Ovarian follicle. Coloured transmission electron micrograph (TEM) of a primary follicle in an ovary. The developing egg (oocyte, ) has a large nucleus and is surrounded by cuboidal follicular cells. 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. Magnification: x600 when printed 10 centimetres wide.