The ultrastructure of early cephalic neural crest cell migration in the mouse.

A study of the ultrastructural changes associated with the detachment of the presumptive neural crest cells from the neuroepithelium in the midbrain region in mouse embryos at 9 and 9 1/2 days of gestation was carried out. The first sign of neural crest cell formation occurred in this region before fusion of the neuroepithelium had occurred. Neural crest cells arose from both the neural plate and the adjoining surface ectoderm. Initially, the cells of the neural plate and the surface ectoderm were attached to each other by zonula occludens and zonula adherans at their apical surfaces however, these junctions disappeared just prior to the beginning of the migration of the crest cells. The first sign of migration of the crest cells was the disappearance of the basal lamina in the region of the presumptive crest cells. Once the basal lamina was lost, cell junctions were formed between the epithelial cells and the underlying mesenchymal cells. Once the crest cells had migrated into the underlying mesenchyme, they tended to form clumps of closely related, irregularly shaped cells. Phagosomes and accumulations of glycogen particles were found within some crest cells when they were still within 50 to 100 microns of the epithelium.