Folliculogenesis

In biology, folliculogenesis is the maturation of the ovarian follicle, a densely packed shell of somatic cells that contains an immature oocyte. Folliculogenesis describes the progression of a number of small primordial follicles into large preovulatory follicles that enter the menstrual cycle.

Contrary to male spermatogenesis, which can last indefinitely, folliculogenesis ends when the remaining follicles in the ovaries are incapable of responding to the hormonal cues that previously recruited some follicles to mature. This depletion in follicle supply signals the beginning of menopause.

The primary role of the follicle is oocyte support. From birth, the ovaries of the human female contain a number of immature, primordial follicles. These follicles each contain a similarly immature primary oocyte. After puberty and commencing with the first menstruation, a clutch of follicles begins folliculogenesis, entering a growth pattern that will end in death or in ovulation (the process where the oocyte leaves the follicle).

During post-pubescent follicular development, and over the course of roughly a year, primordial follicles that have begun development undergo a series of critical changes in character, both histologically and hormonally. Two-thirds of the way through this process, the follicles have transitioned to tertiary, or antral, follicles. At this stage in development, they become dependent on hormones emanating from the host body, causing a substantial increase in their growth rate.

With a little more than ten days until the end of the period of follicular development, most of the original group of follicles have died (a process known as atresia). The remaining cohort of follicles enter the menstrual cycle, competing with each other until only one follicle is left. This remaining follicle, the late tertiary or pre-ovulatory follicle, ruptures and discharges the oocyte (that has since grown into a secondary oocyte), ending folliculogenesis.

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