Light micrograph of a section through an ovary. The outer cortex contains several follicles (round) at various stages of development and two corpus lutea (far right). In each follicle is a developing oocyte (bright) surrounded by ovarian follicular epithelial cells (pink). The bigger the follicle, the more mature the oocyte. 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. This follicle then develops into a corpus luteum, which secretes progesterone to build up the lining of the uterus, ready for a fertilised egg. Magnification: x16 when printed at 15cm wide.