2 Molecular Basis of Mammalian Egg Activation

The chapter discusses signal transduction mechanisms in the activation of mammalian eggs, primarily in the mouse, and explains the sequence of events that accompanies fertilization in the mouse. This information provides a framework within which to examine known and proposed signaling mechanisms that regulate egg activation, helps understand the role of several possible second messengers in egg activation, and helps describe the sperm–egg interaction that may result in the production of second messengers. In the mammal, the first interaction between the sperm and egg is at the level of the zona pellucida (ZP), an extracellular coat that surrounds all mammalian eggs. In the mouse, the ZP is composed of three glycoproteins: ZP1, ZP2, and ZP3. Although acrosome-intact sperm establishes primary binding with ZP3 through an O-linked carbohydrate moiety, the identity of this moiety is controversial. Following sperm binding, ZP3 induces the acrosome reaction of the bound sperm.