The neural crest in presomite to 40-somite murine embryos.

The production of cells by the neural crest is studied light-microscopically in 10 microns and 1 micron serially sectioned mouse and rat embryos, ranging in age from presomite to 40-somite stages. In the head region, mesectoderm formation starts in a pre-neural plate stage. It continues to the 20-somite stage. This implies that the contribution of the neural crest to the head mesoderm must be considerable. In the trunk, the neural crest only produces cells after adhesion of the neural walls. Mesectoderm formation continues for a long time, slowly retreating in a caudal direction. At the 40-somite stage, mesectoderm formation still occurs in the most caudal part of the trunk. Compared to the head, the contribution of the neural crest in the trunk seems to be less important than that of the primitive streak.