Development of ovarian follicle starts when oogonium gets covered by a single layer of flat follicular cells to form primordial follicle that proliferate to form multilaminar (maturing) primary follicle. Small fluid filled cavities appear between follicular cells that fuse together to form antrum. Antrum increases in size. Granulosa cells that surround oocyte are called cumulus oophoricus and those that attach oocyte to wall of the follicle are called discus proligerus. As follicle expands, theca interna and theca externa appears. Matured ovarian follicle is called Graafian follicle.