Neural crest cell migration and pigment pattern formation in urodele amphibians

This review deals with research on the development and differentiation of the neural crest (NC) in amphibians carried out during the past twenty years. First, earlier studies on the migration and differentiation of NC cells in vitro are summarized. These studies include the modes of NC cell migration and their differentiation into chondroblasts, perichondral cells, neurons, Schwann cells and pigment cells (melanophores and xanthophores). Then a summary is given on the development of cranial sensory ganglia and enteric ganglia in Xenopus laevis. In the subsequent sections, mechanisms of NC cell migration are investigated in Ambystoma mexicanum, the Mexican axolotl (wild-type and white mutant) using ultrastructural, immunohistochemical and biochemical methods. In wild-type or dark axolotl embryos, pigment cells leave the NC and migrate out under the epidermis, whereas in the white mutant, pigment cells remain closely confined to the original position of the NC. This system provides an excellent model for analyzing NC cell migration in vertebrate embryos. Further sections deal with the development of larval pigment patterns in Triturus alpestris, (horizontal melanophore stripes) and Ambystoma mexicanum (vertical melanophore bars). Comparing the formation of these patterns shows that two different principles exist in the distribution of pigment derivatives of the NC: patterns following environmental cues (Triturus) and those ignoring these cues, relying solely on cell-cell interactions (Ambystoma). Other studies relate to evolutionary perspectives in pigment pattern formation. They are based on phylogenetic analyses of North American ambystomatids, combined with data on pigment patterns and their formation where such data are available. These studies have shown that vertical bars which develop from aggregates in the NC string are an evolutionary innovation, compared to the more primitive horizontal stripes lacking aggregates in the NC. Thus, in this review we show that the NC of amphibians (T. alpestris, Xenopus laevis, dark and white axolotls and other ambystomatids) may be used for various analyses concerning the migration and differentiation of its derivatives, as well as for studies on the formation and evolution of pigment patterns.