EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
False-colour transmission electron micrograph (TEM) showing penetration of an ovum by a single human spermatozoon, prior to fertilisation. The whole of the sperm is located in a space between the oocyte and the upper zona pellucida and appears lying alongside the oocyte rather than if it were about to penetrate head-first. This is due to the location of immunological recognition & attachment sites on the side of the sperm's head. Following the successful penetration of the oocyte by a single sperm, rapid chemical changes in the membrane surrounding the oocyte cause it to thicken and prevent entry of competing sperm. Magnification: x1000 at 35mm size.
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