Gamete Development Occurs Initially Without Gonadotropin Support and Subsequently with Pulsatile Gonadotropin Secretion
Oocyte proliferation» which occurs by mitotic division during fetal development, ends at about the time of birth in most mammalian species. Oocytes begin the process of reduction of chromosome numbers to the haploid state by meiosis shortly after birth under the influence of meiosis-initiating factor, thought to be produced by the rete ovarii.
The process is soon interrupted at the diplotene, or dictyate, stage of meiosis I by the meiosis-inhibiting factor, which is probably produced by the developing follicle cells. Oocytes remain in this stage until the follicle begins its final development, an interval that can be as long as 50 years or more in humans. The follicle, at this point, is delineated by an outer basement membrane (membrana propria), which is secreted by the follicle cells.The initial development of the follicle involves growth of the oocyte. This growth is accompanied by intense synthetic activity; a large amount of ribonucleic acid (RNA) is synthesized. At the same time, follicle cells begin to divide and form a granulosa that is several cells thick. The granulosa cells then secrete another boundary substance, the zona pellucida, that lies within the granulosa and that immediately surrounds the oocyte. Granulosa cells maintain contact with the oocyte through the zona pellucida by means of the development of cytoplasmic processes. Interaction among granulosa cells is facilitated by the development of gap junctions. This form of communication is important because the granulosa has no blood supply; blood vessels arc excluded at the level of the membrana propria. The thecal layer of the follicle forms around the membrana propria to complete the layers of the follicle. Follicles at this stage are called primary, or preantral, follicles.
Factors that control initial follicle growth are not known. External factors, such as gonadotropins, are not required because preantral follicles can develop in Iiypophysectomized animals. In species such as cattle and horses (perhaps sheep and goats as well) in which several dominant follicles develop during the estrous cycle, it is likely that a few follicles begin to develop each day. In animals in which a cohort of follicles develops synchronously (pigs» cats, dogs), there appears to be less tendency to have competing follicle growth waves during the luteal phase (pig) and a tendency to have only one cohort of follicles during the preovulatory period (cat and dog). Thus the development of a cohort of follicles may limit follicle development from the primordial state, at least during the period of active follicle development leading to ovulation. Initial follicle growth is under genetic control, and the pattern reflects the needs of the particular species.