Sperm Guidance in Mammals: III. Chemotaxis and Thermotaxis Combined
The findings that mammalian spermatozoa are capable to respond to a chemoattractant gradient by chemotaxis and to a temperature gradient by thermotaxis and that both such gradients are established in the female genital tract raised the possibility that spermatozoa guidance is done by both chemotaxis and thermotaxis. It was further suggested that, in vivo, each mechanism is functional in a region of the oviduct where the other mechanism is ineffective (Bahat et al., 2003; Eisenbach and Giojalas, 2006). According to this suggestion (Figure 2), capacitated spermatozoa, released from the sperm storage site at the isthmus (Suarez, 2002), may be first guided by thermotaxis from the cooler sperm storage site towards the warmer fertilization site (Bahat et al., 2003). Passive contractions of the oviduct (Battalia and Yanagimachi, 1979) may assist the spermatozoa to reach there. At this location the spermatozoa may be chemotactically guided to the oocyte-cumulus complex by the gradient of progesterone, secreted from the cumulus cells. In addition, progesterone may inwardly guide spermatozoa, already present within the periphery of the cumulus oophorus (Teves et al., 2006). Spermatozoa that are already deep within the cumulus oophorus may sense the more potent chemoattractant that is secreted from the oocyte (Sun et al., 2005) and chemotactically guide themselves to the oocyte according to the gradient of this chemoattractant. It should be borne in mind, however, that this is only a model. In view of the increasing number of different chemoattractants that are being discovered, the guidance in vivo might be much more complex.
Figure 2. A simplified scheme describing the suggested sequence of sperm guidance events in mammals.
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