THIS is a rare glimpse of how we all started out ā the union of human sperm and egg. Lennart Nilsson, the Swedish photographer who captured this image, titled it The Successful Sperm. In reality the sperm has a few more hurdles to clear before it fertilises the egg, and it could still fail.
The sperm has successfully torn a hole in the eggās outer coating, or zona pellucida, and is working its way through to the eggās plasma membrane. It must also bind to this and then fuse with the egg. Only then has fertilisation truly taken place.
The binding of the sperm to the eggās zona is, however, a crucial first step in fertilisation, and of interest because it is here that the egg recognises the sperm as being from the same species. Though different species can cross-fertilise ā think mules ā it is rare. Proteins on the surface of a āforeignā sperm are not recognised by receptors on the zona surface, so the sperm canāt get a grip on it.
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The binding of the sperm to the zona triggers a leak of zona-digesting enzymes from a compartment in the front of the spermās head in a process called the acrosome reaction. The enzymesā digestive power, combined with the thrust provided by the spermās thrashing tail, drive it through the zona.
The precise sperm binding mechanisms are of much interest to researchers trying to develop new contraceptives. The hunt is under way to identify the proteins and receptors involved, with a view to blocking them and ensuring that the sperm are always unsuccessful.
Nilssonās āsuccessful spermā was captured using a powerful electron microscope. This and other photographs by him tracing the origins of human life appear in his book Life, published by Jonathan Cape in the UK this week and in the US in April (Ā£35, ISBN 0224076914).
