follicular antrum

The follicular antrum begins to form during the transition from secondary to tertiary follicle, typically in the late preantral stage. Accumulating follicular fluid secreted by granulosa cells coalesces into a single fluid-filled cavity. This process, called antrum formation or cavitation, marks a major turning point in folliculogenesis and is estrogen-dependent.

The antrum divides the granulosa cells into two functional populations: the mural granulosa cells lining the follicle wall, and the cumulus granulosa cells surrounding the oocyte. This spatial organization determines how hormonal signals are processed and how the oocyte communicates with supporting cells through gap junctions.

As the follicle matures from early antral to the preovulatory (Graafian) stage, the antrum expands dramatically — from microscopic to over 15 mm. This expansion is driven by continued fluid secretion and is reflected in ultrasound measurements used to monitor follicle development during fertility treatments.