Scientists have discovered how an egg is able to be fertilised by a single sperm only, thereby protecting itself against abnormal development. At the point of fertilisation, the egg may be surrounded by 'countless' sperm, but only one must be able to break through the egg's outer membrane. Polyspermy - the entry into the egg by more than one sperm - is detrimental to normal development after fertilisation, so the outer surface of the egg 'hardens' after fertilisation in order to protect itself.
When penetrated by a single sperm, the egg must undergo a number of chemical changes in order to protect itself against the entry of further sperm. By studying fertilisation in sea urchins, scientists from Brown University, Providence, Rhode Island, US, found that the egg undergoes a 'respiratory burst', which produces a large quantity of hydrogen peroxide near its surface at the point the first sperm penetrates it, and its outer membrane hardens. This fact has been known for some time, but the mechanism that causes it - or why - has not been fully understood, particularly as it is known that hydrogen peroxide can have harmful effects on cells that come into contact with it.
Dr Garry Wessel and colleagues, who have published their findings in the December issue of the journal Developmental Cell, have identified an enzyme in sea urchins that is specifically involved in generating hydrogen peroxide at fertilisation. The enzyme, called urchin dual oxidase 1 (Udx1), works with other cellular components to produce peroxide, but has a dual function - it also acts to keep the peroxide away from the fragile embryo. The enzyme enables the just-fertilised egg to physically block additional sperm from entering.
'Peroxide is what we put on skinned knees to kill germs and on hair to get that summer sun-streaked appearance. It is not something we would expect an egg to be dealing with', said Wessel. But, say the researchers, without the hydrogen peroxide, the egg cannot create its own natural barrier against the 'countless' sperm that may be trying to enter it. Peroxide activity has been identified in the eggs of many other species during fertilisation, they say, and the findings from this study add to the evidence that it may play a widespread functional role during fertilisation and even during early development. 'It is exciting to finally solve this century-old riddle', said Julian Wong, lead author of the study, adding: 'Now we want to take this result from sea urchins, who produce eggs that are easy to obtain and to study, and apply it more broadly'.
Sources and References
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Scientists discover key egg enzyme for blocking sperm entry after fertilization
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The Oxidative Burst at Fertilization Is Dependent upon Activation of the Dual Oxidase Udx1
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Toxic Sperm Blocker
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