Kit and its ligand stem cell factor (SCF) play a fundamental role in hematopoiesis, melanogenesis and gametogenesis. Homozygous W v mutant mice with a mutation in kit show abnormalities in these cell lineages. Fas is a member of the death receptor family inducing apoptosis. In this study, we generated double-mutant mice (W v /W v :Fas À/À ) and analyzed histologically their reproductive organs. In testes and ovaries of the double-mutant mice, testicular germ cells and oocytes were detected,

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... ectively, whereas the same-aged W v /W v mice contained neither cells. In addition, inhibition of Kit signals by administration of anti-Kit mAb, which induces degeneration of testicular germ cells in vivo in wild-type mice, did not cause degeneration in Fas-deficient mice. In testicular germ cells of W v /W v mutant mice, an increase of Fas expression was observed in spermatogonia. Further, in vitro treatment with SCF was shown to downregulate Fas on fibroblasts expressing exogenous Kit through activation of PI3-kinase/Akt. All the results clearly indicate that Fasmediated apoptosis is involved in germ cell degeneration accompanied by defects in Kit-mediated signals, and Kit signaling negatively regulates Fas-mediated apoptosis in vivo.