WHAT'S THE POINT OF ENTRY?
Where a sperm enters an egg has a striking influence on the structure of the early embryo — and in turn on the finished body, researchers at the University of Cambridge report this week Nature (Vol. 409, No. 6819, 25 Jan 01, pp 517-521; News & Views). This is despite the fact that sperm can enter almost anywhere on an egg's surface.
A fertilized egg’s first division occurs along a plane that includes the sperm’s point of entry. At the second cell division, the cell that inherits this point usually divides before its sister. Both the plane of division and the order of division may be important in determining where these cells' descendants end up and what jobs they do.
The embryo goes on to form a ball of cells known as a blastocyst. This contains cells that form the fetus itself, and some that form tissues that nourish and protect it, but later die off.
Karolina Plotrowska and Magdalena Zernicka-Goetz found that the sperm’s point of entry lies along the axis that separates these two cell types. The patterning of the early embryo is also thought to influence the development of the axes of the finished body — left to right, top to bottom and front to back — although the relation between these and sperm entry point is as yet unclear.
Roger Pedersen, of the Reproductive Genetics Unit, University of California, San Francisco, puts this work into context in an accompanying News and Views article.
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