Ovarian follicle

Folliculogenesis proceeds through recognisable stages. Primordial follicles (≈30–50 µm) — a primary oocyte arrested in prophase I surrounded by a single layer of flattened pregranulosa cells — form the resting reserve, laid down before birth. A few are activated each day. In primary follicles the granulosa cells become cuboidal and then multilayered, and the oocyte secretes the glycoprotein zona pellucida. In secondary (preantral) follicles, the granulosa is several layers thick and the surrounding stroma differentiates into a vascular, steroidogenic theca interna (LH receptors, androgen production) and a fibrous theca externa.

Fluid-filled spaces then coalesce into an antrum, forming the antral (tertiary) follicle. Antral follicles of ≈2–5 mm become FSH-dependent; each cycle a cohort is rescued from atresia by the intercycle rise in FSH. By days 5–7 one dominant follicle — the one with most FSH receptors and aromatase — is selected; its estradiol and inhibin B suppress FSH so the others undergo atresia (the fate of >99 % of follicles). The mature (Graafian, preovulatory) follicle reaches ≈18–25 mm; the oocyte sits on a hillock of granulosa cells, the cumulus oophorus, with an inner ring of cells, the corona radiata, attached to the zona pellucida.

Ovulation: the LH surge causes the oocyte to complete meiosis I, cumulus expansion, and prostaglandin- and protease-mediated breakdown of the follicle wall at the stigma; the secondary oocyte with its corona is released ≈24–36 h after surge onset. The remaining wall becomes the corpus luteum.