Migration-Directing Liquid Properties of Embryonic Amphibian Tissues

Abstract

Deep ectoderm, mesoderm and endoderm excised from gastrulating amphibian embryos spontaneously undergo liquid-like movements in organ culture. Cell populations of these tissues on nonadhesive substrata will round up into spheres, spread over one another and segregate (sort out) from one another just as immiscible liquid droplets do. In ordinary liquids, movements like these are controlled by surface tensions; perhaps surface tensions also control the similar movements of liquid-like tissues. One necessary condition for tissue surface tension analysis is that the tissue must be able (just as ordinary liquids are able) to spontaneously relax internal stretching forces (shear stresses). When cellular aggregates of the germ layers were deformed by gentle compression between parallel glass plates, cells within the aggregates were initially stretched. However, the cells soon returned to their original undistorted shapes. Thus, cell stretching forces were gradually relaxed by cell rearrangements. The in vitro spreading movements of the deep germ layers imply that the surface tension of ectoderm should be greater than the surface tension of mesoderm which should be greater than the surface tension of endoderm. Quantitative measurements of tissue surface tensions made by parallel plate compression confirm precisely that relationship. Furthermore, the surface tensions of these tissues remain constant regardless of the amount of aggregate flattening—another necessary condition for valid surface tension measurements. These results demonstrate that amphibian deep germ layers possess fundamental liquid properties which are sufficient to direct their liquid-like rearrangements in organ culture. Furthermore, I also report that one of these properties, surface tension, displays a preliminary correlation with density of cell surface charge (assessed by electrophoretic mobility) and with the onset of in vivo mesodermal involution.