Many principles of eukaryotic DNA replication and its relationship to transcription have been revealed by studying the replication of animal virus chromosomes. Now microinjection of viral DNA into eggs and embryos is providing clues about regulation of chromosomal replication and transcription during early mammalian development.

During mammalian fertilization sperm bind to the egg's zona pellucida (ZP) after undergoing capacitation. Capacitated mouse sperm bind to mZP3 (one of three ZP glycoproteins), undergo the acrosome reaction, penetrate the ZP, and fuse with egg plasma membrane. Sperm protein 56 (sp56), a member of the C3/C4 superfamily of binding proteins, was identified nearly 20 years ago as a binding partner for mZP3 by photoaffinity cross‐linking of acrosome‐intact sperm. However, subsequent research revealed that sp56 is a component of the sperm's (...) acrosomal matrix and, for sperm with an intact acrosome, should be unavailable for binding to mZP3. Recently, this dilemma was resolved when it was recognized that some acrosomal matrix (AM) proteins, including sp56, are released to the sperm surface during capacitation. This may explain why uncapacitated mammalian sperm are unable to bind to the unfertilized egg ZP. (shrink)

The zona pellucida is an extracellular coat that surrounds all mammalian eggs. It is a porous matrix of interconnected filaments that are assembled from glycoproteins synthesized and secreted by growing oocytes. The zona pellucida is responsible both for species‐specific binding of sperm to unfertilized eggs and inducing bound sperm to undergo the acrosome reaction. The latter enables sperm to penetrate the extracellular coat and fertilize the egg. The zona pellucida also aids in prevention of polyspermy following fertilization and in protection (...) of preimplantation embryos. In mice, several of these important functions can now be ascribed to specific zona pellucida glycoproteins that have been purified and characterized. Furthermore, the enzyme responsible for hatching of embryos from the zona pellucida, just prior to implantation, has been identified and characterized. (shrink)