tertiary ovarian follicle

A tertiary ovarian follicle, also called a Graafian follicle, is distinguished by the presence of the follicular antrum — a fluid-filled cavity that forms within the granulosa cell layers. This antrum is absent in primordial, primary, and secondary follicles, making antrum formation the defining milestone of the tertiary stage.

In each menstrual cycle, typically one tertiary follicle becomes dominant and continues to enlarge. At ovulation, the mature Graafian follicle ruptures to release the oocyte. The remaining follicular cells then transform into the corpus luteum, which secretes progesterone to support early pregnancy.

A mature tertiary (Graafian) follicle can reach 15–25 mm in diameter just before ovulation. Transvaginal ultrasound can visualize these follicles during assisted reproductive procedures, allowing clinicians to time ovulation trigger injections accurately.