Countless sperm may surround an egg at one time, yet only one must be allowed to fertilize it. In fact, an egg must undergo a rapid and complex series of changes to prevent entry by more than one sperm, called polyspermy, because it is detrimental to normal development. A study published in the December issue of Developmental Cell provides exciting information about the discovery of an enzyme that enables an egg to physically block additional sperm from entering after fertilization.Preventing polyspermy in sea urchins and many other animals involves a dramatic transformation including a respiratory burst characterized by massive production of hydrogen peroxide near the surface of the egg. Although this phenomenon was initially described over 100 years ago, the mechanism responsible for peroxide generation after fertilization is not well understood. The respiratory burst is not limited to eggs; other cells of the body can also generate peroxide. White blood cells produce it in response to an infection, but its harmful effects often kill the cell producing it and, as a result, these cells must be constantly replaced.Dr. Gary M. Wessel and colleagues from Brown University discovered an enzyme in sea urchins that is separate from those used by white blood cells and is specifically involved in generating hydrogen peroxide at fertilization. The enzyme, named urchin dual oxidase 1 (Udx1), works with other cellular components to produce peroxide and keep it away from the fragile embryo. “Peroxide is what we put on skinned knees to kill germs and on hair to get that summer sun-streaked appearance. It is not something we would expect an egg to be dealing with,” says Dr. Wessel. Yet “deal with it” it does. Without hydrogen peroxide, the egg cannot transform its exterior into a barrier that withstands the onslaught of subsequent sperm or mechanical damage.
