The Neural Crest: a Study On Cell Degeneration and the Improbability of Cell Migration in Mouse Embryos

Mouse embryos (CPB-S strain) aged 6.0-8.3 days post coitum (p.c.) were investigated in 10-μm and 1-μm series by light microscopy with respect to: (a) the occurrence of degenerating cells in the neural crest, and (b) the stage of development in which the neural crest cells start to lose their epithelial arrangement. A high frequency of cell degeneration was found in all embryos examined, starting with the 6.6-day-p.c. stage, i.e., prior to the ectodermal disruption of the neural crest in the 7.3-day-p.c. stage, and continuing through all of the later stages included in the study. In the 7.3-day-p.c. stage the neural plate has not developed yet and the neural crest is present between the neurectoderm and the future surface ectoderm situated just lateral to the notochordal plate. During the transformation of the neurectoderm, via the neural plate and the neural groove, into the neural tube, the neural crest is shifted first laterally and then dorsally and medially, and drops cells. These cells proliferate immediately and eventually differentiate. In this developmental model there is no need for migration of cells to explain the outgrowth of e.g. facial swellings.