Sperm chemotaxis

Sperm chemotaxis is a form of sperm guidance, in which sperm cells (spermatozoa) follow a concentration gradient of a chemoattractant secreted from the oocyte and thereby reach the oocyte.

Since the discovery of sperm attraction to the female gametes in ferns over a century ago (Pfeffer, 1884), 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, any 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) [Eisenbach and Tur-Kaspa (1999) for a review], and when more recent studies showed that mammalian spermatozoa do respond chemotactially (Ralt et al., 1994).

In sperm chemotaxis, the oocyte secretes a chemoattractant, which, as it diffuses away, forms a concentration gradient: a high concentration close to the egg, and a gradually lower concentration as the distance from the oocyte is longer. Spermatozoa can sense this chemoattractant and orient their swimming direction up the concentration gradient towards the oocyte. Sperm chemotaxis was demonstrated in a large number of non-mammalian species, from marine invertebrates (Cosson, 1990; Miller, 1985) to frogs (Al-Anzi and Chandler, 1998).

The sperm chemoattractants in non-mammalian species vary to a large extent. Some examples are shown in Table 1. So far, most sperm chemoattractants that have been identified in non-mammalian species are peptides or low-molecular-weight proteins (1-20 kDa), which are heat stable and sensitive to proteases (Cosson, 1990; Miller, 1985). Exceptions to this rule are the sperm chemoattractants of corals, ascidians, plants such as ferns, and algae (Table 1).

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