The barred pigment pattern (Lehman 1957) of the axolotl larva is best observed from stage 41 onwards, where it already consists of alternating transverse bands of melanophores and xanthophores along the dorsal side of the trunk. The present study investigateswhen the two populations of neural crest derived chromatophores, melanophores and xanthophores become determined andhow they interact to create the barred pigment pattern. The presence of phenol oxidase (tyrosinase) in melanophores (revealed by dopa incubation) and pteridines in xanthophores (visualized by fluorescence) were used as markers for cell differentiation in order to recognize melanophores and xanthophores before they became externally visible. It was found that melanophores and xanthophores were already determined in the premigratory neural crest, at stages 30/31 and 35-36, respectively. Between stages 35-36 and 38 they were arranged in a prepattern of several distinct, mixed chromatophore groups along the dorsal trunk, morphologically correlated in the scanning electron microscope with humps on the original crest cell string. While the occurrence of xanthophores was restricted to the chromatophore groups and around them, melanophores were already uniformly distributed in the dorsolateral flank area, having migrated from trunk neural crest portions including the groups. The bar component of the pigment pattern was subsequently initiated by xanthophores, which caused melanophores in and around the chromatophore groups to fade or become invisible. The barred pattern was established by the formation of alternating clusters of "like" cells, melanophores and xanthophores.
Keywords: Ambystoma mexicanum embryos; Chromatophores; Neural crest; Pattern formation.