Developing egg. Light micrograph of a developing egg (oocyte, right) within a follicle in an ovary. The oocyte is surrounded by granulosa cells (brown). These cells secrete the hormone oestrogen, which forms a cavity (antrum, yellow) within the follicle. The cells also secrete a glycoprotein that forms a membrane (zona pellucida, light brown) 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.
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