Embryonic tissues as elasticoviscous liquids: I. rapid and slow shape changes in centrifuged cell aggregates
Authors: Phillips H. M. , Steinberg M. S. 
Published: April 2021

ABSTRACT
Certain embryonic tissue masses and cell aggregates behave like deformable solids during brief experimental manipulations but like viscous liquids in long-term organ cultures. To investigate these seemingly paradoxical physical properties, we have mechanically deformed cell aggregates derived from several embryonic chick organs by centrifuging them against solid substrata. Aggregate shapes during brief centrifugation were observed directly in a microscope-centrifuge. In addition, techniques were devised for fixing cell aggregates during prolonged centrifugation. Evidence presented here shows that these fixative-injection procedures accurately preserve the prefixation shapes of living centrifuged aggregates.
According to a simple viscous-liquid model for cell aggregates, cohering cells slide past one another when external forces and/or tissue surface tensions cause gradual rearrangements in aggregate conformations. In earlier experiments, 2 types of behaviour predicted from this model were confirmed for several embryonic chick tissues subjected to prolonged centrifugation. First, initially flat aggregates rounded up against the centrifugal force to adopt the same shapes that initially round aggregates reached by flattening. Second, the relative roundness of centrifuged aggregates of different tissues at shape equilibrium correlated with the relative positions that these tissues assumed when they were combined in aggregate-spreading and cell-sorting experiments.
By contrast, the brief centrifugation experiments described here provide some support for a simple elastic-solid model in which aggregate shape changes are accompanied by cell deformations rather than cell redistributions. In particular, since cell migration tends to occur quite slowly, the very rapid aggregate flattening observed during the first few minutes of centrifugation presumably requires cell stretching. Moreover, since they also round up very rapidly following brief centrifugation, these aggregates exhibit considerable elasticity that presumably reflects the swift relaxation of cell stretching as the centrifugal force is removed.
Although both elastic-solid and viscous-liquid properties can be recognized in cell aggregates, we note that, in the prolonged centrifugation experiments described here, rapid initial aggregate flattening is followed by much more gradual, continued flattening. Similarly, after prolonged centrifugation, rapid partial aggregate rounding-up is also followed by much more gradual, continued rounding-up during subsequent culture at 1g. Such rapid-then-slow shape changes contradict both simple elastic-solid and simple viscous-liquid models for cell aggregates. These bimodal shape changes are instead consistent with both compound-viscoelastic-solid and elasticoviscous-liquid models for cell aggregates, although only the latter can also account for long-term liquid-like aggregate behaviour.
While proposing more direct experimental tests to distinguish between these alternative models, we conclude here that these cell aggregates act like elasticoviscous liquids, which first stretch and then flow in response to surface and external deforming forces. In light of the observations reported here, we can distinguish 3 modes of tissue translocation during morphogenesis. In addition to (i) migrations of single cells, and (ii) deformation (expansion, thickening, or bending) of solid tissue sheets by changes in the shapes of their component cells, we suggest (iii) that tissues may also change their contours and positions by flowing from one configuration to another as coherent elasticoviscous liquids.