Sperm thermotaxis

Sperm thermotaxis is a form of sperm guidance, in which sperm cells (spermatozoa) actively change their swimming direction according to a temperature gradient, swimming up the gradient. Thus far this process has been discovered in mammals only.

The discovery of mammalian sperm chemotaxis and the realization that it can guide spermatozoa for short distances only, estimated at the order of millimeters, triggered a search for potential long-range guidance mechanisms. The findings that, at least in rabbits and pigs, a temperature difference exists within the oviduct, and that this temperature difference is established at ovulation in rabbits due to a temperature drop in the oviduct near the junction with the uterus, creating a temperature gradient between the sperm storage site and the fertilization site in the oviduct (Figure 1), led to investigation whether mammalian spermatozoa can respond to a temperature gradient by thermotaxis.

Mammalian sperm thermotaxis was, hitherto, demonstrated in three species: humans, rabbits, and mice. This was done by two methods. One involved a Zigmond chamber, modified to make the temperature in each well separately controllable and measurable. A linear temperature gradient was established between the wells and the swimming of spermatozoa in this gradient was analyzed. A small fraction of the spermatozoa (at the order of ~10%), shown to be the capacitated cells, biased their swimming direction according to the gradient, moving towards the warmer temperature. The other method involved two- or three-compartment separation tube placed within a thermoseparation device that maintains a linear temperature gradient. Sperm accumulation at the warmer end of the separation tube was much higher than the accumulation at the same temperature but in the absence of a temperature gradient. This gradient-dependent sperm accumulation was observed over a wide temperature range (29-41 °C).

Since temperature affects almost every process, much attention has been devoted to the question of whether the measurements, mentioned just above, truly demonstrate thermotaxis or whether they reflect another temperature-dependent process. The most pronounced effect of temperature in liquid is convection, which raised the concern that the apparent thermotactic response could have been a reflection of a passive drift in the liquid current or a rheotactic response to the current (rather than to the temperature gradient per se). Another concern was that the temperature could have changed the local pH of the buffer solution in which the spermatozoa are suspended. This could generate a pH gradient along the temperature gradient, and the spermatozoa might have responded to the formed pH gradient by chemotaxis. However, careful experimental examinations of all these possibilities with proper controls demonstrated that the measured responses to temperature are true thermotactic responses and that they are not a reflection of any other temperature-sensitive process, including rheotaxis and chemotaxis.

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