Hilu ka iʻa, he iʻa noʻenoʻe.

The fish is the hilu, an attractive one.

A quiet, well-behaved person. When a pregnant woman longed for hilu fish, the child born to her would be well-mannered, quiet, and unobtrusive.

ʻŌlelo Noʻeau, compiled by Mary Kawena Pukui, #1007

Introduction

Figure 27.1: Haumea: Haumea is a key deity in Hawaiian cosmogony. She is the mother of many important deities, such as Pele, Kāne Milohai, Kāmohoaliʻi, Nāmaka, Kapo, and Hiʻiaka. She is also connected to sacred space and chronology.

There are numerous events, well before the day of a child’s conception, that rely on critical timing and the healthy functioning of the parents’ reproductive systems. To begin, the parents’ endocrine systems have to secrete the appropriate regulating hormones to induce the production and release of sperm and oocytes, the male and female gametes, which contain the parents’ genetic material. The parent’s reproductive behavior also has to facilitate the transfer of sperm to the female reproductive tract at just the right time to encounter the oocyte. Finally, fertilization–combination of the gametes–has to occur, followed by implantation and development of an embryo.

27.1 Common Features of the Reproductive System

Homologous Structures

The reproductive system is unique in that it does not begin to function continuously or become fully active until puberty. While the male and female reproductive systems are distinctly different, they share many similarities. The male and female reproductive system organs are formed from the same general tissues and thus have homologous (similar) structural components. Both males and females have gonads (primary sex organs) that produce sex hormones and gametes (sperm and oocytes, the specialized cells for reproduction); ducts designed to transport gametes; accessory glands that add liquid secretions to the reproductive tract; and external genitalia. The table below provides basic information on reproductive organs between the sexes.

Table 27.1: Homologous Structures of the Reproductive System

General Functions of the Reproductive System

The reproductive system promotes the survival of the human species through the following functions:

• Forming specialized sexual reproduction cells called gametes (sperm in males and oocytes in females)
• Uniting gametes through sexual intercourse
• Combining gamete genetic material
• Supporting development and birth of offspring

Meiosis

You may recall from Chapter three that there are two different types of cell divisions. Most cells in the body divide to produce identical copies of daughter cells by the process called mitosis. Meiosis is a specialized type of cell division–inclusive of two distinct stages of division that include the phases of prophase, metaphase, anaphase, and telophase–that produces gametes (sperm and oocytes). The primary function of meiosis is genetic diversity with the goal to make daughter cells with half as many chromosomes as the original cell so when a sperm and oocyte unite, a new cell with unique genetics is created. Thus, the end products of meiosis are haploid daughter cells (gametes) with half the number of chromosomes compared to the diploid parent cell (Figure 27.3). Specific details on meiosis and the formation of gametes are discussed in the separate male and female reproductive system sections.

Figure 27.3 Mitosis and Meiosis (from OpenStax Biology 2e)

27.2 Male Reproductive System

Gametes are specialized cells carrying 23 chromosomes. This is half the number of chromosomes that are present in the other cells of the body. At fertilization, the chromosomes in one male gamete (sperm) combine with the chromosomes in one female gamete (oocyte). The male reproductive system’s function is to produce sperm and transfer them to the female reproductive tract. The paired testes are a crucial component in this process, as they produce both sperm and the hormones that support male reproductive physiology. In humans, the most significant male reproductive hormone is testosterone. In this section, we will discuss the different structures and processes necessary for sperm production and transport (Figure 27.4).

Figure 27.4 Male reproductive system (OpenStax)

Scrotum

The testes are in a skin-covered, pigmented, muscular sack called the scrotum (Figure 27.4). On the scrotum surface is a median ridge, the raphe (seam). The scrotum originates at the penis root–where the penis attaches to the pelvis. This location provides the pathway for sperm, which is produced in the testes, to travel up from the testes and into the penis. The dartos muscle makes up the subcutaneous muscle layer of the scrotum. It continues internally to make up the scrotal septum, a wall that divides the scrotum into two compartments, each housing one testis (singular for testes). The dartos muscles can contract, tightening the scrotum and making it appear wrinkled. This reduces heat loss when the testes are exposed to cold external temperatures. Deep to the dartos muscle, the cremaster muscle surrounds each testis like a muscular sling. In cool temperatures or cold water, the cremaster muscles contract and elevate the testes, moving the testes closer to the body to retain heat. Alternatively, in warm temperatures, the dartos and cremaster muscles relax, thinning out the scrotal surface area (reducing the wrinkles) and moving the testes farther from the body core, allowing the testes to cool off.

Figure 27.5 The Scrotum and Testes (OpenStax)

Testes

The testes are the male gonads and produce sperm and testosterone (Figure 27.6). Testosterone is an androgen, a hormone that promotes masculine characteristics. Testosterone also fosters libido (desire for sexual activity). The testes, seated within the scrotum, are paired oval glands measuring 4-5 cm in length. The tunica vaginalis is a serous membrane, with its parietal and visceral layers, and covers most of the testes. Injury to the testes can cause a hydrocele, a noticeable buildup of excess serous fluid in the tunica vaginalis. Often a hydrocele will go away with no treatment. Beneath the tunica vaginalis is the tunica albuginea, a dense connective tissue layer that covers the testis itself. The tunica albuginea also invaginates to form septa that divide the testis into 300-400 structures called lobules. Within the lobules, sperm develop inside structures called seminiferous tubules, tightly coiled structures that form the bulk of each testis. Sperm move from the lumens of the seminiferous tubules through the straight tubules to a meshwork of tubules called the rete testis (rete = network).

Figure 27.6 Anatomy of the Testis This sagittal view shows the seminiferous tubules, the site of sperm production. Formed sperm are transferred to the epididymis, where they mature. They leave the epididymis during an ejaculation via the ductus deferens. (OpenStax)

Inside the seminiferous tubules are two populations of cells, the spermatogonia (stem cells that develop into sperm cells) and Sertoli cells (nurse cells or sustentacular cells) that provide nutrition and support to the spermatogonia. Sertoli cells secrete signaling molecules that promote sperm production and can control whether developing sperm live or die. Sertoli cells extend physically around developing sperm cells from the peripheral basement membrane of the seminiferous tubules to the lumen. Sertoli cells also create the blood-testis barrier, which keeps many blood-borne substances from reaching the developing sperm cells. This barrier also isolates the developing sperm cells from the immune system. The sperm cells’ surface antigens would be recognized, by cells in the blood, as not belonging in the body and the immune system could create a harmful response against the developing sperm.

There are five types of germ cells within the seminiferous tubules. Germ cells give rise to the gametes, and in this case, the sperm. The least mature germ cell is called spermatogonia. Spermatogonia are the stem cells of the testis, which mean that they are still able to differentiate into a variety of different cell types throughout adulthood. Spermatogonia divide to produce primary and secondary spermatocytes, then spermatids, which finally produce formed sperm. The process that begins with spermatogonia and concludes with the production of sperm is called spermatogenesis.

Spermatogenesis

Spermatogenesis, which as noted occurs in the seminiferous tubules, begins at puberty and continues constantly throughout a male’s life (Figure 27.7). One production cycle, from spermatogonia through formed sperm, takes approximately 64 days. A new cycle starts approximately every 16 days, although this timing is not synchronous across the seminiferous tubules. Sperm count–the total number of sperm a male produces–slowly declines after the age of 35. At any age, smoking cigarettes or using nicotine products such as vape pens, reduces sperm count and causes sperm abnormalities.

The process of spermatogenesis begins with mitosis of diploid spermatogonia. The spermatogonia are diploid (2n) stem cells, meaning that they each have a complete copy of the parent cell’s genetic material: 46 chromosomes. The daughter cells of spermatogonia must undergo a second cellular division through the process of meiosis (notice this is not mitosis) because mature gametes–sperm cells–are haploid (n) meaning they contain only 23 chromosomes. As described earlier in this chapter, meiosis is where a single cell divides twice to create four cells with half the number of chromosomes as the original parent cell.

Figure 27.7 Spermatogenesis (a) Mitosis of a spermatogonial stem cell involves a single cell division that results in two identical, diploid daughter cells (spermatogonia to primary spermatocyte). Meiosis has two rounds of cell division: primary spermatocyte to secondary spermatocyte, and then secondary spermatocyte to spermatid. This produces four haploid daughter cells (spermatids). (b) In this electron micrograph of a cross-section of a seminiferous tubule from a rat, the lumen is the light-shaded area in the center of the image. The location of the primary spermatocytes is near the basement membrane, and the early spermatids are approaching the lumen (tissue source: rat). EM × 900. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) (OpenStax)

One of the identical diploid cells that results from spermatogonia mitosis remains a spermatogonium and the other becomes a primary spermatocyte. As in mitosis, DNA is replicated in a primary spermatocyte, before it undergoes a cell division called meiosis I. During meiosis I, each of the 23 pairs of chromosomes separates. This results in two cells called secondary spermatocytes, each with only half the number of chromosomes. During prophase I of meiosis I, an event called crossing over occurs. Crossing over is the exchange of genetic material between homologous chromosomes (chromosomes that share the same structural features). This event produces cells that are unlike each other and unlike the cell they came from, giving rise to genetic diversity.

Next, the second round of cell division called meiosis II occurs in both secondary spermatocytes. During meiosis II, each of the 23 chromosomes divides (similar to what happens during mitosis). Thus, meiosis results in separating the chromosome pairs. This second meiotic division results in a total of four cells with only half the number of chromosomes. Each of these new cells is called a spermatid. Although haploid, early spermatids look similar to cells in the earlier stages of spermatogenesis. They have a round shape, a central nucleus, and a large amount of cytoplasm. A process called spermiogenesis transforms these early spermatids by reducing the cytoplasm and beginning the formation of the parts of true sperm. Spermiogenesis is the last part of spermatogenesis and includes acrosome formation, mitochondrial reproduction, and flagellum formation. Spermatozoa (formed sperm) are the result of the fifth stage of sperm cell formation, which occurs in the portion of the tubule nearest the lumen. Eventually, sperm are released into the lumen and moved along a series of ducts in the testis toward a structure called the epididymis for the next step of sperm maturation.

Structure of Sperm

Sperm are smaller than most cells. A single human egg (ovum) is ten million times larger than a sperm cell. Approximately 100-300 million sperm are produced each day, whereas females typically ovulate only one oocyte per month. As true for most cells in the body, the structure of sperm cells speaks to their function (Figure 27.8). Sperm have a distinctive head, mid-piece, and tail region. The head of the sperm contains an extremely compact haploid nucleus with minimal cytoplasm. A structure called the acrosome covers most of the head of the sperm cell as a cap that is filled with lysosomal enzymes that help prepare the sperm for fertilization. Tightly packed mitochondria fill the midpiece of the sperm. ATP produced by these mitochondria will power the flagellum, which extends from the neck and the midpiece through the tail of the sperm enabling it to move the entire sperm.

Figure 27.8 Parts of a Sperm Sperm Sperm cells are divided into a head, containing DNA; a mid-piece, containing mitochondria; and a tail, providing motility. The acrosome is oval and somewhat flattened. (OpenStax)

Sperm Transport

To fertilize an oocyte, sperm must be moved from the seminiferous tubules in the testes, through the epididymis, and, during ejaculation, through the duct system and along the length of the penis and out into the female reproductive tract (Figure 27.9).

Figure 27.9 Male Reproductive Duct System (Wiki)

Epididymis

From the lumen of the seminiferous tubules, the immotile sperm are surrounded by testicular fluid and moved to the epididymis (Figures 27.6 and 27.9). The epididymis is a coiled tube attached to the testis where newly formed sperm continue to mature. Though the epididymis does not take up much room in its tightly coiled state, it would be nearly 20 feet long if straightened. It takes an average of 12 days for sperm to move through the coils of the epididymis. 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, the sperm further mature and acquire the ability to move under their power. The more mature sperm are then stored in the tail–final section–of the epididymis until ejaculation occurs.

Duct System

During ejaculation, sperm exit the tail of the epididymis and are pushed by smooth muscle contraction to the ductus deferens (vas deferens) (Figures 27.6 and 27.9). The ductus deferens is a thick muscular tube that exits the region of the testis bundled together with connective tissue, blood vessels, and nerves into a structure called the spermatic cord.

From each epididymis, the ductus deferens extends superiorly into the abdominal cavity through the inguinal canal in the abdominal wall. From here, each ductus deferens continues posteriorly into the pelvic cavity, ending posterior to the bladder where it dilates in a region called the ampulla.

Sperm make up only 5% of the final volume of semen, the thick, milky fluid that the male ejaculates. The bulk of the semen is produced by three critical accessory glands:

• seminal vesicles
• prostate
• bulbourethral glands

Seminal Vesicles

As sperm pass through the ampulla of the ductus deferens at ejaculation, they mix with fluid from the seminal vesicle (Figure 27.9). The paired seminal vesicles are glands that contribute approximately 60% of 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 reproductive tract.

The fluid, now containing both sperm and seminal vesicle secretions, moves into the associated ejaculatory duct, a short structure formed from the ampulla of the ductus deferens and the duct of the seminal vesicle (Figure 27.9). The paired ejaculatory ducts transport the seminal fluid through the prostate gland.

Prostate Gland

The medially located prostate gland sits anterior to the rectum at the base of the bladder and surrounds the prostatic urethra–portion of the urethra that runs with the prostate (Figure 27.9).The prostate gland is about the size of a walnut and is formed of both muscular and glandular tissue. It secretes an alkaline, milky fluid to the passing seminal fluid (now called semen). This milky fluid must first coagulate and then decoagulate the semen following ejaculation. The temporary thickening of semen helps retain it within the female reproductive tract, providing time for sperm to utilize the fructose provided by seminal vesicle secretions. When the semen regains its fluid state, sperm can then pass farther into the female reproductive tract.

Bulbourethral Glands

The final addition to semen is made by two bulbourethral glands (Figure 27.9). The bulbourethral glands release a thick, salty fluid that lubricates the end of the urethra and vagina. It helps to clean urine residues from the penile urethra. The fluid from these accessory glands is released after the male becomes sexually aroused and shortly before the release of the semen. It is sometimes called pre-ejaculation. It is important to note that, in addition to the lubricating proteins, bulbourethral fluid can pick up sperm already present in the urethra, and therefore it may be able to cause pregnancy.

Penis

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

The shaft of the penis surrounds the urethra. The shaft is composed of three column-like chambers of erectile tissue that span the length of the shaft (Figure 27.10). Each of the two larger chambers is called the corpora cavernosum (plural = 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 is called the glans penis. The glans penis has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation. The skin from the shaft extends down over the glans and forms a collar called the prepuce (foreskin). The prepuce 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, performed for religious or social reasons, removes the prepuce typically within days of birth. As stated by the American Academy of Pediatrics, “health benefits are not great enough to recommend routine circumcision for all male newborns.”

Figure 27.10: Cross-Sectional Anatomy of the Penis Three columns of erectile tissue make up most of the volume of the penis. (OpenStax)

Male Sexual Response

During sexual arousal, nitric oxide (NO) is released from nerve endings near blood vessels within the corpora cavernosa and corpus spongiosum. The release of NO activates a signaling pathway that relaxes the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood entering the penis and induces the endothelial cells in the penile arterial walls to secrete NO and perpetuate vasodilation. The 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. This results in engorgement of the tissues and stiffening of the penis, called an erection. Penile erections result from vasocongestion, or engorgement of the tissues, because of more arterial blood flowing into the penis than is leaving in the veins. Depending on the flaccid dimensions of a penis, it can increase in size slightly or immensely during erection, with the average length of an erect penis measuring approximately 15 cm. Sexual arousal through thoughts, sights, smells, sounds, and touch can initiate an erection. It is also normal to have an erection during REM (rapid eye movement) sleep. Males may awaken with an erection in the morning due to waking up close to the final REM of the sleep time. An erection can be inhibited due to sadness, anxiety, medications and other conditions. Ejaculation is the process by which semen is expelled from the penis. Although the erection is under the parasympathetic control, ejaculation is under sympathetic control.

Hormonal Regulation of the Male Reproductive System

Testosterone

Testosterone is the main hormone that controls male reproductive physiology, including spermatogenesis. Testosterone, an androgen, is a steroid hormone produced by interstitial endocrine cells (Leydig cells) that are located between the seminiferous tubules in the testes (Figure 27.7). In male embryos, testosterone is secreted by the seventh week of development. In childhood, testosterone levels are low and increase during puberty. Testosterone plays a vital role in the development, maturation, and maintenance of male reproductive organs. The increase in testosterone during puberty initiates spermatogenesis and activates characteristic physical changes such as the growth of pubic, axillary, and facial hair; thickening of vocal cords resulting in a deeper voice; and secretions of the sebaceous gland which can result in acne. The continued presence of testosterone is necessary to keep the male reproductive system working properly. Maintaining these normal concentrations of testosterone promotes spermatogenesis, whereas low levels of testosterone can lead to infertility.

The secretion of testosterone is under the control of a regulatory system involving the hypothalamus and pituitary gland (Figure 27.11). The regulation begins in the hypothalamus. The hypothalamus secretes gonadotropin-releasing hormone (GnRH). Recall from the endocrine system chapter that GnRH stimulates the secretion of gonadotropins: luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland, which stimulates the gonads. In males, 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. LH binds to interstitial endocrine cells and stimulates the release of testosterone. The secreted testosterone enters the bloodstream and affects other organs. For example, it stimulates the maturation of sex organs and the development of secondary sex characteristics mentioned before.

When the levels of testosterone increase, it exerts a negative feedback control on the hypothalamus and anterior pituitary. When concentrations of testosterone in the blood reach a critical threshold, testosterone itself will bind to androgen receptors on both the hypothalamus and the anterior pituitary, inhibiting the synthesis and secretion of GnRH and LH, respectively. Inhibition of GnRH and LH results in inhibition of testosterone release (negative feedback). When blood concentrations of testosterone once again decline, testosterone no longer interacts with the receptors to the same degree and GnRH and LH are secreted, stimulating more testosterone production. This same process occurs with FSH and inhibin to control spermatogenesis.

Figure 27.11 Regulation of Testosterone (OpenStax) The hypothalamus and pituitary gland regulate the production of testosterone and the cells that assist in spermatogenesis. GnRH activates the anterior pituitary to produce LH and FSH, which in turn stimulate interstitial endocrine (Leydig) cells and Sertoli cells, respectively. The system is 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.

27.3 Female Reproductive System

The female reproductive system functions to produce gametes and reproductive hormones. It is the site of fertilization and supports the growth of the developing fetus. When a female reaches puberty, a cascade of physiological events happen. There is an increase in the release of gonadotropin-releasing hormone (GnRH) by the hypothalamus. GnRH stimulates the anterior pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The levels of both hormones, FSH and LH, will change monthly, and it will trigger a sequence of events that will follow a cyclic pattern that leads to the development of the follicles. This cyclic pattern that triggers follicle development is known as the ovarian cycle. Before we talk about the ovarian cycle, we need to understand the anatomy of the ovaries.

Ovaries

The ovaries are paired organs found within the pelvic cavity and lateral to the uterus (Figure 27.12). Although the size of the ovaries varies during the menstrual cycle and pregnancy, they are about 2 cm long and 2 cm wide. The ovaries are supported by the mesovarium, an extension of the peritoneum covering the ovaries to the broad ligament, which is a drape of the peritoneum covering the uterus. Extending from the mesovarium itself is the suspensory ligament that contains the ovarian blood and lymph vessels. Each ovary is attached to the lateral wall of the uterus via the ovarian ligament.

Figure 27.12 Female reproductive structures ( Wiki)

The ovary consists of 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 ovarian cortex. The ovarian cortex is composed of a tissue framework called the ovarian stroma that forms the bulk of the adult ovary. Oocytes (female gametes that can become ova) 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 in the next section. Beneath the cortex lies the ovarian medulla which is composed of areolar connective tissue and is the site of blood vessels, lymph vessels, and the nerves of the ovary.

The Ovarian Cycle

The ovarian cycle is a set of predictable changes in a female’s oocytes and ovarian follicles. During a female’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 ovarian cycle includes two interrelated processes: oogenesis (production of female gametes) and folliculogenesis (growth and development of ovarian follicles).

Oogenesis and ovarian follicles

Gametogenesis in females is called oogenesis and it is the process of maturation of a primary oocyte into a secondary oocyte. The process begins before birth with the ovarian stem cells, or oogonia. Oogonia are formed during fetal development, and divide via mitosis, much like spermatogonia in the testis. Oogonia are stem cells that are diploid (2n) meaning that they, similar to spermatogonia, have a complete copy of the parent cell’s genetic material. Unlike spermatogonia, however, oogonia form primary oocytes in the fetal ovary before birth. The primary oocytes are then arrested in this stage of meiosis I, only to resume it years later, beginning at puberty and continuing until the female is near menopause. The number of primary oocytes present in the ovaries declines from one to two million in an infant, to approximately 400,000 at puberty, and zero by the end of menopause.

Ovarian follicles are oocytes and their supporting cells. The follicles grow and develop in a process called folliculogenesis, which typically leads to ovulation of one primary follicle approximately every 28 days, along with the death of multiple other follicles. The death of ovarian follicles is called atresia and can occur at any point during follicular development. Female infants at birth will have one to two million oocytes within her ovarian follicles and this number declines throughout life until menopause when no follicles remain. Menopause is the natural decline in female reproductive hormones and the ceasing of menstrual periods. As you will see next, follicles progress from primordial to primary, to secondary, and tertiary stages before ovulation–with the oocyte inside the follicle remaining as a primary oocyte until right before ovulation. The following are the different types of ovarian follicles and their corresponding phases:

• Primordial follicles are present in newborn females and are the prevailing follicle type in the adult ovary. Folliculogenesis begins with follicles in a resting state. Primordial follicles have only a single flat layer of support cells, called granulosa cells, surrounding the oocyte, and they can stay in this resting state for years–some until right before menopause. Within primordial follicles, a primary oocyte is arrested in the prophase of meiosis I (see meiosis in Figure 27.3).
• 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 (number 1 in Figure 27.13) 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–increase in diameter, adding a new outer layer of connective tissue, blood vessels, and theca cells, which are cells that work with the granulosa cells to produce estrogens and help control the development of the follicles. As they mature, the primary follicles secrete estrogen which triggers changes in the uterus lining. Within the growing secondary follicle, the primary oocyte now secretes a thin acellular membrane called the zona pellucida, a translucent structure that will play a critical role in fertilization. A thick fluid, called follicular fluid, which has formed between the granulosa cells also begins to collect into one large pool called the antrum.
• Follicles in which the antrum has become large and fully formed are considered tertiary follicles or antral follicles. Two key structures surround the primary oocyte. These are the zona pellucida and the corona radiata, which are found externally to the zona pellucida.
• Finally, a mature follicle (Graafian follicle) (number 3 in Figure 27.13), develops from the tertiary follicle. This follicle is formed by the secondary oocyte, with the zona pellucida and corona radiata, many granulosa cells, and a large antrum. A secondary oocyte finished meiosis I, and it is arrested in the metaphase of meiosis II. Each month, only one mature follicle will form and will continue to grow and develop until ovulation (number 4 in the Figure below), when it will expel its secondary oocyte surrounded by several layers of granulosa cells from the ovary. Keep in mind that most follicles do not make it to this point. Roughly 99% of the follicles in the ovary will undergo atresia, which can occur at any stage of folliculogenesis.
• After ovulation, the remnants of the follicle become a structure that is yellow in color and is known as corpus luteum (number 6 in Figure 27.13). This structure secretes estrogen and progesterone. This hormone is vital for the development of the lining of the uterus and the preparation of the uterus for implantation of the oocyte, in case fertilization has happened.
• If fertilization does not occur, the corpus luteum regresses and becomes a white structure called the corpus albicans that will be resorbed.

Figure 27.13 Ovarian Cycle (Wiki)

The initiation of ovulation marks the transition from puberty into reproductive maturity for females. From then on, throughout a female’s reproductive years, ovulation occurs approximately once every 28 days. Just before ovulation, a surge of luteinizing hormone (LH) 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 27.14, 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–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 and remains viable for reproduction purposes.

Figure 27.14 Oogenesis 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. (OpenStax)

Phases of the ovarian cycle

The events described above can be organized into the ovarian cycle’s three phases–follicular phase, ovulation, and luteal phase. Each phase is associated with different levels of reproductive hormones and is related to the uterine cycle phases (Figure 27.14)

Follicular phase

The follicular phase happens during the first 13 days of a 28-days cycle. In this phase, primordial follicles develop into primary follicles. While the ovarian follicles develop, a hormone called inhibin is released from the granulosa cells. Inhibin inhibits the further production of FSH. This process limits the production of too many follicles and allows the primary follicle to mature. After that, the primary follicles become secondary follicles. The increase in the levels of the hormones FSH and LH triggers the process of follicular development from secondary to tertiary, and from tertiary to mature follicles. With the increase in the volume of fluid within the antrum of the mature follicle, the oocyte is pushed to one side of the follicle. The primary oocyte ends meiosis I and there is a formation of two cells (Figure 27.14). The smaller cell, called the first polar body, eventually disintegrates. The other cell receives most of the cytoplasm and becomes the secondary oocyte. This secondary oocyte reaches meiosis II and becomes arrested in metaphase. This secondary oocyte will only complete meiosis II if it is fertilized by a sperm (Figure 27.14).

Ovulation

The release of a secondary oocyte from the mature follicle approximately once every 28 days is known as ovulation. This process is triggered by the peak in LH secretion. With the increase in the volume of fluid within the antrum of the mature follicle, the oocyte is pushed to one side of the follicle. The follicle walls become thin and they rupture, which expels the secondary oocyte (Figures 27.13 and 27.15).

Luteal phase

The phase when the remnants of the follicle cells become the corpus luteum is called the luteal phase. This phase happens in the second part of the ovarian cycle. The corpus luteum is an endocrine gland that temporarily secretes progesterone and estrogen to aid with the development of the lining of the uterus and preparation for the implantation of the fertilized oocyte. The corpus luteum also produces inhibin to negatively regulate the secretions of GnRH, FSH, and LH. If not fertilized, the corpus luteum lives for about 10 days and then regresses and becomes the corpus albicans. With the regression of the corpus luteum, there is a significant decrease in the levels of progesterone and estrogen, which then leads to the shedding of the lining of the uterus, a process known as menstruation. A female’s first menstrual cycle is called menarche and it represents the peak of puberty. Menarche happens around 11 or 12 years of age. In Figure 27.15, the phases of the ovarian and uterine cycles are arranged to demonstrate how the two processes influence each other.

Figure 27.15 Ovarian and Uterine Cycles (OpenStax Bio 2e)

Hormonal Regulation of the Ovarian Cycle

The ovarian cycle is regulated by many of the same hormones that regulate the male reproductive system, including GnRH, LH, and FSH. The steps of this regulation are the following:

• As in males, the hypothalamus produces GnRH, a hormone that signals the anterior pituitary gland to produce the gonadotropins FSH and LH (Figure 27.16).
• FSH and LH 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 and trigger the development of the follicles (number 1 in Figure 27.16).
• 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 the follicular phase.
• The more granulosa and theca cells a follicle has, the more estrogen it will produce in response to LH stimulation. Following a classic negative feedback loop, elevated levels of estrogen will inhibit the hypothalamus and pituitary production of GnRH, LH, and FSH. Inhibin also decreases production of 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. Estrogen also helps with the development of the dominant or mature ovarian follicle.
• When only the one dominant follicle remains in the ovary, it again begins to secrete estrogen. It produces more estrogen than all the developing follicles did together before the negative feedback occurred. It produces so much estrogen that normal negative feedback does not occur. Instead, these particularly high concentrations of estrogen trigger a regulatory switch in the anterior pituitary that responds by secreting large amounts of LH and FSH into the bloodstream (Figure 27.16). This positive feedback loop by which more estrogen triggers the release of more LH and FSH occurs only at this point in the cycle.
• It is this peak of LH that leads to ovulation of the dominant follicle (number 2 in Figure 27.16). 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. Estrogen levels decline right after ovulation.
• 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 (number 3 in Figure 27.16). 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 critical hormone 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 postovulatory phase of progesterone secretion is known as the luteal phase of the ovarian cycle that was described previously in this text.

Figure 27.16 Hormonal Regulation of Ovulation The hypothalamus and pituitary gland regulate the ovarian cycle and ovulation. GnRH activates the anterior pituitary to produce LH and FSH, which stimulate the production of estrogen and progesterone by the ovaries. (OpenStax)

Uterine tubes, Uterus, and Vagina

Uterine Tubes

The uterine tubes (fallopian tubes or oviducts) extend laterally from both sides of the uterus to the ovaries and serve as the conduit of the oocyte from the ovary to the uterus (Figure 27.17). They are also the place where fertilization usually occurs. Each of the two uterine tubes is close to, but not directly connected to the ovary, and divided into sections. These sections are described below:

• The wide infundibulum has a funnel shape and flares out with slender, finger-like projections called fimbriae. These fimbriae cover the ovary at the time of ovulation only.
• The middle region of the tube, called the ampulla, is where fertilization often occurs
• The isthmus is the narrow medial end of each uterine tube that is connected to the uterus

The walls of the uterine tubes 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 moving the oocyte. This mucosa is composed of simple ciliated columnar epithelium and an underlying layer of areolar connective tissue. The middle layer, composed of smooth muscle, also helps to propel the oocyte via peristaltic contractions.

Figure 27.17 Ovaries, Uterine Tubes, and Uterus This anterior view shows the relationship of the ovaries, uterine tubes (oviducts), and uterus. Sperm enter through the vagina, and fertilization of an ovulated oocyte usually occurs in the distal uterine tube. From left to right, LM × 400, LM × 20. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) (OpenStax)

Uterus

Within the pelvic cavity, the uterus is the muscular organ that nourishes and supports the growing embryo (Figure 27.17). Its average size is approximately 5 cm wide by 7 cm long (approximately 2 by 3 inches) when a female is not pregnant. It connects to the uterine tubes laterally and to the vagina inferiorly. The uterus is separated into three areas:

• The fundus is the uterus area superior to the opening of the uterine tubes
• The body of the uterus is the middle region of the uterus and is formed by smooth muscle
• 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 estrogen concentrations, and these secretions can facilitate sperm movement through the female reproductive tract.
• The isthmus is a narrow region of the body and superior to the cervix

Several ligaments maintain the position of the uterus within the abdominopelvic cavity (Figure 27.12).

• The broad ligament is a fold of the peritoneum that serves as 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

In addition to the above-mentioned ligaments, some muscles secure the uterus and the vagina in place. There are muscles of the pelvic floor, such as the pelvic diaphragm and the urogenital diaphragm.

The wall of the uterus is composed of three tunics or layers.

• The most superficial layer is the serous membrane, or perimetrium, which consists of epithelial tissue that covers the exterior portion of the uterus. This layer is continuous with the broad ligament.
• The middle layer, or myometrium, is a thick smooth muscle layer responsible for uterine contractions. Most of the uterus is myometrial tissue, and the muscle fibers allow for the powerful contractions that occur during labor and the less powerful contractions (or cramps) that help to expel menstrual blood during a female’s period.
• The innermost layer of the uterus is known as the endometrium. The endometrium is composed of simple columnar epithelium and an underlying lamina propria made up of connective tissue and tubular or uterine glands that become bigger with the uterine cycle. Structurally, the endometrium is composed 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 is permanent and 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. Starting with puberty, when the ovarian follicles secrete progesterone and estrogen, the functional layer grows from the basal layer. The functional layer will shed as menses if fertilization and implantation does not occur. After the end of each menses, the basal layer generates a new functional layer.

Vagina

The vagina is a fibromuscular tube that forms the most inferior region of the female reproductive tract (Figure 27.17). The walls of the vagina are expandable and are composed of three tunics:

• Mucosa: An inner tunic made up of non-keratinized stratified squamous epithelium and a lamina propria rich in blood vessels; the inferior part of the vagina has folds, called rugae.
• Muscularis: composed of smooth muscle
• Adventitia: composed of areolar connective tissue and elastic fibers

The vaginal orifice is localized in the proximity of the external opening of the vagina. The folds from the rugae project into the lumen to form a thin membranous structure known as the hymen, which sometimes partially covers the entrance to the vagina. The hymen has many blood vessels and can be perforated during the first sexual intercourse, tampon use, exercise, or medical examinations. 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 vagina is home to a population of microorganisms that help to protect against infection by pathogenic bacteria, yeast, or other organisms that can enter the vagina. The most predominant type of healthy 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; washing the labia with antibacterial soaps; and cleaning the vaginal and urethral regions with harsh or perfumed chemicals (as in some wipes) can disrupt the normal balance of healthy microorganisms and increase proneness to yeast infections and irritation.

The Uterine Cycle

The uterine cycle (menstrual cycle) consists of the cyclical changes that happen in the lining of the uterus, specifically within the endometrium. These changes are regulated by the hormones progesterone and estrogen that are released from the developing follicles and the corpus luteum (Figure 27.15). The uterine cycle has three phases, as follows:

• Menstrual phase: This phase is characterized by the shedding of the functional layer of the endometrium. This phase happens approximately on days 1-5 or whatever are the first through last days of menstrual bleeding (menstruation or menses).

• Proliferative phase: This phase is characterized by the growth of a new functional layer, along with the growth of a follicle, and the release of estrogen by the ovaries. This phase occurs around days 6-14 of the cycle.

• Secretory phase: In response to progesterone secretion from the corpus luteum, there is an increase in blood vessels and the growth of the uterine glands. This phase happens around days 15-28.

Integration of the Uterine and Ovarian Cycles

Although we separated both the ovarian and uterine cycles to make each a little easier to understand, in reality, both cycles overlap each other. Now is the time to put them together in an integrative way (Figure 27.18). Next, you will read a description of both cycles, starting with the phases of the uterine cycle and the influence of the hormones on the uterine and ovarian cycles.

• The menstrual phase of the uterine cycle: This phase is characterized by the shedding of the functional layer of the endometrium (menses). This phase happens on days 1-5 approximately and lasts through the menstrual bleeding. 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, because of the degradation of the corpus luteum, marking the end of the luteal phase of the ovarian cycle. This decline in progesterone triggers the shedding of the stratum functionalis of the endometrium.

• Proliferative phase of the uterine cycle: This phase is characterized by the growth of the new functional layer, along with the growth of the follicle and the release of estrogen by the ovaries. This phase occurs around days 6-14 of the cycle. These increasing estrogen concentrations stimulate the endometrial lining to grow and rebuild. Recall that the high estrogen concentrations will eventually lead to a decrease in FSH because 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 of the ovarian cycle.
• The secretory phase of the uterine cycle: 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 released from the corpus luteum triggers the secretory phase, and as a result, there is an increase in blood vessels and the formation of uterine glands. The uterine 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. This phase happens around days 15-28.

If no pregnancy occurs, within approximately 10 to 12 days, the corpus luteum will degrade into the corpus albicans. At the same time, 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 uterine or menstrual cycle. Hormonal birth control pills take advantage of the negative feedback system that regulates the ovarian and menstrual cycles to stop ovulation and prevent pregnancy. The pills 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 happen.

Figure 27.18 Integration of Ovarian and Uterine Cycles The correlation of the hormone levels and their effects on the female reproductive system is shown in this timeline of the ovarian and menstrual cycles. The menstrual cycle begins at day one with the start of menses. Ovulation occurs around day 14 of a 28-day cycle, triggered by the LH surge. (OpenStax)

External genitalia

The external female reproductive structures are referred to collectively as the vulva (Figure 27.19). 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 medially to the labia majora. Although they naturally vary in shape and size from female to female, 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 vital in sexual sensation and orgasm. The vaginal opening is located between the opening of the urethra and the anus. It is flanked by outlets to the greater vestibular glands or Bartholin’s glands that release mucus for lubrication during intercourse. They are homologous to the bulbourethral glands in males. The paraurethral glands open laterally to the external urethral orifice and secrete mucus. Their homologous structure in males is the prostate.

Figure 27.19: The Vulva The external female genitalia are referred to collectively as the vulva. (OpenStax)

Mammary glands

The breasts are considered accessory organs of the female reproductive system. The function of 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, 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 an infant 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. The clusters can change in size depending on the amount of milk in the alveolar lumen. Once the milk is made in the alveoli, stimulated myoepithelial cells contract to push milk to the lactiferous sinuses. Thereafter, the infant can draw milk through the lactiferous ducts by suckling. The lobes 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.

Human milk contains lots of distinct molecules and antibodies that protect infants from infection, contribute to immune system maturation, organ development, and help to create healthy microbial colonization. Breastfeeding provides passive natural immunity from mother to child. Exclusive human breast milk feeding for the first six months of life and continued breast milk feeding with supplemented solid food from six months up to 1-2 years of age is considered the normative standard for feeding infants. Human milk is dynamic, this means that it changes in composition depending on the age and feeding needs of the infant.

Figure 27.20: Anatomy of the Breast OpenStax]

Female sexual response

Sexual intercourse occurs when the erect penis is inserted into the vagina. In females, during the excitement phase, the clitoris and the labia swell. The breasts also become engorged (filled with blood) and the nipples become erect. In the excitement phase, there is also an increase in the secretion of fluid that lubricates the walls of the vagina. Other changes during the excitement phase include increased heart rate and blood pressure, increased skeletal muscle tone, and hyperventilation. In the plateau phase, the changes that occur during the excitement phase continue and are maintained at an intense level. The briefest phase is the orgasm, with rhythmic muscular contractions and a further increase in blood pressure, heart rate, and respiratory rate. Females can experience two or more orgasms in succession which involve rhythmic contractions of the muscles underlying the vulva. The last phase is the resolution in which a sense of relaxation occurs. Genital tissue, heart rate, blood pressure, and breathing return to the unaroused state.

Menopause

Female fertility peaks when females are in their twenties and slowly reduces until a female reaches the age of thirty-five. After that time, fertility declines more rapidly, until it ends completely at menopause. Menopause is the cessation of the menstrual cycle that occurs because of the loss of ovarian follicles and the hormones that they produce. A female is considered postmenopausal when they have not menstruated for a full year. In the years before menopause, ovaries begin to produce decreasing levels of hormones, resulting in perimenopause, a transition period in which menstrual cycles become erratic, irregular, and different inflow. Common symptoms of perimenopause include periods that are heavier or lighter than usual, irregular periods, vaginal dryness, vaginal atrophy, sleep disturbances, and mood changes. These symptoms are often uncomfortable and can be treated–females should discuss treatment with their physician. The average age for postmenopausal females worldwide is between 50 and 52 years of age, with perimenopause beginning as early as the mid-to-late-30s. Poor health can lead to early loss of fertility and earlier menopause.

Birth Control Methods

The term birth control is used to describe methods to control fertility, prevent fertilization, and pregnancy. There are several methods available for birth control. Each has its advantages and disadvantages.

Surgical methods

Surgical methods of birth control include vasectomies in males (as described previously), where a portion of the ductus deferens are ligated with stitches to prevent sperm from entering the semen. In females, the surgical sterilization method is termed tubal ligation, where the uterine tubes are tied and closed to prevent secondary oocytes from passing through.

Hormonal methods

Hormonal methods of birth control involve hormones to prevent ovulation and pregnancy. They work by providing a constant level of 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 LH, ovulation does not occur. There are many ways to administer hormones to an individual. The most common method is birth control pills, which are taken orally. The small amount of estrogen and progesterone in these pills prevents ovulation (LH surge) but also changes the characteristics of the cervical mucus and endometrium to make it difficult for fertilization and implantation to occur. Other methods of hormonal contraception are skin patches, vaginal rings, hormone injections, and the morning after pill. Hormonal contraception does not protect from sexually transmitted diseases.

Physiological methods

Physiological methods of contraception include intrauterine devices (IUDs), barrier methods (such as condoms and vaginal pouches), and spermicides. Intrauterine devices are small objects that are inserted into the cavity of the uterus. The barrier method involves a physical barrier designed to prevent sperm from entering the uterus. IUDs prevent fertilization by blocking sperm access to the secondary oocyte.

Figure 27.21: Intrauterine Device IUD in the uterus (Wiki)

Spermicides include foams, creams, and jellies that make the vagina and cervix unfavorable for sperm survival. These agents are placed in the vagina before sexual intercourse. Spermicides kill sperm by disrupting their plasma membrane and work best when used with the barrier method of contraception.

Chapter Summary

Attribution

https://openstax.org/books/anatomy-and-physiology/pages/27-2-anatomy-and-physiology-of-the-female-reproductive-system

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3586783/

Key Terms

alveoli

(of the breast) milk-secreting cells in the mammary gland

ampulla

(of the uterine tube) middle portion of the uterine tube in which fertilization often occurs

antrum

fluid-filled chamber that characterizes a mature tertiary (antral) follicle

areola

highly pigmented, circular area surrounding the raised nipple and containing areolar glands that secrete fluid important for lubrication during suckling

bulbourethral glands

(also, Cowper’s glands) glands that secrete a lubricating mucus that cleans and lubricates the urethra prior to and during ejaculation

cervix

elongate inferior end of the uterus where it connects to the vagina

clitoris

(also, glans clitoris) nerve-rich area of the vulva that contributes to sexual sensation during intercourse

corpus albicans

nonfunctional structure remaining in the ovarian stroma following structural and functional regression of the corpus luteum

corpus luteum

transformed follicle after ovulation that secretes progesterone

corpus spongiosum

(plural = corpora cavernosa) column of erectile tissue in the penis that fills with blood during an erection and surrounds the penile urethra on the ventral portion of the penis

cremaster muscle

surrounds each testis like a muscular sling; contracts to elevate the testes when exposed to cold

dartos muscle

subcutaneous muscle layer of the scrotum; capable of tightening and wrinkling the scrotum when exposed to cold

diploid

having two complete sets of chromosomes

ductus deferens

(also, vas deferens) duct that transports sperm from the epididymis through the spermatic cord and into the ejaculatory duct; also referred as the vas deferens

ejaculatory duct

duct that connects the ampulla of the ductus deferens with the duct of the seminal vesicle at the prostatic urethra

endometrium

inner lining of the uterus, part of which builds up during the secretory phase of the menstrual cycle and then sheds with menses

epididymis

(plural = epididymides) coiled tubular structure in which sperm start to mature and are stored until ejaculation

fertilization

a sperm combines with an oocyte

fimbriae

fingerlike projections on the distal uterine tubes

follicle

ovarian structure of one oocyte and surrounding granulosa (and later theca) cells

folliculogenesis

development of ovarian follicles from primordial to tertiary under the stimulation of gonadotropins

fundus

(of the uterus) domed portion of the uterus that is superior to the uterine tubes

gamete

haploid reproductive cell that contributes genetic material to form an offspring

glans penis

bulbous end of the penis that contains a large number of nerve endings

gonads

reproductive organs (testes in men and ovaries in women) that produce gametes and reproductive hormones

granulosa cells

supportive cells in the ovarian follicle that produce estrogen

greater vestibular glands

(also, Bartholin’s glands) glands that produce a thick mucus that maintains moisture in the vulva area

haploid

having a single set of unpaired chromosomes

homologous

similar in function, location, and/or structure; derived from the same embryological structure

hymen

membrane that covers part of the opening of the vagina

infundibulum

(of the uterine tube) wide, distal portion of the uterine tube terminating in fimbriae

inguinal canal

opening in abdominal wall that connects the testes to the abdominal cavity

isthmus

narrow, medial portion of the uterine tube that joins the uterus; narrow region of the body of the uterus superior to the cervix

labia majora

hair-covered folds of skin located behind the mons pubis

labia minora

thin, pigmented, hairless flaps of skin located medial to the labia majora

lactiferous ducts

ducts that connect the mammary glands to the nipple and allow for the transport of milk

lactiferous sinus

area of milk collection between alveoli and lactiferous duct

interstitial endocrine cells

(also Leydig cells) cells between the seminiferous tubules of the testes that produce testosterone

mammary glands

glands inside the breast that secrete milk

meiosis

type of cell division that makes daughter cells with half as many chromosomes as the parent cell; type of cell division for gamete production

menarche

first menstruation in a pubertal female

menopause

the natural decline in female reproductive hormones and the ceasing of menstrual periods

menses

shedding of the inner portion of the endometrium out though the vagina; also referred to as menstruation or menstrual period

menses phase

phase of the menstrual cycle in which the endometrial lining is shed

menstrual cycle

approximately 28-day cycle of changes in the uterus consisting of a menses phase, a proliferative phase, and a secretory phase

mitosis

type of cell division that makes daughter cells that are identical to the parent cell; common type of cell division for tissue growth

mons pubis

mound of fatty tissue located at the front of the vulva

myometrium

smooth muscle layer of uterus that allows for uterine contractions during labor and expulsion of menstrual blood

oocyte

cell that results from the division of the oogonium and undergoes meiosis I at the LH surge and meiosis II at fertilization to become a haploid ovum

oogenesis

process by which oogonia divide by mitosis to primary oocytes, which undergo meiosis to produce the secondary oocyte and, upon fertilization, the ovum

oogonia

ovarian stem cells that undergo mitosis during female fetal development to form primary oocytes

ovarian cycle

approximately 28-day cycle of changes in the ovary consisting of a follicular phase and a luteal phase

ovaries

female gonads that produce oocytes and sex steroid hormones (notably estrogen and progesterone)

ovulation

release of a secondary oocyte and associated granulosa cells from an ovary

ovum

haploid female gamete resulting from completion of meiosis II at fertilization

paraurethral glands 

structures that open laterally to the female external urethral orifice and secrete mucus

penis

male organ of sexual intercourse

perimetrium

outer epithelial layer of uterine wall

polar body

smaller cell produced during the process of meiosis in oogenesis

prepuce

(also, foreskin) flap of skin that forms a collar around, and thus protects and lubricates, the glans penis

primary follicles

ovarian follicles with a primary oocyte and one layer of cuboidal granulosa cells

primordial follicles

least developed ovarian follicles that consist of a single oocyte and a single layer of flat (squamous) granulosa cells

proliferative phase

phase of the menstrual cycle in which the endometrium proliferates

prostate gland

doughnut-shaped gland at the base of the bladder surrounding the urethra and contributing fluid to semen during ejaculation

puberty

life stage during which a male or female adolescent becomes anatomically and physiologically capable of reproduction

rugae

(of the vagina) folds of skin in the vagina that allow it to stretch during intercourse and childbirth

scrotum

external pouch of skin and muscle that houses the testes

secondary follicles

ovarian follicles with a primary oocyte and multiple layers of granulosa cells

secondary sex characteristics

physical characteristics that are influenced by sex steroid hormones and have supporting roles in reproductive function

secretory phase

phase of the menstrual cycle in which the endometrium secretes a nutrient-rich fluid in preparation for implantation of an embryo

semen

ejaculatory fluid composed of sperm and secretions from the seminal vesicles, prostate, and bulbourethral glands

seminal vesicle

gland that produces seminal fluid, which contributes to semen

seminiferous tubules

tube structures within the testes where spermatogenesis occurs

Sertoli cells

(also, nurse cells and sustentacular cells) cells that support germ cells through the process of spermatogenesis

sperm

(also, spermatozoon (plural spermatozoa)) male gamete

spermatic cord

bundle of nerves and blood vessels that supplies the testes; contains ductus deferens

spermatid

immature sperm cells produced by meiosis II of secondary spermatocytes

spermatocyte

cell that results from the division of spermatogonium and undergoes meiosis I and meiosis II to form spermatids

spermatogenesis

formation of new sperm, occurs in the seminiferous tubules of the testes

spermatogonia

(singular = spermatogonium) diploid precursor cells that become sperm

spermiogenesis

transformation of spermatids to spermatozoa during spermatogenesis

tertiary follicles

(also, antral follicles) ovarian follicles with a primary or secondary oocyte, multiple layers of granulosa cells, and a fully formed antrum

testes

(singular = testis) male gonads

theca cells

estrogen-producing cells in a maturing ovarian follicle

uterine tubes

(also, fallopian tubes or oviducts) ducts that facilitate transport of an ovulated oocyte to the uterus

uterus

muscular hollow organ in which a fertilized egg develops into a fetus

vagina

tunnel-like organ that provides access to the uterus for the insertion of semen and from the uterus for the birth of a baby

vulva

external female genitalia