The development of the neural crest in amphibians

Our review deals with the development of the neural crest (NC) in amphibians. We will consider relevant aspects of evolution, ontogeny, migration and differentiation, and investigate principal problems such as the regulation of NC cell determination, pathway selection and destination recognition. Earlier data and more recent findings will be presented. The NC probably evolved about 440 million years ago from the anlagen of epidermal nerve plexuses in protochordates. In urodele amphibians, the prospective NC is already present in the early gastrula as a narrow band of ectodermal cells between the prospective epidermis and the prospective neural plate. The NC proper develops later from the apices of the neural folds and forms, after neural fold fusion, a transient cellular ridge on the dorsal surface of the neural tube. NC cells migrate extensively into various body regions and give rise to a wide variety of derivatives including the mesenchymal elements of the skull, the neural and glial precursors of the peripheral nervous system and the pigment cells. NC cell migration is stimulated by components of the extracellular matrix (ECM) and may conveniently be analyzed in the system of wild-type (dark) and white mutant axolotl embryos. Skeletal elements of the head derive from cranial NC cells following an interaction with pharyngeal endoderm. Other derivatives of the NC are the ganglia of the peripheral nervous system in the head (sensory ganglia; mostly mixed origin with placodal material) and trunk (spinal, sympathetic and enteric ganglia). Pigment cells also derive from the NC and become arranged into uniform or banded pigment patterns.