NO is necessary and sufficient for egg activation at fertilization

The early steps that lead to the rise in calcium and egg activation at fert ilization are unknown but of great interest-particularly with the advent of in vitro fertilization techniques for treating male infertility and whole- animal cloning by nuclear transfer. This calcium rise is required for egg a ctivation and the subsequent events of development in eggs of all species(1 ,2). Injection of intact sperm or sperm extracts can activate eggs, suggest ing that sperm-derived factors may be involved. Here we show that nitric ox ide synthase is present at high concentration and active in sperm after act ivation by the acrosome reaction. An increase in nitrosation within eggs is evident seconds after insemination and precedes the calcium pulse of ferti lization. Microinjection of nitric oxide donors or recombinant nitric oxide synthase recapitulates events of egg activation, whereas prior injection o f oxyhaemoglobin, a physiological nitric oxide scavenger, prevents egg acti vation after fertilization. We conclude that nitric oxide synthase and nitr ic-oxide-related bioactivity satisfy the primary criteria of an egg activat or: they are present in an appropriate place, active at an appropriate time , and are necessary and sufficient for successful fertilization.