Melanin synthesis activation dependent on inductive influences.

Gene activity in melanin-synthesising cells of albino periodic (a) mutants ofXenopus laevis is expressed phenotypically in the framework of the following cycle: a period of complete albinism succeeds the short peak of pigmentation, and melanosomes which have formed disappear. Skin and choroid coat melanophores as well as pigmented epithelium melanocytes are involved in this cycle.Parabiosis experiments allowed hormonal regulation of the melanin-synthesising gene activity to be excluded. Neural fold transplantations have shown that there is no inhibitory action on melanophore differentiation from the side of the a/a recipient.Melanin synthesis in pigmented epithelium of a mutants can be activated to level comparable with that of wild-type animals, if eye vesicles of a/a embryos have been brought into contact with endomesodermal derivatives of +/+ embryos at the early tail bud stage. Contact of eye vesicles of +/+ embryos with the endomesoderm of mutants prevents normal melanogenesis in pigmented epithelium of transplanted eyes. Eye transplantations made after the early tail bud stage have shown that gene expression in pigmented epithelium is independent of any external influences.Data obtained here demonstrate a selective induction of a separate cell type (melanocytes) and the stage-specificity of this process. In the a mutant the abnormal melanin synthesis is apparently predetermined by deficiency in the inducer of melanogenesis. Inhibition of melanogenesis by endomesoderm seems to be less probable. Data are discussed in the light of current ideas on the play of gene activity.