The differentiation of neural crest cells into visceral cartilages and odontoblasts in Amblystoma , and a re-examination of the germ-layer theory

Abstract

In the axolotl, A. mexicanum serial photomicrographs from embryonic stage 20 to 35 indicate that ectodermal cells of the neural crest (identified by pigment granules) develop into mesenchyme of ectodermal origin (ectomesenchyme) which differentiates into the cartilages of the visceral arches (with the exception of the second basibranchial) into odontoblasts and probably into osteoblasts of dermal bones. Mesodermal and endodermal cells may be identified by lack of pigment and abundance of large yolk granules (10jll). The enamel organs of teeth can be formed from the ectodermal cells of the stomodeal collar, from the endodermal cells of the gut wall, or from both. There is no invariable correlation between the germ layer and either the presumptive organ-forming regions or the formed structures. Germ layers are not determinants of differentiation in development, but are embryonic structures which resemble one another closely in different forms although they may contain materials differing in origin and in fate.