Light micrograph of a section through an ovary showing a primary follicle (centre). In the follicle is a developing oocyte (bright) surrounded by ovarian follicular epithelial 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 and release its egg (ovum) at ovulation. Magnification: x100 when printed at 15cm wide.