Cell behaviour during active cell rearrangement: evidence and speculations.

The cell behaviour and motility underlying cell rearrangement during gastrulation in amphibian and sea-urchin embryos are discussed. In particular, the cell behaviour of deep (non-epithelial) and epithelial cell populations that undergo cell rearrangement is compared and contrasted. Deep cell rearrangement in Xenopus laevis involves both convergence of cells towards the future dorsal midline and simultaneous axial extension of the mesodermal cell mass. Time-lapse cinemicrography and scanning electron microscopy suggest that asynchronous, repetitive motions of individual deep cells, involving local extensions and retractions of their margins, may provide the motive force for rearrangement. Such protrusive activity may be guided by local differences in cell-cell contacts in the marginal zone. Epithelial cell rearrangement in the sea-urchin embryo both elongates the archenteron and simultaneously closes the blastopore. Cell rearrangement is accompanied by stage-specific changes in protrusive activity and cell shape of the basal surfaces of cells in the wall of the gut rudiment, in contrast to the apical surfaces, which show little activity. These basal protrusions may be involved in the rearrangement process.