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A newborn baby girl has about a million primary oocytes in each ovary. By the time she reaches puberty, she has only about 200,000; the rest have degenerated. During a woman’s fertile years, her ovaries go through about 450 ovarian cycles. During each cycle, 10 to 20 primary oocytes begin to mature, but usually only one matures completely and is ovulated; the others degenerate (see Figure 42.13A). At around the age of 50, a woman reaches menopause—the end of fertility—and may have few oocytes left in each ovary. For more than 50 years, the gradual age-related decline in oocytes was taken as evidence that oogenesis in humans is limited to prenatal life. However, in recent years oogonial stem cells (OSCs) have been isolated from adult ovaries of mice and of humans. These OSCs have been shown to be capable of division and production of oocytes. Thus menopause may not be the absolute end of fertility.

A primary oocyte surrounded by a layer of ovarian cells is the functional unit of the ovary—the follicle (see the photo in Figure 42.13A). The follicle cells surrounding the oocyte supply it with nutrients and growth factors. The follicle cells are also the main site of production of the female hormones estrogen and progesterone. Usually only one follicle matures completely and reaches the stage of ovulation, releasing its egg at midcycle. Events that lead up to and follow ovulation are illustrated in Figure 42.13B. Follicle maturation occurs in the first half of the cycle, therefore called the follicular phase. The midcycle point at which the follicle ruptures to release the oocyte is marked by a vertical yellow bar. The notable hormonal event leading up to ovulation is the gradual and then escalating increase in estrogen, which peaks just prior to midcycle (see Figure 42.13B).

The second half of the ovarian cycle is called the luteal phase because the follicle cells left in the ovary after the egg is ovulated develop into an endocrine gland—the corpus luteum (yellow body)—secreting estrogen and progesterone. If the egg is not fertilized and does not embed into the endometrium, the corpus luteum becomes inactive and degenerates in 12 to 14 days.