Microtubules and Microfilaments in Amphibian Neurulation

In the salamander embryo, the morphogenetic movements of neurulation are correlated with two cell shape changes in the neural epithelium: elongation and apical constriction of the columnar neural plate cells. Cells first elongate to form the flat open neural plate and then constrict apically as the plate rolls up to form the neural tube. Evidence is presented that these cell shape changes are intrinsic to the cells themselves and that they play a causal role in the morphogenetic movements. Neural plate cells contain numerous microtubules oriented parallel to the axis of elongation. These microtubules are critical to the elongation process. Possible mechanisms for microtubule function in cell elongation are considered. During apical constriction the cells contain bundles of microfilaments which encircle the cell apex in purse-string fashion. Evidence is presented which suggests that microfilament bundles play an active role in apical constriction, and that this localized contraction is produced by filament sliding.