EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
False-colour transmission electron micrograph (TEM) of an early stage of fertilisation, showing a human spermatozoon beginning to penetrate the corona radiata, the layers of follicular cells that surround the oocyte (the female sex cell - not visible). The cap over the head of each sperm, called the acrosome (light purple) contains enzymes which aid penetration of the egg. The dart-shaped head (dark purple) contains the 23 chromosomes of the male sex cell. Following the successful penetration of the egg by a single sperm, rapid chemical changes in the membrane surrounding the oocyte prevent entry of competing sperm. Magnification: x1000 at 35mm size.
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