STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Ovarian follicle. Coloured transmission electron micrograph (TEM) of a secondary follicle in an ovary. The developing egg (oocyte, pink) is surrounded by granulosa cells (blue and green). These cells secrete the hormone oestrogen, which forms a cavity (antrum, white, upper right) within the follicle. The cells also secrete a glycoprotein that forms a membrane (zona pellucida, orange) around the oocyte. This membrane binds sperm during fertilisation. Once sexual maturity is reached, up to 20 oocytes start developing into eggs (ovums) every menstrual cycle. Only one will reach full maturity and release its egg at ovulation. Magnification: x1000 when printed 10 centimetres tall.
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