Sperm-induced calcium oscillations: Soluble factor, factors or receptors?

The two questions posed by Tesarik (1998) and Swann et al. (1998) on the nature of sperm factor are: (i) if glucosamine6-phosphate deaminase is not the ‘so-called oscillogen’, as shown by the recombinant studies of Wolosker et al. (1998), what is the nature of sperm factor?; and (ii) is there a dual mechanism for oocyte activation involving signals from soluble sperm factors together with signals generated by the interaction of the spermatozoon with receptors on the oocyte plasma membrane. The debate really started in Edwardian times when it was shown that cortical granule exocytosis and oocyte activation in sea urchins were triggered by a component of the spermatozoon that enters the oocyte cytoplasm (Robertson, 1912; Loeb, 1913). Contrasting models emphasized the interaction of the spermatozoon with the external surface of the oocyte membrane (Lillie, 1922; Mazia et al., 1975). In the early 1980s, direct experimentation re-kindled the controversy. Direct evidence for a soluble sperm factor was shown by microinjecting soluble components from spermatozoa into sea urchin and ascidian oocytes (Daleet al., 1985; Dale, 1988). Several activation events including cortical granule exocytosis and gating of plasma membrane currents were triggered. These initial studies were supported with experiments on mammals, where it was shown that activation events, including calcium oscillations, were triggered by soluble sperm extracts (Stice and Robl, 1990; Swann, 1990; Dale t al., 1996; Parringtonet al., 1996). Further credibility was gained with the development of intracytoplasmic sperm injection (ICSI) in the 1990s, since injection of a spermatozoon directly into the oocyte, leading to normal embryogenesis and live birth, seemed to preclude the membrane receptor theory. Since sperm factor was shown to induce a series of calcium oscillations in mammalian oocytes, the term ‘oscillogen’ was coined (Parringtonet al., 1996). Before entering into the debate we would like to suggest that this term is inadequate and does not represent this physiological trigger that spans the animal kingdom. We re-propose the use of ‘soluble sperm factor’ (Dale et al., 1985). There are several reasons for this. Firstly, the capacity for mammalian oocytes to generate oscillatory calcium transients does not depend on the spermatozoon but is an intrinsic oocyte characteristic acquired during oocyte maturation. In fact, sperm-induced activation in immature