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Primordial follicles form during fetal development and consist of a primary oocyte surrounded by a single layer of squamous pre-granulosa cells. These follicles remain dormant in late prophase I of meiosis until puberty, when hormonal changes trigger their maturation into primary follicles.

At puberty, follicle-stimulating hormone (FSH) prompts primordial follicles to mature into primary follicles. The squamous pre-granulosa cells become cuboidal, the oocyte enlarges, and the oocyte secretes the zona pellucida, a protective glycoprotein layer. This transformation is essential for early follicular development and progression.

Secondary follicles are characterized by proliferation of pre-granulosa cells into multiple layers of granulosa cells and formation of a thickened outer wall called theca folliculi. These changes are driven by increased estrogen and FSH levels, preparing the follicle for further maturation and development.

The antrum is a fluid-filled cavity that forms in tertiary or vesicular follicles as follicular fluid accumulates between granulosa cells. This fluid contains hormones necessary for continued follicular development and supports the growth of the follicle during the hormonal control of the ovarian cycle.

A Graafian follicle is the mature, dominant follicle that develops from tertiary follicles and is poised for ovulation. It contains a secondary oocyte arrested at metaphase II, ready for release from the ovary when triggered by a surge in luteinizing hormone.

The zona pellucida is a protective glycoprotein layer secreted by the oocyte during primary follicle formation. This transparent barrier surrounds the oocyte and remains intact throughout follicular development, protecting the oocyte and later playing a role in fertilization and early embryonic development.

Folliculogenesis is essential because it produces mature follicles containing secondary oocytes ready for ovulation and fertilization. This process, beginning in fetal development and accelerating at puberty, ensures a continuous supply of viable eggs throughout a woman's reproductive years for potential conception.