Illustration showing fertilisation of a human egg (ovum). The egg is surrounded by numerous spermatozoa (pink). In fertilisation, only a single sperm may successfully penetrate the egg cell to fuse with the female nucleus. Sperm must pass through the layers of follicular cells which constitute the corona radiata (blue), and an underlying glycoprotein membrane, the zona pellucida (green). The membrane is digested by enzymes released from the acrosome, a cap on the head of the sperm: subsequent rapid chemical changes in the zona pellucida prevent competing sperm from entering.
September 27th, 2018
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