Ovarian follicle. Coloured scanning electron micrograph (SEM) of a secondary follicle in the ovary. The developing egg is at centre (orange) and is surrounded by cuboidal follicular cells (light brown). 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 fill maturity and release its egg (ovum) at ovulation.