On the search for the sperm oscillogen

Activation during fertilization in all animal oocytes is mediated by Ca²⁺ changes. In mammals, a series of Ca²⁺ oscillations is observed lasting a number of hours (Miyazaki et al., 1993; Swann and Ozil, 1994; Schultz and Kopf, 1995) which, when blocked, inhibits all the events of oocyte activation. There is current interest (and certainly much argument) as to the relative importance of the two main hypotheses which explain how the spermatozoa cause Ca²⁺ oscillations in the oocyte. The first is that spermatozoa act via plasma membrane receptors to generate the Ca²⁺ mobilizing messenger inositol 1,4,5-trisphosphate (InsP₃) (Foltz and Shilling, 1993; Shilling et al. 1994). The alternative hypothesis suggests that the spermatozoa introduce a soluble and presumably cytosolic factor that diffuses into oocytes after the sperm-oocyte membrane fusion (Dale et al., 1985; Stice and Robl, 1990; Swann, 1990; Stricker, 1997; Fissore et al., 1998). This is the so called 'sperm factor' hypothesis that we and others have supported for a number of years. Here we present a history of the sperm factor model, which may offer some insight into the current status of the hypothesis