Factors controlling the time of onset of the migration of neural crest cells in the fowl embryo

Transmission electron microscopy of fowl embryos during the 7-10 h preceding migration of trunk-level neural crest (NC) cells revealed extracellular material near the NC-cells. In contrast to the cells of the neural tube, the basal surfaces of NC-cells possessed projections, and were neither contiguous nor covered by a complete basal lamina. The apical zones of NC-cells showed intercellular junctions at the stage of neural-fold fusion, but such junctions were absent in some NC-cells 5 h before migration. The basal laminae of the neural tube and the ectoderm were fused lateral to the NC before migration. In vitro, NC-cell migration commenced immediately when neural anlagen were explanted onto fibronectin-rich matrices, but only when the neural anlagen were from a level where migration had commenced in vivo. Migration was delayed 4-8 h when premigratory-level expiants were used. Short-term cell-adhesion assays showed that NC-cells of both premigratory and migratory levels could adhere to fibronectin-rich matrices and to collagen gels, but only migratory NC-cells could be detached from the neural anlage. The results suggest that the precise schedule of the onset of NC-cell migration correlates with a decrease in the intercellular adhesion of NC-cells.