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Absence of species specificity for mammalian sperm capacitation in vivo

Summary. Spermatozoa and eggs of one species were transferred into the oviducts of other species to examine the specificity of mammalian sperm capacitation in vivo. Using mice, rats, hamsters and rabbits, 11 different combinations of gametes and host were tested, with cumulus-intact and cumulus-free eggs being placed in opposite oviducts of a recipient. In all combinations compatible with the maintenance of sperm motility over a period sufficient for capacitation, fusion occurred between homologous gametes without regard to the identity of the host environment. The period required for capacitation, as judged by the timing of egg penetration, was a function of the specific character of the spermatozoon more than of the 'oestrous' oviduct. Fertilization level was not enhanced significantly by the presence of the cumulus oophorus, with the single striking exception of mouse eggs in the hamster. Sperm viability was not always optimal in a foreign tract; mouse and hamster spermatozoa became immotile within 2 h in the rabbit. Despite this limitation on sperm viability, some mouse eggs were fertilized in the rabbit, indicating that capacitation is possible even when the environment is suboptimal for motility maintenance. Therefore, while the results indicate that there is no specificity for capacitation as such, it should not be inferred that there is a total absence of specificity in the relationship between the female reproductive tract and gametes.