Neural crest cell behavior in white and dark embryos of Ambystoma mexicanum: Epidermal inhibition of pigment cell migration in the white axolotl

Melanophores in larvae of the white (dd) strain of the Mexican axolotl (Ambystoma mexicanum) are confined to the dorsal midline of the trunk and dorsal posterior part of the head, whereas those in dark larvae (D_-) are distributed over the flank as well. Our results show that this phenotype of white larvae is the result of the failure of the melanophores or their neural crest precursor cells to migrate laterally due to an inhibition of or a failure in the support of their migration in the subepidermal space by the overlying epidermis. Correlated light and scanning electron microscopy of dissected larvae showed melanophores occupying the subepidermal space on the flank of dark larvae, whereas these cells were restricted to the dorsal midline of white larvae. Grafting experiments in which patches of epidermis, the underlying mesoderm, or both, were exchanged between dark and white embryos suggested that white epidermis alone can prevent the integration of pigment cells on the flank of dark larvae and, conversely, that grafts of dark epidermis alone can support their migration on the flank of white larvae. Mesoderm, when grafted alone, could not be shown to have similar effects.