Large-scale mechanical properties of Xenopus embryonic epithelium.

Epithelia are planar tissues that undergo major morphogenetic movements during development. These movements must work in the context of the mechanical properties of epithelia. Surprisingly little is known about these mechanical properties at the time and length scales of morphogenetic processes. We show that at a time scale of hours, Xenopus gastrula ectodermal epithelium mimics an elastic solid when stretched isometrically; strikingly, its area increases twofold in the embryo by such pseudoelastic expansion. At the same time, the basal side of the epithelium behaves like a liquid and exhibits tissue surface tension that minimizes its exposed area. We measure epithelial stiffness (~1 mN/m), surface tension (~0.6 mJ/m(2)), and epithelium-mesenchyme interfacial tensions and relate these to the folding of isolated epithelia and to the extent of epithelial spreading on various tissues. We propose that pseudoelasticity and tissue surface tension are main determinants of epithelial behavior at the scale of morphogenetic processes.