Embryonic tissues as elasticoviscous liquids. II. Direct evidence for cell slippage in centrifuged aggregates.

Certain embryonic cell aggregates display both solid-like and liquid-like properties in organ culture. When centrifuged against solid substrata, these aggregates undergo sudden initial deformations followed by more gradual shape changes. Thus, they are either compoundviscoelastic solids (in which cells first stretch rapidly, then slowly) or elasticoviscous liquids (in which cells first quickly stretch, but then gradually slide by one another). To distinguish between these alternatives, we have examined cell shapes in centrifugally deformed chick liver aggregates. Light and electron micrographs show that initially stretched cells within flattening aggregates gradually reassume their original undisturbed shapes during prolonged centrifugation. Therefore, although the cells themselves may react to stretching forces like elastic solids, they slowly slip past one another within aggregates to relax stretching forces, endowing the aggregates with liquid properties. Slow cell slippage can account not only for temporary elastic solidity and viscous liquid flow in cell aggregates, but also for stress-free changes in the positions and configurations of tissues migrating within developing embryos. Regulation of cell slippage properties may shift the morphogenetic dependence of embryonic tissues in successive developmental stages from intracellular to intercellular force-generating mechanisms, or vice versa.