EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
False-colour transmission electron micrograph (TEM) of an oocyte (a female sex cell) following its penetration by a single human spermatozoon during fertilisation. Numerous dark granules are visible within the cytoplasm, moving towards the cell's surface. These granules contain enzymes that are involved in the mechanism which causes the membrane surroundng the oocyte to thicken following penetration by a single sperm. The mobilisation of the enzyme granules with the subsequent thickening of the membrane takes place rapidly following penetration and forms a barrier against competing sperm. Magnification: x1000 at 35mm size.
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