GERM-LAYER SURFACE TENSIONS AND TISSUE AFFINITIES IN RANA-PIPIENS GASTRULAE - QUANTITATIVE MEASUREMENTS

The morphogenetic properties causing germ-layer spreading and stratifi cation in amphibian gastrulation were called ''tissue affinities'' by Holtfreter. The differential adhesion hypothesis (DAH) attributes such liquidlike tissue rearrangements to forces generated by intercellular adhesions within and between the migrating cell populations. This the ory predicts that, among the primary germ layers, the cohesiveness of deep ectoderm should be the greatest, that of deep mesoderm should be intermediate, and that of deep endoderm should be the least. Also, the cohesiveness of differentiating neural ectoderm should increase after induction, causing it to internalize and segregate from epidermis. Th e DAH also explains why the cohesiveness of ''liquid'' tissues, whose cells are free to rearrange, should be measurable as tissue surface te nsions. Using a specially designed tissue surface tensiometer, we demo nstrate that (i) aggregates of Rana pipiens deep germ layers do posses s liquid-like surface tensions, (ii) their surface tension values lie in precisely the sequence necessary to account for germ-layer stratifi cation in vitro and in vivo, and (iii) the surface tension of deep ect oderm just underlain by the archenteron roof is twice that of not-yet- underlain deep ectoderm. These measurements provide direct, quantitati ve evidence that the ''tissue affinities'' governing germ-layer now du ring early stages of vertebrate morphogenesis are reflected in tissue surface tensions. (C) 1997 Academic Press.