Ovarian follicle. Coloured light micrograph (LM) of a section through a secondary follicle in an ovary. At centre is the large nucleus of the developing egg (oocyte, pink). Surrounding the egg are cuboidal follicular cells (purple). 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: x100 when printed 10cm wide.