Ovarian follicle. Coloured scanning electron micrograph (SEM) of a fracture of a secondary follicle in the ovary. This is the follicular cavity, which holds the developing egg (not seen). At centre is the glycoprotein membrane, the zona pellucida (brown), that surrounds the oocyte. This membrane binds sperm during fertilisation. At birth there are tens of thousands of primordial follicles, the precursors to primary follicles, present in the ovaries. Once sexual maturity is reached, up to 20 primordial follicles are triggered to start developing into primary follicles every menstrual cycle. Only one will reach full maturity and release its egg (ovum) at ovulation.