Chemotaxis: role in egg-sperm interaction and its evaluation

Abstract

Communication between spermatozoa and egg before contact by chemotaxis is prevalent throughout the animal kingdom. In most animal species, the prospects of the spermatozoa coming in contact with eggs are very slim in the absence of some guidance mechanism usually chemical in nature that has been acquired gradually. Such chemical guidance, sperm chemotaxis, is now recognized in many marine invertebrates, fish, amphibians, and a few mammals including humans. This suggests that sperm chemotaxis is a general guidance mechanism, irrespective of whether the fertilization is external, like in most marine species, or whether it is internal, as in mammals. In spite of this generality, there are some basic differences between sperm chemotaxis of mammals and that of marine invertebrates, the most pronounced one being the fractional chemotactic response in the former; namely, the restriction of the chemotactic response to a subpopulation of the spermatozoa. Unlike spermatozoa of marine species, spermatozoa of mammals must undergo a process of maturation, termed capacitation, for acquiring the ability to bind to the egg and penetrate it. At any given time, the percentage of capacitated spermatozoa is small due to both the limited time for which spermatozoa remain in this stage and the continuous replacement of capacitated cells in the sperm population. Since the role of sperm chemotaxis is to bring to the egg spermatozoa that are capable of fertilizing it, therefore, unlike in marine invertebrates where most, if not all, the spermatozoa appear to be chemotactically responsive, in mammals only the small fraction of capacitated spermatozoa are chemotactic. Because of the massive chemotactic response in marine invertebrates and its much earlier discovery, most of our, limited, knowledge about the molecular mechanism of sperm chemotaxis is in these species. The present paper and attempts have been made to compile the various known aspects of chemotaxis in egg-seprm interaction of non-mammals and mammalians and assays methods for evaluation of chemo-attractants. This will be useful for the researcher, students and others who interested in understanding of chemotaxis and its role in egg-sperm interaction.