The spatio-temporal framework of melanogenic induction in pigmented retinal cells of Xenopus laevis.

Using methods involving transplantation of neuroectoderm and eye rudiments between ap/ap mutants and +/+ embryos of Xenopus laevis, new aspects of melanogenic induction have been ascertained. The possibility of separate induction of a melanocyte cell type and of the product of terminal differentiation in it-melanosomes, has been demonstrated. Melanogenic induction starts at the late gastrula stage. It has an irreversible character which has been shown using the technique of double transplantation. Melanogenic factor (MgF) is produced only by the region of endomesoderm which is adjacent to eye rudiments in the course of normal development. Competence for melanogenic induction lasts till stage NF 30. Melanogenic induction appears to be species-specific; at least after transplantation of eye vesicles from ap/ap Xenopus laevis to Rana temporaria the melanoprotein synthesis cannot be switched on. L-Tyrosine, under the conditions of the experiments, is not a factor in the induction of melanogenesis.