The pigmentary system of developing axolotls. II. An analysis of the melanoid phenotype

The melanoid mutant in the Mexican axolotl (Ambystoma mexicanum) is analysed with respect to the differentiation of pigment cells. Pigment cells were observed with the transmission electron microscope in order to determine any unusual structural characteristics and to determine what happens to each of the cell types as development proceeds. Chemical analysis of pteridine pigments was also carried out, and changes in pteridine biosynthesis were found to correlate well with changes in xanthophore morphology and number. In melanoid axolotls, as development proceeds, melanophore numbers increase, xanthophores decrease, and iridophores fail to differentiate at all. This is considered to result from: (a) conversion of xanthophores (that are present in young larvae) to melanophores; (b) the gradual programming of the majority of chromatoblasts to become, exclusively, melanophores, and (c) the failure of some chromatoblasts (possibly iridoblasts) to differentiate altogether. The ultrastructural and chemical evidence presented in this study is compared to similar data for wild-type axolotls, and a mechanism regarding how the melanoid gene might act is suggested.