■ 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).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.