Graafian follicle A fluid-filled, spherical vesicle in the ovary of a mammal, containing an oocyte attached to its wall, and responsible for oestrogen production in the ovary. A cavity first appears amongst follicle cells surrounding the oocyte, around the end of the period of cytoplasmic growth of the oocyte. In mammals with an oestrus cycle, it enlarges during the early part of the cycle, so that follicle cells separate into two layers, one around the oocyte and the other an external layer, until the follicle bursts on to the surface of the ovary, discharging the oocyte (ovulation). Most follicles, however, will not ovulate but will simply degenerate; the rest form the corpus luteum in most mammals. Follicle growth is under the influence of the pituitary gland.

Graafian follicle (ovarian follicle) The fluid-filled cavity that surrounds and protects the developing egg cell (oocyte) in the ovary of a mammal. The oocyte is surrounded by granulosa cells, which secrete fluid into the cavity of the follicle. After the release of the ovum the follicle develops into a corpus luteum. It is named after the Dutch anatomist Reinier de Graaf (1641–73).

Graafian follicle (grah-fi-ăn) n. a mature follicle in the ovary prior to ovulation, containing a large fluid-filled cavity that distends the surface of the ovary. The oocyte develops inside the follicle, attached to one side. [ R. de Graaf (1641–73), Dutch physician and anatomist]