Folliculogenesis
Graafian follicles develop from a pool of primordial follicles that are at rest through a process known as folliculo- genesis. Throughout a female’s entire reproductive life, this process is ongoing.
Three physiologically separate stages can be distinguished in the development of a primordial follicle into a Graafian follicle: the stages from primordial to preantral, preantral to antral, and antral to Graafian follicle.(i) Primordial to preantral follicle development: The typical number of primordial follicles in a species, most likely controlled by an unidentified intraovarian factor, begins to increase daily. An initial developing primordial follicle persists until ovulation or degenerates. It is said that a primordial follicle needs anywhere from 20 days to 6 months to mature to the ovulatory stage. Every day, a number of follicles reach the preantral stage, regardless of the time it takes for them to grow from the primordial to the preantral stage or the hormonal and reproductive status of the animal. To the degree that the growth of primordial follicles is an ongoing process, gametogenesis in females is thus similar to spermatogenesis in males. Gonadotropins have no effect on the prean- tral stage of primordial follicle development. After completing this stage of hormone-independent growth, primordial follicles still need to produce receptors for LH in the theca, FSH, and estrogen in the granulosa cells, and other hormones to move into the hormone-dependent phase of development and reach the Graafian follicle stage.
(ii) Preantral to Antral follicle development: Each species’ characteristic number of preantral follicles is attracted by the FSH surge, which in all farm animals takes place 20-30 hours after the preovulatory LH surge. Preantral follicles undergo antrum formation in response to this FSH surge, which happens before the CL starts secreting progesterone.
Some of these follicles go on to ovulate in a matter of cycles. The number of antral follicles on the ovary about 17 days later and the ovulation rate are related to the magnitude of this FSH surge in farm animals. Either 17 or 34 days pass from the start of antrum formation to the time needed for ovulation. The granulosa cells produce their own receptors as a result of FSH. It could be this capacity of FSH that gives rise to the term “follicle recruitment by FSH.” Increased FSH also causes granulosa cell LH receptors to grow. LH stimulates theca cells, which then release androgens. Granulosa cells use FSH to change these androgens into estrogens. In granulosa cells, estrogen stimulates FSH receptors even more. Additionally, the amount of estrogen secreted dictates which follicle will acquire the LH receptors required for luteinization and ovulation. In granulosa cells, FSH and estrogen stimulate mitosis. It is also in charge of gap junction formation within and between granulosa cells as well as between them. Under the effect of FSH, granulosa cells produce and secrete substances that lead to granulosa cell separation and the development of an antrum.(iii) Antral to Graafian follicle development: Few follicles that were encouraged to form an antrum reach the ovulatory stage; the rest experience atresia, most likely as a result of a drop in FSH levels that takes place during the luteal phase. It is true that injecting FSH or hormones similar to FSH during the luteal phase of an animal’s cycle can cause super ovulation in domesticated animals. During ovulation, multiple ovum are released due to the administration of FSH exogenously. Domestic animals’ luteal phases last comparatively long because progesterone from the CL can cause follicular growth to decline for up to 17 days. It’s interesting to note that when progesterone levels are lowered through hormonal or manual removal of the corpus luteum, follicular growth accelerates and the next estrus soon ensues. This is the physiological underpinning of the widely recognized practice of synchronizing estrus following prostaglandin-induced luteolysis. Progesterone therapy can be used to extend the life of CL and lengthen the cycle. The transformation of preantral follicles into Graafian follicles is largely dependent on FSH. Every estrous cycle, a crop of developing follicles is stimulated to continue growing and maturing by the pituitary gland’s adequate release of FSH. Throughout the early stages of the estrous cycle, many follicles grow, but only a small number mature before the ovum. On the granulosa, FSH increases LH receptors while LH decreases them, particularly during the preovulatory LH surge. The transition of granulosa from estrogen secretion (during follicular phase) to progesterone secretion (during luteal phase) is dependent on these receptor alterations.
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