Structure of the Graafian Follicle

The Graafian follicle is the mature ovarian follicle, ready to release its egg at ovulation. Inside it, an eccentrically placed secondary oocyte is surrounded by a non-cellular layer called the zona pellucida, secreted by the vitelline membrane of the oocyte. The outermost covering of the follicle is a protective, fibrous layer called the theca externa; just inside it lies a cellular layer, the theca interna, which produces the hormone estrogen. Inner to the theca interna, the follicular cells form a layer called the membrana granulosa; cells of the membrana granulosa differentiate further into the discus proligerus and the corona radiata, the layer of cells immediately surrounding the oocyte. The oocyte together with its surrounding granulosa cells is together called the cumulus oophorus. Between th …