The Spark of Life: Electricity in the Human Body by Ashcroft Frances

Author:Ashcroft, Frances [Ashcroft, Frances]
Language: eng
Format: epub, mobi
Publisher: Norton
Published: 2012-09-16T21:00:00+00:00

Raising the Barriers

It is vital that only a single sperm fertilizes an egg because if multiple sperm do so the resulting cell fails to develop normally. Thus the egg has developed defences to ensure that only the first sperm to arrive is welcomed and that all subsequent hopefuls are excluded. How this block to polyspermy is produced was first studied in sea urchin eggs, which are easier to work with, as they are very large and can even be seen with the naked eye. Back in 1976, while still a young student, Rindy Jaffe discovered that as soon as the first sperm penetrates a sea urchin egg, the potential across the egg membrane rapidly flicks from being negatively charged on the inside to being positive. This voltage difference prevents further sperm from entering.

The surprise came when scientists looked at mammalian eggs and discovered that the mechanism was different. Here, the block to polyspermy is not an electrical but a physical one – a mechanical barrier that the sperm cannot penetrate, which develops only slowly after fertilization. The difference in strategy reflects the very different environments in which fertilization takes place. In the ocean, many millions of sperm arrive almost simultaneously at the egg so an electrical block to polyspermy is ideal as it is very fast. In mammals, the long and difficult journey up the female tract ensures that only a few sperm make it to the egg and that they rarely do so simultaneously. Hence a slower block is adequate.

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