Sperm Guidance in Mammals: I. Chemotaxis
Following the findings that human spermatozoa accumulate in follicular fluid (Ralt et al., 1991; Villanueva-Díaz et al., 1990) and that there is a remarkable correlation between this in vitro accumulation and oocyte fertilization (Ralt et al., 1991), chemotaxis was substantiated as the cause of this accumulation (Ralt et al., 1994). Sperm chemotaxis was later also demonstrated in mice (Giojalas and Rovasio, 1998; Oliveira et al., 1999) and rabbits (Fabro et al., 2002). In addition, sperm accumulation in follicular fluid (but without substantiating that it truly reflects chemotaxis) was demonstrated in horses (Navarro et al., 1998) and pigs (Serrano et al., 2001). A key feature of sperm chemotaxis in humans is that this process is restricted to capacitated cells (Cohen-Dayag et al., 1995; Eisenbach, 1999) — the only cells that possess the ability to penetrate the oocyte and fertilize it (Jaiswal and Eisenbach, 2002). This raised the possibility that, in mammals, chemotaxis is not solely a guidance mechanism but it is also a mechanism of sperm selection (Cohen-Dayag et al., 1995; Eisenbach, 1999). Importantly, the fraction of capacitated (and, hence, chemotactically responsive) spermatozoa is low (~10% in humans), the life span of the capacitated/chemotactic state is short (1–4 hours in humans), a spermatozoon can be at this state only once in its lifetime, and sperm individuals become capacitated/chemotactic at different time points, resulting in continuous replacement of capacitated/chemotactic cells within the sperm population, i.e., prolonged availability of capacitated cells (Cohen-Dayag et al., 1994; 1995). These sperm features raised the possibility that prolonging the time period, during which capacitated spermatozoa can be found in the female genital tract, is a mechanism, evolved in humans, to compensate for the lack of coordination between insemination and ovulation (Cohen-Dayag et al., 1995; Eisenbach, 1999; Eisenbach and Giojalas, 2006; Giojalas et al., 2004).
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