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 tw enty years. First, earlier studies on the migration and differentiatio n of NC cells in vitro are summarized. These studies include the modes of NC cell migration and their differentiation into chondroblasts, pe richondral cells, neurons, Schwann cells and pigment cells (melanophor es and xanthophores). Then a summary is given on the development of cr anial sensory ganglia and enteric ganglia in Xenopus laevis. In the su bsequent 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. I n wild-type or dark axolotl embryos, pigment cells leave the NC and mi grate out under the epidermis, whereas in the white mutant, pigment ce lls remain closely confined to the original position of the NC. This s ystem provides an excellent model for analyzing NC cell migration in v ertebrate embryos. Further sections deal with the development of larva l pigment patterns in Triturus alpestris, (horizontal melanophore stri pes) and Ambystoma mexicanum (vertical melanophore bars). Comparing th e formation of these patterns shows that two different principles exis t in the distribution of pigment derivatives of the NC: patterns follo wing environmental cues ( Triturus) and those ignoring these cues, rel ying solely on cell-cell interactions (Ambystoma). Other studies relat e to evolutionary perspectives in pigment pattern formation. They are based on phylogenetic analyses of North American ambystomatids, combin ed with data on pigment patterns and their formation where such data a re available. These studies have shown that vertical bars which develo p from aggregates in the NC string are an evolutionary innovation, com pared to the more primitive horizontal stripes lacking aggregates in t he NC. Thus, in this review we show that the NC of amphibians (T. alpe stris, Xenopus laevis, dark and white axolotls and other ambystomatids ) may be used for various analyses concerning the migration and differ entiation of its derivatives, as well as for studies on the formation and evolution of pigment patterns.