The ovaries contain a large number of follicles, each of which encloses an ovum. Some of these follicles mature during the ovarian cycle, and the ova they contain progress to the secondary oocyte stage of meiosis. At ovulation, the largest follicle breaks open to extrude a secondary oocyte from the ovary. The empty follicle then becomes a corpus luteum, which ultimately degenerates at the end of a nonfertile cycle.
The two ovaries (fig. 20.25), about the size and shape of large almonds, are suspended by means of ligaments from the pelvic girdle. Extensions called fimbriae of the uterine (fallopian) tubes partially cover each ovary. Ova that are released from the ovary—in a process called ovulation—are normally drawn into the uterine tubes by the action of the ciliated epithelial lining of the tubes. The lumen of each uterine tube is continuous with the uterus (or womb), a pear-shaped muscular organ held in place within the pelvic cavity by ligaments.
The uterus consists of three layers. The outer layer of connective tissue is the perimetrium, the middle layer of smooth muscle is the myometrium, and the inner epithelial layer is the endometrium. The endometrium is a stratified, squamous,
1. Describe the effects of castration on FSH and LH secretion in the male. Explain the experimental evidence suggesting that the testes produce a polypeptide that specifically inhibits FSH secretion.
2. Describe the two compartments of the testes with respect to (a) structure, (b) function, and (c) response to gonadotropin stimulation. Describe two ways in which these compartments interact.
3. Using a diagram, describe the stages of spermatogenesis. Why can spermatogenesis continue throughout life without using up all of the spermatogonia?
4. Describe the structure and proposed functions of the Sertoli cells in the seminiferous tubules.
5. Explain how FSH and androgens synergize to stimulate sperm production at puberty. Describe the hormonal requirements for spermatogenesis after puberty.
Suspensory ligament of ovary
Broad ligament of uterus
Ampulla of uterine
Infundibulum of uterine tube
Fimbriae Egg cell Follicle
The uterus, uterine tubes, and ovaries. The supporting ligaments can also be seen in this posterior view.
Posterior portion of vaginal fornix
Cervix of uterus
Cervix of uterus
■ Figure 20.26 The organs of the female reproductive system. These are shown in sagittal section.
nonkeratinized epithelium that consists of a stratum basale and a more superficial stratum functionale. The stratum functionale, which cyclically grows thicker as a result of estrogen and progesterone stimulation, is shed at menstruation.
The uterus narrows to form the cervix (= neck), which opens to the tubular vagina. The only physical barrier between the vagina and uterus is a plug of cervical mucus. These structures— the vagina, uterus, and fallopian tubes—constitute the accessory sex organs of the female (fig. 20.26). Like the accessory sex organs of the male, the female reproductive tract is affected by gonadal steroid hormones. Cyclic changes in ovarian secretion, as will be described in the next section, cause cyclic changes in the epithelial lining of the tract.
The vaginal opening is located immediately posterior to the opening of the urethra. Both openings are covered by longitudinal folds—the inner labia minora and outer labia majora (fig. 20.27). The clitoris, a small structure composed largely of erectile tissue, is located at the anterior margin of the labia minora.
The germ cells that migrate into the ovaries during early embryonic development multiply, so that by about 5 months of gestation (prenatal life) the ovaries contain approximately 6 million
■ Figure 20.27 The external female genitalia. The labia majora and clitoris in a female are homologous to the scrotum and penis, respectively, in a male.
■ Figure 20.28 Photomicrographs of the ovary. (a) Primary follicles and one secondary follicle and (b) a graafian follicle are visible in these sections.
to 7 million oogonia. Most of these oogonia die prenatally through a process of apoptosis (chapter 3). The production of new oogonia stops at this point and never resumes again. The oogonia begin meiosis toward the end of gestation, at which time they are called primary oocytes. Like spermatogenesis in the prenatal male, oogenesis is arrested at prophase I of the first meiotic division. The primary oocytes are thus still diploid.
Primary oocytes decrease in number throughout a woman's life. The ovaries of a newborn girl contain about 2 million oocytes—all she will ever have. Each oocyte is contained within its own hollow ball of cells, the ovarian follicle. By the time a girl reaches puberty, the number of oocytes and follicles has been reduced to 400,000. Only about 400 of these oocytes will ovulate during the woman's reproductive years, and the rest will die by apoptosis. Oogenesis ceases entirely at menopause (the time menstruation stops).
Primary oocytes that are not stimulated to complete the first meiotic division are contained within tiny primary follicles (fig. 20.28a). Immature primary follicles consist of only a single layer of follicle cells. In response to FSH stimulation, some of these oocytes and follicles get larger, and the follicular cells divide to produce numerous layers of granulosa cells that surround the oocyte and fill the follicle. Some primary follicles will be stimulated to grow still more, and they will develop a number of
■ Figure 20.29 Photomicrographs of oocytes. (a) A primary oocyte at a metaphase I of meiosis. Notice the alignment of chromosomes (arrow) (b) A human secondary oocyte formed at the end of the first meiotic division. Also shown is the first polar body (arrow).
fluid-filled cavities called vesicles; at this point, they are called secondary follicles (fig. 20.28a). Continued growth of one of these follicles will be accompanied by the fusion of its vesicles to form a single fluid-filled cavity called an antrum. At this stage, the follicle is known as a mature, or graafian, follicle (fig. 20.28b).
As the follicle develops, the primary oocyte completes its first meiotic division. This does not form two complete cells, however, because only one cell—the secondary oocyte—gets all the cytoplasm. The other cell formed at this time becomes a small polar body (fig. 20.29), which eventually fragments and disappears. This unequal division of cytoplasm ensures that the ovum will be large enough to become a viable embryo should fertilization occur. The secondary oocyte then begins the second meiotic division, but meiosis is arrested at metaphase II. The second meiotic division is completed only by an oocyte that has been fertilized.
The secondary oocyte, arrested at metaphase II, is contained within a graafian follicle. The granulosa cells of this follicle form a ring around the oocyte and form a mound that supports the oocyte. This mound is called the cumulus oopho-rus. The ring of granulosa cells surrounding the oocyte is the corona radiata. Between the oocyte and the corona radiata is a thin gel-like layer of proteins and polysaccharides called the zona pellucida (see fig. 20.28b). The zona pellucida is significant because it presents a barrier to the ability of a sperm to fertilize an ovulated oocyte.
Under the stimulation of FSH from the anterior pituitary, the granulosa cells of the ovarian follicles secrete increasing amounts of estrogen as the follicles grow. Interestingly, the granulosa cells produce estrogen from its precursor testosterone, which is supplied by cells of the theca interna, the layer immediately outside the follicle (see fig. 20.28b).
Usually by the tenth to fourteenth day after the first day of menstruation only one follicle has continued its growth to become a fully mature graafian follicle (fig. 20.30). Other secondary follicles during that cycle regress and become atretic—a term that means "without an opening" in reference to their failure to rupture. Follicle atresia, or degeneration, is a type of apoptosis that results from a complex interplay of hormones and paracrine regulators. The gonadotropins (FSH and LH), as well as various paracrine regulators and estrogen act to protect follicles from atresia. By contrast, paracrine regulators that include androgens and FAS ligand (chapter 15) promote atresia of the follicles.
The follicle that is protected from atresia and that develops into a graafian follicle becomes so large that it forms a bulge on the surface of the ovary. Under proper hormonal stimulation, this follicle will rupture—much like the popping of a blister— and extrude its oocyte into the uterine tube in the process of ovulation (fig. 20.31).
The released cell is a secondary oocyte, surrounded by the zona pellucida and corona radiata. If it is not fertilized, it will degenerate in a couple of days. If a sperm passes through the corona radiata and zona pellucida and enters the cytoplasm of the secondary oocyte, the oocyte will then complete the second meiotic division. In this process, the cytoplasm is again not divided equally; most remains in the zygote (fertilized egg), leaving another polar body which, like the first, degenerates (fig. 20.32).
Changes continue in the ovary following ovulation. The empty follicle, under the influence of luteinizing hormone from the anterior pituitary, undergoes structural and biochemical
(graafian) Primary Germinal follicle follicle epithelium
Growing Corpus primary albicans follicle
■ Figure 20.30 An ovary containing follicles at different stages of development. An atretic follicle is one that is dying by apoptosis. It will eventually become a corpus albicans.
Fimbria of uterine tube
Fimbria of uterine tube
■ Figure 20.31 Ovulation from a human ovary. Notice the cloud of fluid and granulosa cells surrounding the ovulated oocyte.
changes to become a corpus luteum (= yellow body). Unlike the ovarian follicles, which secrete only estrogen, the corpus luteum secretes two sex steroid hormones: estrogen and progesterone. Toward the end of a nonfertile cycle, the corpus luteum regresses to become a nonfunctional corpus albicans. These cyclic changes in the ovary are summarized in figure 20.33.
The term pituitary-ovarian axis refers to the hormonal interactions between the anterior pituitary and the ovaries. The anterior pituitary secretes two gonadotropic hormones—follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—both of which promote cyclic changes in the structure and function of the ovaries. The secretion of both gonadotropic hormones, as previously discussed, is controlled by a single releasing hormone from the hypothalamus—gonadotropin-releasing hormone (GnRH)—and by feedback effects from hormones secreted by the ovaries. The nature of these interactions will be described in detail in the next section.
Primary oocyte (46 chromosomes)
Test Yourself Before You Continue
1. Compare the structure and contents of a primary follicle, secondary follicle, and graafian follicle.
2. Define ovulation and describe the changes that occur in the ovary following ovulation in a nonfertile cycle.
3. Describe oogenesis and explain why only one mature ovum is produced by this process.
4. Compare the hormonal secretions of the ovarian follicles with those of a corpus luteum.
First meiotic division
First polar body degenerates
Secondary oocyte (23 chromosomes)
Spermatozoon fertilizes oocyte
Second meiotic division
Second polar body degenerates
■ Figure 20.32 Oogenesis. During meiosis, each primary oocyte produces a single haploid gamete. If the secondary oocyte is fertilized, it forms a second polar body and its nucleus fuses with that of the sperm cell to become a zygote.
Since one releasing hormone can stimulate the secretion of both FSH and LH, one might expect always to see parallel changes in the secretion of these gonadotropins. This, however, is not the case. FSH secretion is slightly greater than LH secretion during an early phase of the menstrual cycle, whereas LH secretion greatly exceeds FSH secretion just prior to ovulation. These differences are believed to result from the feedback effects of ovarian sex steroids, which can change the amount of GnRH secreted, the pulse frequency of GnRH secretion, and the ability of the anterior pituitary cells to secrete FSH and LH. These complex interactions result in a pattern of hormone secretion that regulates the phases of the menstrual cycle.
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