  The notion of tissue surface tension has provided a physical understanding of
morphogenetic phenomena such as tissue spreading or cell sorting. The
measurement of tissue surface tension so far relied on strong approximations on
the geometric profile of a spherical droplet compressed between parallel
plates. We solved the Laplace equation for this geometry and tested its
solution on true liquids and embryonic tissue fragments as well as
multicellular aggregates. The analytic solution provides the surface tension in
terms of easily and accurately measurable geometric parameters. Experimental
results show that the various tissues and multicellular aggregates studied here
are incompressible and, similarly to true liquids, possess effective surface
tensions that are independent of the magnitude of the compressive force and the
volume of the droplet.
