folliculogenesis

Folliculogenesis is the process by which ovarian follicles mature from dormant primordial follicles into preovulatory follicles capable of ovulation. It occurs in cycles and is regulated by endocrine signals from the pituitary and ovaries as well as intra-ovarian factors that coordinate growth, differentiation, and steroid production.

The developmental sequence starts with primordial follicles, each containing an oocyte arrested in prophase I. Activation recruits a subset of these follicles to grow, forming primary follicles with a single layer of cuboidal granulosa cells. As growth continues, follicles become secondary (preantral), with multiple granulosa cell layers and the beginnings of the theca layer. The transition to the antral stage involves the formation of fluid-filled spaces that coalesce into a single antrum, yielding early and then late antral follicles. A dominant follicle is selected during the follicular phase, enlarges to mature preovulatory (Graafian) status, and eventually responds to the luteinizing hormone surge to ovulate. The remaining structures undergo luteinization or atresia.

Regulation of folliculogenesis involves systemic hormones and local ovarian factors. Follicle-stimulating hormone (FSH) supports growth and granulosa cell proliferation, while luteinizing hormone (LH) influences theca cell steroidogenesis and ovulation. Intra-ovarian factors include anti-Müllerian hormone (AMH), which modulates initial recruitment; oocyte-derived factors such as growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15) that regulate granulosa cell function; and other signaling peptides like activins and inhibins. AMH levels reflect pool size and can influence recruitment rates.

Dysregulation of folliculogenesis is linked to fertility disorders such as anovulation and polycystic ovary syndrome. Understanding its mechanisms aids in managing reproductive health and informing assisted reproductive technologies.