actions occur when the environment is warm.
• Use the post-it notes or labeling tape to label each structure on the models. Take pictures and insert these below. Alternatively, you can sketch and label.

• Slide of Human Testis
• Model of Meiosis
• Obtain a prepared slide or a histological section of the testes. Identify the seminiferous tubules . Multiple tubules may be identifiable in the preparation. It is within these structures where sperm are produced. Look for the triangular clusters of cells in between each tubule. These are the interstitial ( Leydig ) cells . They will produce the male sex hormone, testosterone .  Sketch and label these tubules and interstitial structures in the space below:

2. Examine the seminiferous tubules under high magnification. You should be able to see an outer row of cells, known as the spermatogonia . These cells will divide by the process of mitosis, giving rise to primary spermatocytes . The primary spermatocytes will then undergo meiosis, or reduction division, to eventually produce spermatozoa . To do so, primary spermatocytes will initially divide to form secondary spermatocytes , which are found closer to the lumen of the seminiferous tubules. These cells will then become spermatids . Ultimately, the spermatids will lose their remaining cytoplasm and mature into functional spermatozoa . Locate the primary and secondary spermatocytes, spermatids, and spermatozoa. You may be able to see sustentacular ( Sertoli ) cells , which help nourish, support, and move the sperm during development.

3. Draw an example of what you see at high magnification of what is listed in step 2 in the space provided below:

• Slide of Human Sperm
• Examine a prepared slide of sperm.
• Identify the different components of the sperm cell. Each sperm consists of a head , midpiece , and tail . Sketch and label these structures below:  
• Torso Models.

• Now, examine a model or chart of a cross section of the penis. Notice that the penis contains three distinct cylinders of erectile tissue that are anchored to the body proximally. Identify the corpus spongiosum , the cylinder of erectile tissue that contains the spongy ( penile ) urethra . Located dorsal to the spongiosum, two cylinders of corpus cavernosa are located. Follow the corpus spongiosum as it extends distally. At the most distal region of the penis, this tissue expands to form the glans penis. Note the dorsal arteries and deep ( cavernosal ) arteries. Together, these vessels take blood to the penis. When these vessels dilate, the erectile tissue of the penis engorge with blood, making the penis erect. Also locate the dorsal vein and venules of the penis. These vessels will remove blood from the penis, except for when the penis is erect. At this point, these vessels are compressed, preventing venous drainage of the penis.

Post-laboratory Questions

• The __________________ is the inner epithelial lining of the uterus.
• A follicle is comprised of _________________ cells, _____________ cells, and the _____________.
• The finger-like structures on the fallopian tube that help sweep the ovum into the ampulla are called __________________.
• The layer of cells surrounding the ovulated ovum are called the _________________________.
• The _______________________ holds the uterus and ovaries in place within the body.
• The ____________________________ is the layer of the endometrium that is shed every month.
• The ___________ is a tube that allows sperm cells to travel from the testes to the urethra.
• Which cells are responsible for the production of testosterone in response to LH from the anterior pituitary?
• The _____________, _____________, and _____________, provide important fluids to the sperm before ejaculation.
• True or False: warmer temperatures (above body temp) are essential for spermatogenesis.

Anatomy and Physiology Laboratory Manual for Nursing and Allied Health Copyright © by Aylin Marz; Ganesan Kamatchi; Joseph D'Silva; Krishnan Prabhakaran; Rajeev Chandra; and Solomon Isekeije is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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22.4: Male Reproductive System

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• Page ID 53847

• Rosanna Hartline
• West Hills College Lemoore

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Male Reproductive System

The testes produce spermatozoa (sperm), the male reproductive cells. Spermatozoa are produced in the seminiferous tubules and collected by the rete testis (rete = net). Spermatozoa travels from the rete testis to the head of the epididymis where they mature and are stored until ejaculation. The vas deferens transports spermatozoa from the tail of the epididymis to the prostate gland . Seminal glands, the prostate gland, and the bulbourethral glands add fluid secretions to spermatozoa to form semen. These additional fluids support the spermatozoa and their survival as well as facilitate their transfer to the female reproductive tract.

Above: Male reproductive system, lateral view of the left side with structures shown with sagittal sections.

The epididymis, vas deferens, ejaculatory duct and urethra form a system of tubules for the transport of spermatozoa from testes to the pelvic cavity. There they will be combined with the secretions of the accessory glands to form semen.

Above: Structures of the male genitalia.

In order for proper and efficiently development of spermatozoa, the testes must be cooler than core body temperature (95°F is optimal for spermatogenesis and 98.6°F is core body temperature). The testes cool some just from being suspended away from the body, but the core temperature arterial blood from the gonadal artery must also be cooled before reaching the testis capillary beds. As blood is cooled in the peripheral capillary beds of the scrotal walls , it collects into a network of gonadal veins called the pampiniform plexus (pampiniform = shaped like a vine or tendril). As it ascends to the abdomen, this venous plexus wraps around the gonadal artery, cooling the arterial blood to sufficient temperature.

Above: Internal structure of the testis and epididymis.

Sperm cells are produced in the seminiferous tubules in the testes. A cross section through the process of spermatogenesis (a type of meiosis). Close examination of a cross section of a seminiferous tubule shows that sperm cells develop beginning close to the inner lining of the seminiferous tubule, beginning with specialized stem cells called spermatogonia , and develop as they move closer to the lumen of the seminiferous tubules. Primary spermatocytes (cells that undergo the first meiotic division) and secondary spermatocytes (cells that undergo the second meiotic division) are intermediate cells produced as spermatozoa develop. Spermatogenesis produces immature spermatids that ultimately mature into spermatozoa.

Above: The testis is both an exocrine (producing spermatozoa) and an endocrine (producing androgens) gland. This immature testis, cut in cross section, provides an overview of the testis proper, its enclosing tunics, and its location in the scrotum. Also seen are the epididymis and the ductus deferens lying posterior to the testis. Tissue is magnified by 10x.

Above: The exocrine function of the testis is performed by the epithelium lining the convoluted portions of seminiferous tubules. Each convoluted tubule is lined by a stratified epithelium composed of two cell types. Germ cells divide and cytodifferentiate to form haploid spermatozoa. Sertoli cells nourish and protect germ cells during their formation before releasing them into the lumen of the tubule. Tissue is magnified by 1000x.

Above: Structures of the spermatic cord and structures enveloping the testis.

Above: (Left) External male genitalia. (Right) Cross section of the penis.

Attributions

• "Anatomy 204L: Laboratory Manual (Second Edition)" by Ethan Snow , University of North Dakota is licensed under CC BY-NC 4.0
• "Anatomy and Physiology Lab Reference" by Laird C Sheldahl , OpenOregonEducational Resources , Mt. Hood Community College is licensed under CC BY-SA 4.0
• "Digital Histology" by Department of Anatomy and Neurobiology and the Office of Faculty Affairs , Virginia Commonwealth University School of Medicine and the ALT Lab at Virginia Commonwealth University is licensed under CC BY 4.0
• "Male Reproductive System" by Dongho Kim is licensed under CC BY-NC-SA 4.0
• "Rete testis.jpg" by KDS444 is licensed under CC BY-SA 3.0
• "Wiki Images" by https://www.scientificanimations.com/wiki-images/ is licensed under CC BY-SA 4.0

Female Reproductive System Anatomy and Physiology

Step into the powerful realm of the female reproductive system anatomy and physiology. Nursing students, discover the miraculous cycles and stages that cradle and nurture life.

Table of Contents

Fallopian tubes, mons veneris, labia minora, labia majora, skene’s glands, bartholin’s gland, perineal body, internal structures.

• The ovaries are the ultimate life-maker for the females.
• For its physical structure, it has an estimated length of 4 cm and width of 2 cm and is 1.5 cm thick. It appears to be shaped like an almond. It looks pitted, like a raisin, but is grayish white in color.
• It is located proximal to both sides of the uterus at the lower abdomen.
• For its function, the ovaries produce, mature, and discharge the egg cells or ova.
• Ovarian function is for the maturation and maintenance of the secondary sex characteristics in females.
• It also has three divisions: the protective layer of epithelium, the cortex, and the central medulla.
• The fallopian tubes serve as the pathway of the egg cells towards the uterus.
• It is a smooth, hollow tunnel that is divided into four parts: the interstitial, which is 1 cm in length; the isthmus, which is2 cm in length; the ampulla, which is 5 cm in length; and the infundibular, which is 2 cm long and shaped like a funnel.
• The funnel has small hairs called the fimbria that propel the ovum into the fallopian tube.
• The fallopian tube is lined with mucous membrane, and underneath is the connective tissue and the muscle layer.
• The muscle layer is responsible for the peristaltic movements that propel the ovum forward.
• The distal ends of the fallopian tubes are open, making a pathway for conception to occur.
• The uterus is described as a hollow, muscular, pear-shaped organ.
• It is located at the lower pelvis, which is posterior to the bladder and anterior to the rectum.
• The uterus has an estimated length of 5 to 7 cm and width of 5 cm. it is 2.5 cm deep in its widest part.
• For non-pregnant women, it is approximately 60g in weight.
• Its function is to receive the ovum from the fallopian tube and provide a place for implantation and nourishment.
• It also gives protection for the growing fetus.
• It is divided into three: the body, the isthmus, and the cervix. f
• The body forms the bulk of the uterus, being the uppermost part. This is also the part that expands to accommodate the growing fetus.
• The isthmus is just a short connection between the body and the cervix. This is the portion that is cut during a cesarean section .
• The cervix lies halfway above the vagina, and the other half extends into the vagina. It has an internal and external cervical os, which is the opening into the cervical canal.

External Structures

• The mons veneris is a pad of fat tissues over the symphysis pubis.
• It has a covering of coarse, curly hairs, the pubic hair .
• It protects the pubic bone from trauma .
• The labia minora is a spread of two connective tissue folds that are pinkish in color.
• The internal surface is composed of mucous membrane and the external surface is skin.
• It contains sebaceous glands all over the area.
• Lateral to the labia minora are two folds of fat tissue covered by loose connective tissue and epithelium, the labia majora.
• Its function is to protect the external genitalia and the distal urethra and vagina from trauma .
• It is covered in pubic hair that serves as additional protection against harmful bacteria that may enter the structure.
• It is a smooth, flattened surface inside the labia wherein the openings to the urethra and the vagina arise.
• The clitoris is a small, circular organ of erectile tissue at the front of the labia minora.
• The prepuce, a fold of skin, serves as its covering.
• This is the center for sexual arousal and pleasure for females because it is highly sensitive to touch and temperature.
• Also called as paraurethral glands, they are found lateral to the urethral meatus and have ducts that open into the urethra.
• The secretions from this gland lubricate the external genitalia during coitus.
• Also called bulbovaginal gland, this is another gland responsible for the lubrication of the external genitalia during coitus.
• It has ducts that open into the distal vagina.
• Both of these glands secretions are alkaline to help the sperm survive in the vagina.
• This is a ridge of tissue which is formed by the posterior joining of the labia minora and majora.
• During episiotomy , this is the tissue that is cut to enlarge the vaginal opening.
• This is a muscular area that stretches easily during childbirth.
• Most pregnancy exercises such as Kegel’s and squatting are done to strengthen the perineal body to allow easier expansion during childbirth and avoid tearing the tissue.
• This covers the opening of the vagina.
• It is tough, elastic, semicircle tissue torn during the first sexual intercourse.

Craving more insights? Dive into these related materials to enhance your study journey!

• Anatomy and Physiology Nursing Test Banks . This nursing test bank includes questions about Anatomy and Physiology and its related concepts such as: structure and functions of the human body, nursing care management of patients with conditions related to the different body systems.

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9.1: Lab 9- Urinary and Reproductive Systems

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• Page ID 10707

• Malgosia Wilk-Blaszczak
• University of Texas at Arlington via Mavs Open Press

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Measurable Outcomes

• Analyze reproductive and urinary organ tissues under the microscope.
• Complete the dissection of the kidney and accurately identify (with a pin) all structures of the kidney using the corresponding vocabulary list.
• Learn about the organs of the urinary system: kidney, ureter, urinary bladder and urethra, as well as the structural elements of the nephron and label on any available models.
• Compare and contrast the elements of the male and female reproductive systems and their associated accessory glands.
• Recognize homologous structures of the male and female reproductive systems.
• Demonstrate an adequate understand of the material in this section.

The urinary system is one of excretion, elimination and reabsorption. It is made from four organs, only one of which produces urine (the kidney ). Nephrons , the smallest functional unit of the kidneys, are found in numbers of one to two million within the kidney and can filter up to 400 gallons of cycled blood, daily. The kidneys receive more blood than the heart, liver, or even the brain and have vital functions such as the regulation of pH, blood pressure, concentration of blood solutes and concentration of red blood cells. The remaining three organs ( ureters, urinary bladder , and urethra ) facilitate urine storage and secretion. Of these organs, only the urethra is anatomically distinct between males and females.

The reproductive system is designed to propagate a species and therefore has two primary functions: the production of gametes ( n ) and sex hormones. Male gametes are referred to as sperm cells, whereas female gametes are called ova . Reproduction is very metabolically taxing especially for the female. To illustrate, mature ovum can contain as many as 600,000 mitochondria; to reference, liver cells and cardiac muscles cells contain 2,000 and 5,000 mitochondria respectively. The role of the male reproductive system is to produce sperm and transfer them to the female reproductive tract. Although they originate from similar primordial tissues, the female and male reproductive systems differ in gonad type, ducts, accessory glands, and external genitalia. Male gonads are referred to as testes while the female gonads as ovaries ; both are the sites of their respective gametogenesis. The hormones produced by the gonads are crucial to the reproductive system and sexual development, including primary and secondary sexual development, tissue regeneration, and production of gametes.

Humans are a sexually dimorphic species, which mean that there are distinguishing secondary sex characteristics. The hormones that influence male primary and secondary sexual development are called androgens. The hormones that influence female primary secondary sexual development are called estrogens. In females, this entails the development of breasts which are specialized sweat glands. Males also have mammary tissue but their development is arrested early. Similarly, the thyroid cartilage is enlarged and commonly referred to as an Adam’s apple in males but not so in females.

A developing fetus remains anatomically undifferentiated a will either develop characteristically male or female anatomy. At some point of gestation, the fetus will develop both Wolffian and Müllerian ducts, anlagen of the male and female reproductive systems. As a result, there are several elements of the male and female reproductive systems which are homologous. Such structures share developmental and evolutionary origins but are not necessarily similar in function. The following are the homologous structures of the male and female reproductive system: labia majora – male scrotum ; labia minora – shaft of penis ; clitoris – glans penis ; paraurethral gland – prostate gland ; greater vestibular gland – bulbourethral gland. Vocabulary for the Urinary and Reproductive systems on page(s) 172 and 168.

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Male Reproductive System

The male reproductive system consists of the testis which are the sites of sperm generation and the male reproductive tract which deliver sperm from the testis to the outside world. Along the reproductive tract, several secretory organs deliver nutrients and other factors that facilitate survival of sperm.

Hormonal Regulation of Spermatogenesis

Similar to oocyte development in ovaries, development of sperm depends upon the actions of several different hormones, including follicle-stimulating hormone (FSH), leutenizing-hormone (LH), and testosterone. Recall from the reading in Female Reproductive System the role the hypothalamus and anterior pituitary play in producing FSH and LH. The production of testosterone is described below.

The testes are a source of gametes and steroid sex hormones. Each testis is a compound tubular gland contained within a thick connective tissue coat called the tunica albuginea. Thin septa radiate from the dorsal portion of the tunica albuginea to separate the testis into lobules. Each lobule contains between one and four seminiferous tubules that are the site of sperm production. Myofibroblasts surround each seminiferous tubule and rhythmically contract to help move sperm, which are immotile in the testes, through the seminiferous tubules.

Spermatogenesis

The development of a mature sperm is divided into two steps, spermatogenesis and spermiogenesis. Spermatogenesis is the process by which an undifferentiated spermatogonium, the stem cell of the testis, develops into a spermatid. During spermatogenesis, the number of chromosomes is halved through meiosis. It involves the following stages.

Spermatogenesis occurs within the epithelium of seminiferous tubules which is also called a germinal epithelium. The epithelia is divided into two compartments: basal and adluminal. The basal compartment is closest to the basement membrane whereas the adluminal compartment is closer to the lumen of the seminiferous tubule. The border between the two compartments is defined by Sertoli cells (see below).

Spermatogonia rest on the basement membrane of the seminiferous tubule and divide mitotically to produce more spermatogonia and primary spermatocytes. Primary spermatocytes represent the first differentiation step along the spermatogenesis pathway. Spermatogonia remain in the basal compartment while spermatocytes migrate away from the basement membrane and cross into the adluminal compartment toward the lumen of the seminiferous tubule.

Primary spermatocytes enter meiosis and have a prolonged prophase that facilitates the exchange of genetic material between homologous chromosomes. The first meiotic division gives rise to secondary spermatocytes that have 23 pairs of chromatids. This stage is short-lived and consequently, secondary spermatocytes are rarely seen in histological images. This stage ends with the second meiotic division.

The second meiotic division produces spermatids that are haploid. Despite being the product of meiosis, spermatids remain connected to one another by cytoplasmic bridges. These bridges result from incomplete cytokinesis and allow exchange of material for synchronous maturation.

Spermiogenesis

Spermiogenesis is the process by which a spermatid matures into a spermatozoan. This process involves the following cellular changes to spermatids.

• An acrosome, containing hydrolytic enzymes, develops and comes to overlie the dense, elongated nucleus.
• A flagellum grows out of the pole opposite the acrosome, facing the tubular lumen. This flagellum is a modified cilium that develops from the centrioles of the spermatid.
• Mitochondria become arranged around the flagellum.
• The bulk of the cytoplasm is cast off as a residual body , leaving only a thin rim of cytoplasm around the nucleus. Sertoli cells consume the residual body.

Sertoli Cells

Sertoli cells are located within the germinal epithelium and play a supportive role in the development of spermatozoa. These cells have abundant cytoplasm and extend from the basement membrane to the lumen of seminiferous tubules. Sertoli cells have a characteristic oval nucleus with a dark nucleolus.

Sertoli cells facilitate spermatogenesis by providing structural and chemical support to the developing spermatogonia, spermatocytes and spermatids. In addition, Sertoli cells synthesize androgen-binding protein that keeps testosterone levels high within the seminiferous tubules. Follicle-stimulating hormone (FSH) produced in the anterior pituitary stimulates Sertoli cells to synthesize androgen-binding protein. Sertoli cells also produce inhibin that decreases production of FSH in the anterior pituitary. Thus, Sertoli cells are part of a negative feedback loop that keeps the concentration of FSH within a defined range.

Blood-Testis Barrier

This electron micrograph provides a better view of the structure of Sertoli cells and the blood-testis barrier. The basement membrane on which all Sertoli cells rest is visible, as is a narrow myofibroblast which contracts rhythmically. The Sertoli cells are connected to one another via junctional complexes (both tight and adhesion junctions) close to the basement membrane; these complexes divide the germinal epithelium into basal and adluminal comparments. The basal compartment resides between the basement membrane and junctional complexes whereas the adlimunal compartment is defined as the region from the junctional complexes to lumen of the seminiferous tubules. The basal compartment contains diploid spermatogonia that rest upon the basement membrane. These cells develop by migrating into the adluminal compartment, which contains primary spermatocytes, spermatids, and spermatozoa.

The primary function of the blood-testis barrier is to create a protected environment, adluminal compartment, for the development of sperm. The junctional complexes of Sertoli cells prevent the diffusion of antibodies that might bind the surface of sperm and inhibit their motility or ability to fertilize an egg. The junctional complexes also inhibit the diffusion of small molecules that may disrupt the development of sperm or be toxic to sperm.

Rete Testis

The rete testis connects the seminiferous tubules to the ductus efferentes and the rest of the male gentic tract. It is lined by ciliated, cuboidal epithelial cells that also contain microvilli. The activity of the cilia helps to move the spermatozoa along the tube, as spermatozoa are immobile until they reach the epididymis. The microvilli absorb excess materials, including protein and potassium, from the seminal fluid.

Leydig Cells

Interstitial or Leydig cells are located in the connective tissue surrounding the seminiferous tubules. They produce testosterone, the male sex hormone responsible for the growth and maintenance of the cells of the germinal epithelium and the development of secondary sex characteristics. Leydig cells often display cytoplasmic crystals of Reinke; the function of these crystals is unknown.

Male Reproductive Tract

The male reproductive tract is a long tube that brings the spermatozoa from the testes to the outside of the body. The tract comprises several segments with different structures and functions. As discussed previously, spermatozoa are produced in the seminiferous tubules in the testis. Spermatozoa flow from the seminiferous tubules into the rete testis and then the ductili efferentes, which is the first segment of the reproductive tract. The ductili efferentes merge to form the epididymis which is the site where spermatozoa gain motility. The epididymis transitions into the ductus deferens which receives secretions from the seminal gland and prostate. The ductus deferens from each side merge with the urethra.

Ductili Efferentes

The ductuli efferentes emerge from the dorso-superior margin of each testis. They originate from the rete testis and gradually fuse to form the ductus epididymis. The epithelium of the ductuli efferentes has a characteristic scalloped appearance that results from a lining that contains both cuboidal and columnar epithelial cells. A layer of smooth muscle surrounds the walls. The non-ciliated cells reabsorb testicular fluid, while the ciliated cells propel the immobile sperm to the epididymis, where they gain the ability to swim.

The epididymis is a muscular and highly convoluted tubule that stores spermatozoa and is the site at which they acquire their motility. It is lined by a pseudostratified epithelium whose cells contain non-motile stereocilia. These stereocilia absorb much of the excess fluid surrounding the spermatozoa. The epithelium of the epididymis also contains mitotic basal cells. In this section, the spermatozoa can be seen in the lumen throughout the epididymis.

Ductus Deferens

The ductus deferens is another muscular tubule that carries sperm downstream from the epididymis. Its wall is thicker than that of the epididymis and contains three muscular layers: inner longitudinal, middle circular, and outer longitudinal. The epithelium of the ductus deferens is similar to that of the epididymis, with pseudostratified cells bearing stereocilia.

The distal portion of the ductus deferens is called the ampulla and receives secretions from the seminal vesicles. The duct is now referred to as the ejaculatory duct and ducts from each side will merge and join the urethra as it runs through prostate gland.

The urethra is lined primarily by stratified or psueodstratified columnar epithelial cells, but its opening displays a stratified squamous epithelium. Erectile tissue surrounds the urethra and contains numerous blood vessels. During an erection, the arteries dilate to fill the sinuses, which obstruct venous outflow and traps blood in the penis.

Seminal Vesicle

Seminal vesicles are glandular sacs that produce a secretion that composes 80% of the seminal fluid and contains fructose, fibrinogen, and prostaglandins. The secretion empties via a short duct into the ampulla of the ductus deferens. The seminal vesicles appear as honeycombed saccules with thin, highly branched folds of mucosa, lined by a pseudostratified columnar epithelium. Observe the coat of smooth muscle surrounding the saccular dilation of the gland. Its contraction expels the accumulated secretion during ejaculation.

Prostate Gland

The prostate is a walnut-sized conglomeration of tubulo-acinar glands that surrounds the initial segment of the urethra. This gland produces a secretory product containing citric acid and proteolytic enzymes that prevent coagulation of semen. The epithelium that lines the glands is usually columnar with numerous flattened basal cells also visible. The lumen of the glands often contain prostatic concretions that accumulate over time. Their significance is unknown but they make a useful marker for identifying the prostate. The glands are surrounded by a stroma that contains connective tissue and smooth muscle.