The pigmentary system of developing axolotls. III. An analysis of the albino phenotype.

The albino 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 analyses of pteridine pigments were also carried out, and the pattern of pteridines in albino animals was found to be more complex than, and quantitatively enhanced (at all developmental stages examined) over, the pattern observed in comparable wild-type axolotls. The golden colour of albino axolotls is due primarily to sepiapterin (a yellow pteridine) and secondarily to riboflavin (and other flavins). Coincident with enhanced levels of yellow pigments, xanthophore pigment organelles (pterinosomes) in albino skin reach a mature state earlier than they do in wild-type axolotl skin. This morphology is conserved throughout development in albino animals whereas it is gradually lost in the wild type. Unpigmented melanophores from albino axolotls are illustrated for the first time, and in larval albino axolotls the morphology of these cells is shown to be very similar to xanthophore morphology. In older animals xanthophores are easily distinguished from unpigmented melanophores. Iridophores seem to appear in albino skin at an earlier stage than they have been observed in wild-type skin. Morphologically, wild-type and albino iridophores are identical.