The activation of egg development by the fertilizing spermatozoa is produced by a rapid rise in cytosolic free Ca²⁺ concentration in all plant and animal species examined thus far (Stricker, 1999). In mammals, sperm-egg fusion results in a remarkable series of cytosolic Ca²⁺ oscillations that have been shown to be essential for normal development of the early embryo (Stricker, 1999). This spectacular Ca²⁺ signalling event arises from transient elevations in inositol 1,4,5-trisphosphate (IP₃) levels that cause the activation of IP₃ receptor-mediated Ca²⁺ release from intracellular stores in the egg. However, the precise mechanism describing how the stimulation of IP₃ production occurs within the egg following fusion by sperm has remained undefined.

The ‘sperm factor’ hypothesis is a proposal for the signalling mechanism occurring at fertilization in which spermatozoa release a soluble Ca²⁺-releasing factor that enters the egg upon sperm-egg fusion and this ‘sperm factor’ somehow generates the observed cytosolic Ca²⁺ oscillations (Swann, 1990). This is in contrast with another proposal, the ‘egg receptor’ hypothesis (Runft et al. 2002), a mechanism that involves a specific, sperm-surface ligand interacting with a receptor on the egg plasma membrane to trigger activation of IP₃ generation via an egg cytosolic phosphoinositide-specific phospholipase C (PLC). An advantage of the ‘egg receptor’ hypothesis is that it recruits an established signalling mechanism, well-characterised in somatic cells, whereby hormone ligand interaction with surface receptors triggers a signalling cascade culminating in PLC activation and IP₃ elevation. Conversely, significant empirical evidence supports the ‘sperm factor’ proposal, as the microinjection of soluble sperm extracts (Swann, 1990), or of a single spermatozoa (Nakano et al. 1997), triggers Ca²⁺ oscillations in mammalian eggs indistinguishable from those at fertilization, and the cytoplasmic fusion of sperm and egg precedes the first Ca²⁺ rise (Lawrence et al. 1997). Analysis of mammalian sperm extracts has indicated a high level of PLC activity that coelutes with Ca²⁺ releasing ability, suggesting a sperm PLC may be a component of the ‘sperm factor’ (Jones et al. 1998), although it appears not to be a previously identified PLC isoform (Parrington et al. 2002).

We have employed a contemporary strategy whereby novel, PLC-related, short nucleotide sequences present in mammalian testis, were identified in the public domain database of expressed sequence tags (EST) using the heuristic Blast similarity search algorithm. An unique isoform of PLC, termed zeta (PLCζ), was thus identified and characterised from mouse (Saunders et al. 2002) and human and monkey (Cox et al. 2002) sperm. These studies have shown that (a) PLCζ triggers Ca²⁺ oscillations in mouse eggs indistinguishable from those at fertilization, (b) PLCζ removal from sperm extracts abolished Ca²⁺ release in eggs, (c) the PLCζ content of a single sperm was sufficient to produce Ca²⁺ oscillations as well as normal embryo development to blastocyst. Our results are consistent with sperm PLCζ as a component of the physiological trigger for development of a fertilized egg into an embryo.

This work was supported by a SIF grant to F.A.L. from the University of Wales College of Medicine. The studies were part of a collaboration with the laboratory of Dr Karl Swann, University College London.