Sperm Guidance - Background

Background

Since the discovery of sperm attraction to the female gametes in ferns over a century ago (Pfeffer, 1884), sperm guidance in the form of sperm chemotaxis has been established in a large variety of species (Miller, 1985). Although sperm chemotaxis is prevalent throughout the Metazoa kingdom, from marine species with external fertilization such as sea urchins and corals, to humans (Cosson, 1990; Eisenbach and Tur-Kaspa, 1994; Miller, 1985), most of the current information on sperm chemotaxis is derived from studies of marine invertebrates, primarily sea urchin and starfish (Kaupp et al., 2006). As a matter of fact, until not too long ago, the dogma was that, in mammals, guidance of spermatozoa to the oocyte was unnecessary. This was due to the common belief that, following ejaculation into the female genital tract, large numbers of spermatozoa 'race' towards the oocyte and compete to fertilize it. This belief was taken apart when it became clear that only few of the ejaculated spermatozoa — in humans, only ~1 of every million spermatozoa — succeed in entering the oviducts (Fallopian tubes), and when more recent studies showed that mammalian spermatozoa employ two different guidance mechanisms: chemotaxis (Ralt et al., 1994) and thermotaxis (Bahat et al., 2003).