STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Ovarian follicle. Coloured transmission electron micrograph (TEM) of a section through a primary follicle in an ovary. A central primary oocyte (developing egg, green) can be seen containing a large nucleus (blue), within which is a smaller nucleolus (circular). The oocyte is surrounded by follicular cells (purple). The whole follicle is surrounded by ovarian stromal cells (brown). 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: x1200 when printed 10 centimetres wide.
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