EDELMANN / SCIENCE PHOTO LIBRARY EDELMANN / SCIENCE PHOTO LIBRARY
Light micrograph of the encounter between sperm and ovum during in vivo fertilisation (IVF). A single sperm is seen in centre of image approaching the circular oocyte. Other competing sperm and cells comprising the corona radiata, which forms a protective halo around the central oocyte, are visible at bottom. Only one sperm will be successful in penetrating the oocyte; at the instance this occurs the oocyte membrane thickens to construct a barrier to competing sperm. In IVF, egg and sperm meet in a petri dish and subsequent events are monitored through a microscope to check for successful fertilisation.
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