This lead follicle is termed the “dominant follicle,” Cataldo said, because it acts as the dominant force in the ovary. “It basically makes the smaller ones die,” he said, and it does this by producing estradiol and possibly inhibin B as well, both of which are hormones that signal the pituitary gland to cut back on its production of FSH. With less FSH arriving in the ovaries, the smaller follicles do not have enough of the hormone to keep developing and avoid atresia. The dominant follicle, in contrast, is able to survive because of a mechanism that increases its sensitivity to FSH and allows it to keep growing with less FSH than the other follicles need. This process of selecting a single dominant follicle is the reason that women tend to ovulate only one egg per cycle, Cataldo explained. This limit is known to biologists as the ovulatory quota, and it is different from species to species. For example, pigs generally have litters of 6 to 12 piglets; and cows typically have 1 or, at most, 2 calves.
Although only the dominant follicle survives to produce an oocyte in a normal monthly cycle, the other antral follicles can also survive and grow if there is enough circulating FSH. This is basic of the hormone therapy used to stimulate the ovaries and increase the number of oocytes that a woman can provide for assisted reproduction or for research.
The standard hormone therapy involves daily injections of gonadotropins—most often hormones with an action similar to FSH—beginning on about the third day of menstruation and lasting for about 10 days (see Figure 2-2), after having begun injections of a gonadotropin-releasing hormone (GnRH) to prevent premature oocyte release from the follicles. An alternative is to give the drug clomiphene to induce the body’s pituitary gland to release more FSH. With the right amount of hormones, all or almost all of the antral follicles will continue to grow. Occasionally, a GnRH agonist or a GnRH antagonist is added to the mix in order to prevent the body’s normal LH surge. Then, when ultrasound shows that the follicles have all reached the proper stage of maturation, with their oocytes ready for ovulation and fertilization, yet another hormone—human chorionic gonadotropin (hCG)—is given. This hormone would normally cause the follicles to ovulate and release their eggs in about 36 hours, but in practice the physician will retrieve the eggs from the follicles before that happens.