A human oocyte passes following stages of development: primordial follicle, primary follicle, secondary follicle, tertiary follicle and Graafian follicles. In the transition from secondary to tertiary follicle, the follicular antrum (see brown cavity) is formed by an accumulation of liquid (follicular liquor) in the follicle epithelium (stratum granulosum). The oocyte itself is surrounded by the corona radiata (see meat-colored cell layers), a follicular epithelium crown, and bulges into the follicular cavity (Cumulus oophorus, see picture). The growing oocyte is surrounded by stroma that is called theka folliculi, and differentiates into an inner, estrogen producing and an outer layer (theca folliculi interna et externa). In the secondary follicle, the glycol-protein-rich zona pellucida is purple, the stratum granulosum is pink and the basal lamina is ochre.
