Injection of a Sperm Extract Triggers Egg Activation in the Newt Cynops pyrrhogaster

Abstract

Unfertilized eggs of the newt Cynops pyrrhogaster are arrested at the second meiotic metaphase. The primary signal for egg activation is a transient increase in [Ca2+]i, which is triggered by the fertilizing sperm and propagates over the egg cortex as a Ca2+ wave. We injected an extract of Cynops sperm (SE) into unfertilized eggs and induced a wave-like [Ca2+]i increase which resulted in activation and resumption of meiosis. The SE-injected eggs showed degradation of cyclin B1 and DNA replication. When SE was boiled or treated with proteinase K before injection, it was unable to cause egg activation. Preinjection of Ca2+-chelator BAPTA before SE injection inhibited egg activation. These results indicate that a heat-labile and proteinaceous factor in the sperm cytoplasm induces a transient increase in [Ca2+]i which is required for egg activation. Injection of IP3 into unfertilized eggs caused an increase in [Ca2+]i and egg activation, but injection of cADP-ribose did not. These results support the hypothesis that Ca2+ release at fertilization occurs via IP3 receptors.