13.7: Female System - Structures and Functions

The reproductive system in women contains certain paired structures, including the ovaries, the tubular oviducts, external genital structures (labia minora and labia majora), and breasts (Figure 13.1, Figure 13.7, Figure 13.8, Figure 13.12). Important female structures that occur singly include the uterus, vagina, and clitoris (Figure 13.11).

Figure 13.7 Structure of the female reproductive system: Mid-sagittal section of female pelvis - two versions. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Figure 13.8 3D view - Structure of the female reproductive system. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Figure 13.11 Female genitalia. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Figure 13.12 Structure of the breast – sagittal view and internal anatomy. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

This section is abbreviated because reproductive functioning in women ends at menopause. Menopause usually occurs some time between ages 45 and 55 and is evidenced by the absence of menstrual periods for at least 1 year.

Ovaries

The ovaries are held in place within the lower region of the abdominal cavity by several ligaments (Figure 13.9). Each ovary is shaped like a slightly flattened oval and is approximately the size of a large almond (2.5 to 5.0 cm long, 1.0 to 2.5 cm wide, 0.5 to 1.0 cm thick). The bulk of the ovary consists of the stroma, which contains fibrous material with many blood vessels (Figure 13.10). Embedded within the stroma and near its surface are many small clusters of cells called follicles. Each follicle contains an immature ovum. The entire ovary is surrounded by a thin layer of cells (germinal epithelium)..

Figure 13.9 Ovaries, oviducts, uterus and associated structures. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Ovarian Cycles

Unlike the testes, which produce hormones and sperm cells at a fairly steady rate, ovarian functioning consists of a sequence of events during which hormones and ova are produced periodically. Since this sequence is repeated over and over, it is called the ovarian cycle (Figure 13.10).

Figure 13.10 Ovarian structure, follicle development, and ovulation. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

An ovarian cycle begins when two hormones (LH and FSH) from the pituitary gland stimulate the cells in a few follicles to make more follicle cells and secrete the hormones estrogen and progesterone. The blood levels of these hormones rise as the follicles grow and increase their hormone production. These hormones also cause the immature ovum in each stimulated follicle to begin to mature. For unknown reasons, one of the follicles develops faster than do the others, and after several days the other stimulated follicles begin to degenerate and become masses of scar tissue.

Approximately 14 days after the cycle has begun, elevated blood estrogen levels cause the pituitary gland to increase production of LH and FSH. The surge in these hormones causes the fully mature follicle to rupture and release its ovum in a process called ovulation. The freed ovum is then transported down the oviducts, where it degenerates unless it is fertilized within 3 days.

After ovulation, LH and FSH cause the ruptured follicle, which remains in the ovary, to grow into a mass called a corpus luteum. The corpus luteum produces estrogen and great quantities of progesterone for about 10 days after ovulation. Then, as high levels of progesterone cause blood levels of the pituitary hormones to decline, the corpus luteum degenerates. As it does so, the production and blood levels of estrogen and progesterone fall sharply. The degenerated corpus luteum remains in the ovary as a pale mass of scar tissue. Once estrogen and progesterone levels have become very low, the pituitary gland initiates the beginning of the next ovarian cycle. Though a typical ovarian cycle spans 28 days, each cycle may vary by several days.

The estrogen and progesterone produced by the ovaries are required for the complete development and maintenance of female reproductive system structures and other female characteristics (e.g., body contour). The ovary also produces a very small amount of testosterone, which seems to stimulate interest in sexual activity (libido), just as it does in men. These three hormones also influence several other activities (Chapter 14).

Approximately 200,000 immature follicles are present in each ovary when ovarian cycles begin and sexual maturation occurs during adolescence. Once begun, ovarian cycles are repeated until menopause. Since usually only one follicle matures fully and releases its ovum during each ovarian cycle, not more than approximately 500 follicles release ova before ovarian cycles cease. Most of the other follicles degenerate into atretic follicles.

Oviducts

An ovum that has been ovulated enters the funnel-shaped opening of the nearby oviduct (uterine tube, fallopian tube) (Figure 13.9). Each oviduct is approximately 10 cm (4 inches) long and extends from the region near its corresponding ovary to its point of entry into the upper part of the uterus. The wall of each oviduct consists of an inner lining of cells, a middle layer containing smooth muscle, and an outer layer containing fibrous material that helps hold the oviduct in place.

The cells covering the open end of the oviduct and lining its interior have motile projections called cilia. The cilia beat in an organized pattern that sweeps fluids from the body cavity into the funnel-shaped opening of the oviduct. The current produced carries each ovulated ovum into the oviduct. Movement of the cilia and peristaltic contractions of the smooth muscle keep the ovum moving toward the uterus, a journey that takes almost 9 days. The cells in the lining of the oviduct also secrete fluid that seems to nourish the ovum.

The oviducts also serve as an upward passageway for sperm cells deposited into the female system during sexual intercourse. This function is important because an ovum is viable for not more than 3 days after ovulation. Therefore, for fertilization to occur, the sperm must reach the ovum while it is in the upper third of the oviduct. The mechanism by which sperm cells move quickly up the oviduct while an ovum is carried downward toward the uterus is not clearly understood.

Ova that are not fertilized degenerate. However, once an ovum is fertilized, the embryo begins to develop immediately. It divides into many cells and begins to form a hollow sphere of cells before reaching the uterus. Secretions from the oviduct help support the development of the new individual by providing it with nutrients.

Uterus

The uterus is suspended near the bottom of the abdominal cavity and has a broad upper region into which the oviducts enter. It is held in place by several ligaments and receives support from the muscular floor of the pelvic cavity, the urinary bladder, and the end of the large intestine. The uterus tapers to a narrow lower portion, the cervix, which protrudes into the vagina (Figure 13.7, Figure 13.8). An average uterus in a young adult woman is 7.5 cm (3 inches) long and 5.0 cm (2 inches) wide at its broadest point.

Like the oviducts, the uterine wall is composed of three layers, but the two inner layers are much thicker. The innermost layer (endometrium) becomes especially thick when its growth is stimulated by estrogen and progesterone. The middle layer of smooth muscle (myometrium) is the thickest layer. The outermost layer contains much fibrous material that attaches to ligaments that hold the uterus in place. The uterine wall surrounds a narrow space called the uterine cavity, which connects the passageways in the oviducts with the central channel in the vagina.

Since the uterine cavity extends from the vagina to the oviducts, it serves as a passageway for sperm cells in the vagina to reach the ovum. Several days after fertilization, the embryo reaches and enters the uterus. The embryo remains adrift in the uterine cavity for a few days, after which it embeds itself into the endometrium.

The endometrium contributes to the formation of the placenta and thus nourishes the developing child until birth. The placenta also produces estrogen and progesterone, which further stimulate breast development and help maintain pregnancy. As the developing child and the placenta grow, the myometrium stretches to accommodate them. When prenatal development is complete, contractions of the myometrium (labor contractions) push the infant through the vagina and out of the mother's body.

Menstrual (Uterine) Cycles

Because of hormonal changes during an ovarian cycle, the uterus also undergoes cyclic changes. These changes constitute a menstrual cycle or uterine cycle.

A menstrual cycle begins within 3 days to 4 days after blood levels of estrogen and progesterone start to fall, near the end of the previous ovarian cycle. Since the endometrium is no longer strongly stimulated by these hormones, the arteries serving it constrict, resulting in inadequate blood flow to this thickened layer. Then, except for a thin layer of endometrial cells close to the myometrium, the endometrium dies and is shed along with some blood from the damaged vessels. This material passes through the central passageways in the cervix and the vagina and leaves the woman's body as the menstrual flow. The period of 3 days to 5 days required for endometrial shedding is often called the menstrual period, and the woman is said to be menstruating or "having a period."

By the time menstruation is completed, the next ovarian cycle has begun and blood levels of estrogen and progesterone rise. These rising hormone levels stimulate the remaining endometrial cells to proliferate, and the endometrium thickens considerably for the next 20 days. This prepares the endometrium to receive and nourish an embryo if fertilization and the embedding of an embryo occur. If embedding does not occur, estrogen and progesterone levels fall and the menstrual cycle ends approximately 3 days thereafter, when the next menstrual period begins. Thus, the end of one menstrual cycle is marked by the beginning of the next. Since these cycles are controlled by and parallel ovarian cycles, both cycles take approximately 28 days.

If an embryo is embedded in the endometrium, the developing placenta produces hormones that stimulate the corpus luteum to continue hormone production so that menstruation does not occur. Therefore, the developing child is retained and pregnancy continues. Hormones from the placenta and corpus luteum also inhibit the production of LH and FSH by the pituitary gland and prevent additional ovarian cycles until birth has occurred.

Vagina

The vagina is a tube approximately 7.5 to 10.0 cm (3 to 4 inches) long that extends downward behind the urinary bladder and urethra. It leads from the cervix to the outside of the body (Figure 13.7, Figure 13.8).

The wall of the vagina is thin and is composed of an inner lining of cells covering a layer containing smooth muscle, blood vessels, and much fibrous elastic material. Under resting conditions, the wall of the vagina is wrinkled and collapsed inward so that the inner surfaces touch and close its central channel. However, the wrinkles (rugae) and the elasticity of the wall allow the vagina to be stretched considerably in both length and width. This allows the entrance of a penis during sexual intercourse and the exit of an infant during birth.

The vagina makes five contributions to the reproductive functioning of the female system. It permits the menstrual flow to leave the woman's body; serves as part of the passageway for sperm cells to reach an ovum; helps sperm cells reach an ovum by accommodating the entrance of a penis and permitting the cells to be deposited close to the opening to the uterus; provides a warm moist environment for sperm cell survival; and provides a birth canal through which an infant can leave the mother's body during birth.

Since the vagina undergoes physical trauma during sexual intercourse and provides a relatively wide entry into a woman's body, its lining has three adaptations to resist abrasion and the entry of microbes. First, the lining cells form many layers which resemble the epidermis except that no keratin is present. The surface cells of the lining steadily peel away and are replaced by underlying cells. Second, lubricating fluids, which seem to seep through the lining from underlying blood vessels, help reduce friction during sexual intercourse. Third, lining cells contain glycogen, which is released from these cells after they peel away. Healthful bacteria in the vagina use the glycogen as a nutrient to produce acidic waste products that prevent the growth of harmful microbes. Sperm cells survive the acids because alkaline materials in the vaginal lubricating fluid and in semen neutralize the acids.

External Structures (Genitalia)

Externally, the vaginal opening is flanked by a pair of thin fleshy folds called the labia minora (Figure 13.7, Figure 13.8, Figure 13.11). These folds also flank the urethral opening, which lies in front of the vaginal orifice. The labia minora meet a short distance in front of the urethral opening.

Under resting conditions, the free edges of the labia minora tend to meet at the midline and cover the vaginal orifice, inhibiting the entrance of microbes and foreign materials. The labia are very sensitive to touch because they have many sensory nerve endings. Their surface also contains many sebaceous glands. Between the rear of the labia minora and the vaginal orifice lie a pair of Bartholin's glands. During sexual arousal, these glands secrete a small amount of lubricating fluid.

To the side of the labia minora lie the labia majora, two thick fleshy folds that contain fat and have hair (pubic hair) on their exposed surfaces. These labia meet in front of the junction of the labia minora and blend with the mons pubis, a fatty hair-covered pad overlying the front of the pelvis at the center. As in the labia minora, the free edges of the labia majora meet under resting conditions and help block the entrance to the vagina.

The junction of the labia minora marks the location of the clitoris. Most of the clitoris is embedded between the front limits of the labia minora and the junction of the labia majora. However, the tip of the clitoris (glans) protrudes slightly just behind the junction of the labia minora. The clitoris is approximately 2.5 cm long and less than 1.3 cm wide.

The clitoris consists primarily of two masses of erectile tissue. Though much smaller, these masses correspond to the two masses of erectile tissue along the top of the penis (corpus cavernosa). Also like the penis, the clitoris is very sensitive to touch and its erectile tissue becomes engorged with blood during sexual arousal.

Except for preventing the entrance of foreign materials and providing a small amount of lubricant, the external genitalia contribute little to the reproductive role of the female system. However, they make major contributions to the pleasurable sensations derived from sexual activity.

Breasts

Figure 13.13 Age changes in the structure of the breast. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

The breasts are attached to the layer of fibrous material that overlies the large chest muscles (pectoralis major) (Figure 13.13). Except when a woman is pregnant, each breast consists mostly of fat tissue. The breast is divided by sheets of fibrous material into approximately 20 segments, each of which contains some glandular material (mammary glands). The glands remain small unless the woman becomes pregnant. During pregnancy, the very high blood levels of estrogen and progesterone stimulate them to enlarge. When a woman is not pregnant, the fat tissue makes the breast firm and the sheets of fibrous material support the breast, causing the breast to protrude from the chest wall.

The circular pigmented patch of skin on the front of each breast is called an areola. The nipple is the protrusion at the center of the areola. Both structures contain many sensory nerve endings which make them especially sensitive to touch. When the areola or nipple is stimulated by touch or other factors (e.g., cold) or when a woman becomes sexually aroused, smooth muscle cells contract and cause the nipple to become firmer and protrude farther, a process called erection of the nipple.

The reproductive role of the breasts is to help support the development of a child by providing nourishment after birth. This is accomplished when a hormone (prolactin) produced by the mother's pituitary gland after giving birth causes the enlarged mammary glands to produce milk. Another pituitary hormone (oxytocin) causes the breasts to eject the milk through ducts leading out of the nipples.