The chances of an individual human sperm fertilising an egg are miniscule. A man may produce up to half a billion – 500,000,000 – sperm every time he ejaculates; for every one that fertilises an egg, there are dozens of “also-rans” that reach it only just too late. Scientists have been puzzling for decades over the molecular changes that instantly erect a “keep out” sign when the first sperm penetrates the egg, preventing genetic chaos. But now, thanks to a technique called X-ray crystallography, pieces of this jigsaw puzzle are beginning to fall into place.
Sperm swimming for the egg.
Mammalian eggs are surrounded by a coat called the zona pellicula that has two jobs: to bind the first sperm, but then prevent others from binding. It contains filaments formed from many copies of each of two proteins, ZP2 and ZP3. What we know now is the exact shape of – the positions of the atoms in – just one part of the ZP3 protein. The scientists who solved the structure were able to use this to predict that ZP2 contains parts with similar shapes. We already know that sperm binding causes the egg to release enzymes called proteases which cut up ZP2, and we can now guess how this might trigger a shape change allowing the zona pellicula to wrap itself tighter round the egg and turn itself into a barrier for sperm.
This development should interest those women who find the Pill difficult to take, as a molecule that binds to ZP2 or ZP3 preventing it from working normally might make a different, effective chemical contraceptive.
Find out more about this development in episode 12 of Breaking Science.