Species difference in the expression of Fas and Fas ligand in mature mammalian spermatozoa

Although it has been proposed that the Fas and Fas ligand (FasL) may protect ejaculated spermatozoa against apoptosis induced by lipoperoxidative damage and against lymphocytes present in the female genital tract, studies reported conflicting results on the presence of Fas receptors in ejaculated human spermatozoa. Furthermore, the expression of Fas/FasL on mature spermatozoa has not been observed in several important mammals. Using seven species, we observed the possibility for species difference in Fas/FasL expression on mature spermatozoa by both immunofluorescence microscopy and western blot analysis. Whereas intensive signals of Fas immunolabelling were detected in sperm head and middle piece and weak signals observed in the tail in 86-100% of the mouse, rat, bull, ram, and buck spermatozoa, only weak signals were detected on the whole body of 27% boar spermatozoa and in the head of 21% human spermatozoa. The pattern of FasL localization was identical to that of Fas in spermatozoa from human, mouse, rat, ram, and buck, but boar and bull spermatozoa showed weak and intensive FasL signals, respectively, only in the head. Western blotting further confirmed the Fas and FasL expression in mouse, rat, bull, ram, and buck, but not in human and boar spermatozoa. Taken together, the results revealed a marked species difference in Fas/FasL expression and an extensive co-expression of Fas and FasL among mature mammalian spermatozoa, suggesting that whereas spermatozoa from most species may be protected by Fas/FasL, those from human and boar may not use the Fas system for protection.