Ovarian follicles. Coloured scanning electron micrograph (SEM) of secondary follicles in the ovary. The developing eggs are sectioned at centre (red) and are surrounded by cuboidal follicular cells (pink). 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 as a secondary follicle and release its egg (ovum) at ovulation. Magnification: x400 when printed 10 centimetres wide.