Eggs is Eggs

Scientists in Sweden have cracked the structure of the coat that surrounds mammalian eggs.

Known as the zona pellucida the mammalian "egg shell" has long been a scientific enigma.  Sperm seemed to know how to recognise the egg's surface and how to dock with it, but at the same time as soon as a single sperm penetrated the egg no others were allowed in.  This crucial mechanism ensures that a fertilised egg contains the right amount of genetic material, but how the system worked was not known.

Now Luca Jovine, together with his colleagues from the Karolinska Institute in Sweden, have provided some new insights by uncovering the three-dimensional structure of a major part of the zona pellucida that docks with sperm, called ZP3.  Writing in this week's Nature the team describe how they used a chemical trick to prepare crystals of the protein and then measured the patterns produced when beams of x-rays were shone through the crystals, enabling them to unpick the protein structure.  The researchers then used the structure to build a model of a related molecule called ZP2 which, together with ZP3, forms a molecular docking station for sperm on the egg's surface.

When the model was assembled it revealed an insight into how the egg prevents more than one sperm from fertilising it.  The docking of a sperm with the receptor causes the proteins to change shape.  This produces a molecular domino effect that instantly triggers the formation of a compact layer of protein filaments at the base of the zona pellucida which forms a barrier to further sperm entry and also masks the sperm receptor so other sperms cannot dock.

This discovery will almost certainly shed fresh light on a number of human infertility syndromes and also may reveal new avenues for the production of non-hormonal contraceptives which can exploit this system to temporarily block sperm entry.