31.17 : Folliculogenesis

Folliculogenesis is the development of ovarian follicles, the specialized structures within the ovarian cortex where oogenesis, or egg development, occurs. This process is essential for female reproductive health and begins during fetal development when primordial follicles are formed. Each primordial follicle comprises a primary oocyte in the center, surrounded by a single layer of squamous pre-granulosa cells. These follicles remain dormant in late prophase I of meiosis until triggered by hormonal changes at puberty.

At puberty, folliculogenesis accelerates due to the action of follicle-stimulating hormone (FSH). Primordial follicles transform into primary follicles, characterized by the cuboidal shape of pregranulosa cells and the enlargement of the oocyte. During this stage, the oocyte secretes a glycoprotein layer known as the zona pellucida, which acts as a protective barrier. The primary follicles then develop into secondary follicles, marked by the proliferation of pre-granulosa cells into a multi-layered structure and forming a thickened outer wall called theca folliculi. These changes are driven by increased estrogen and FSH levels.

As development progresses, secondary follicles evolve into tertiary or vesicular follicles, distinguished by an antrum, a fluid-filled cavity formed by accumulating follicular fluid between granulosa cells. The follicular fluid also contains hormones necessary for continued follicular development. Typically, one follicle becomes dominant—the mature Graafian follicle—which carries the secondary oocyte arrested at metaphase II. This mature follicle is poised for ovulation, triggered by a surge in luteinizing hormone (LH). Upon ovulation, the oocyte is released and ready for fertilization.