IDENTIFICATION AND CHARACTERIZATION OF SPERM STIMULATING FACTORS FROM FOLLICULAR FLUID

Abstract

Chemotaxis or movement towards the chemical attractant is important for diverse processes such as bacterial attraction to nutrients, morphogenesis in Dictyostelium discodium, neutrophil migration towards bacterial infections and neuronal growth cone projection. Chemotaxis of sperm towards an egg is yet another phenomenon which is vital during fertilization. During sperm chemotaxis, a chemoattractant source re-orients sperm motility towards the source, and hence towards the egg. This type of sperm behaviour is reported in various organisms, mostly marine invertebrates (such as sponges and sea urchins) and in some vertebrates (such as ascidians, amphibians, and possibly mammals). Inspite of the reports indicating occurrence of sperm chemotaxis in mammals (Villanueva-Diaz et ah, 1990; Rait et ah, 1991) there are some basic differences between sperm chemotaxis of mammals and that of marine invertebrates, the most pronounced being the fractional chemotactic response in the mammals. Unlike spermatozoa of marine species, spermatozoa of mammals must undergo a process of maturation, termed as capacitation, for acquiring the ability to bind to the egg and penetrate it (Yanagimachi, 1994). At any given time, the percentage of capacitated spermatozoa is small due to both limited time span of the capacitated stage and the continuous replacement of the capacitated cells in the sperm population (Cohen-Dayag et ah, 1994). Chemotaxis as a form of sperm guidance in case of mammals was not easily accepted initially because of the inconsistent interpretations drawn from the initial studies, one of them being the failure to distinguish between chemotaxis and other causes of sperm accumulation (such as chemokinesis and trapping). This was largely on account of lack of fool proof assay methods available to study sperm chemotaxis and inconsistence results obtained during the studies (Eisenbach, 1999). Besides, in highermammals, chemotaxis was believed to be unnecessary because fertilization is Chapter 1 internal and very large numbers of spermatozoa are ejaculated directly into the female reproductive tract offemale (4-40 x 107 in humans) (WHO, 1992). This implied that sufficient number of spermatozoa would reach the egg coincidentally. In contrast to this view, it was found that the spermatozoa that actually reach the oviduct is extremely low, about six orders of magnitude less than the number of spermatozoa deposited in the female genital tract (Harper, 1982; Barrat and Cooke, 1991; Williams et ah, 1993) reducing the possibility of coincidental successful collision between a spermatozoa and the egg. It is therefore likely that additional mechanisms are involved in directing the spermatozoa to the egg. Recent studies have revealed that chemoattractant signal triggering sperm chemotaxis are synthesized by the egg or, in some cases, by the somatic cells associated with the egg and are believed to exert influence over a short distance to enhance the efficiency of sperm-egg contact in higher animals (Oliveira et ah, 1999). The events of sperm chemotaxis are of interest for two main reasons. As this marks the first non-contact interaction of sperm with egg derived factors, the response can be considered to be the initial step towards fertilization. This process also enables the recruitment ofselected population ofsperm to fertilize the oocyte. Thus, a study on sperm chemotaxis could improve our understanding of the processes of natural fertilization. By identifying natural molecules that are involved it would be possible to get some insight into the signalling pathways of sperm chemotaxis. These molecules could also be exploited for sperm selection in assisted reproduction programs.