The origin and development of retinal pigment cells and melanophores analyzed by Xenopus black-white chimeras

At the 16 cell stage, three kinds of borealis-laevis and eight kinds of laevis-laevis chimeric embryos were produced by replacing a particular blastomere of albino embryos of Xenopus laevis with that of wild-type embryos of X. borealis or X. laevis, and then leaving the embryos to develop into frogs. In the borealis-laevis chimera frogs, we found that all the melanized cells (retinal pigment cells and melanophores) were derived from a transplanted wild-type blastomere with a nuclear marker of X. borealis and that all the albino-mutant cells derived from the host did not become melanized. Thus, retinal pigment cells and melanophores differentiated according to their own genotype. We then examined the origin of these two types of cells, using melanin as a cell-marker in the borealis-laevis and laevis-laevis chimeras. Retinal pigment cells derive from A1 (dorso-animal) and A2 (latero-animal) blastomeres. A1 of one side contributes to retinal pigment cells in both eyes. Though the blastomeres of one side contribute to the formation of bilateral melanophores, the major contribution is to melanophores of the same side. A1, A2 and V2 (latero-vegetal) form the anterior part of the neural fold, and A2 and V2 contribute to melanophores of the head region. The most anterior part of the neural fold derived from A1 does not make a significant contribution to melanophores. Though V2 is a vegetal blastomere, it forms the anterior part of the neural fold by upward movement against the downward movement for gastrulation. A3 forms the middle and posterior parts of the neural fold and contributes to melanophores of the trunk and hindlimbs. Melanophores of hindlimbs also come from A2, A4 and V2. It is to be noted that A4 contributes to melanophores of hindlimbs, despite no apparent contribution to the neural fold. Development of the retinal pigment cells and melanophores is discussed from the point of pigmentation patterns of the chimeras.