During amphibian gastrulation, migrating subsurface germ layers may flow past one another like immiscible viscous liquids in response to tissue surface tension forces. We describe here two physical tests for liquid-tissue morphogenesis in cultured aggregates of subsurface ectoderm (E), mesoderm (M) and endoderm (N) excised from mid-yolk-plug Rana pipiens gastrulae. (i) Liquids are coherent substances in which subunits can slip past one another to relax internal shear stresses. We find, in cross-sections of cell aggregates fixed during compression, that cells within flattened aggregates are intially deformed, but do, as predicted, gradually reassume their original, undistorted shapes, (ii) Surface tensions (γ's) govern ordinary liquid-droplet spreading; e.g., if equal-sized droplets A and B fuse in medium O, B spreads around A when γAO γBO. When pairs of subsurface aggregates are cultured together, N surrounds M and E, and M surrounds E. To see if γEO > γMO > γNO. we flatten aggregates with quartz fibers calibrated to measure the force of compression. As predicted, under the same flattening force, E aggregates are rounder than M aggregates, which are rounder than N aggregates. Furthermore, a second surface tension relationship can account for the autonomous involution of M between E and N; and these surface tension relationships can also explain the inversion of E, M and N by coated ectoderm to produce normal gastrular germ-layer arrangements. We conclude that, combined with active cell shape changes in solid-like surface cell layers and also with autonomous elongation of dorsal lip mesoderm, tissue surface tension control of liquid-tissue flow in subsurface germ layers is a key morphogenetic mechanism in amphibian gastrulation which might be regulated by changes in intercellular adhesiveness.
HERBERI M. PHILLIPS (Wed,) studied this question.