Pigment cell pattern formation in amphibian embryos: a reexamination of the dopa technique

Neural crest-derived melanophores form species-specific patterns in the dermis of amphibian embryos. Melanophore patterns may be generated by one of two general mechanisms: pigment cell precursors disperse throughout the embryo, with melanophores differentiating in certain regions due to environmental cues, or melanoblasts may localize in different regions as a result of a hierarchy of tissue affinities. Both of these mechanisms have been proposed to be responsible for the dorso-ventral patterning of melanophores in Xenopus laevis. We have reexamined the distribution of melanoblasts in X. laevis and Taricha torosa using the dopa (3,4-dihydroxyphenyl-alanine)-staining technique. We have found that many of the dopa-positive cells identified as melanoblasts by some researchers are actually not derived from the neural crest: dopa-positive cells in T. torosa were identified in the transmission electron microscope to be either leukocytes or erythrocytes, in X. laevis dopa-positive cells are found between the ectoderm and somites where neural crest cells are not found, and X. laevis embryos surgically depleted of neural crest have dopa-staining patterns identical to control embryos. Melanoblasts are apparently not found in the ventralmost regions of early T. torosa and X. laevis embryos, providing additional evidence for the role of differential tissue affinities in directing the formation of embryonic pigment cell patterns.