Abstract

It has long been established that biological tissues are viscoelastic materials, and that embryonic tissues in particular have an ability to flow over large distances on long time scales. The mechanical properties of these tissues likely play an important role in cell movements and pattern formation during embryogenesis, and they can be measured using tissue surface tensiometry (TST). Over the past 40 years, two theories have been advanced to explain the microscopic origins of tissue surface tension: the differential adhesion hypothesis (DAH) and the differential interfacial tension hypothesis (DITH). Several recent experiments appear to support the first theory, while others seem to support the second. We show that a simple mechanical model which accounts for adhesion, cortical tension, and fluid incompressibility can explain the two types of experimental data within a single framework, and that surface tension in tissues is a careful balance between adhesive and tensile forces. In addition, the model predicts that cells on the surface of an aggregate alter their morphology as the surface tension changes, and preliminary experimental observations are consistent with these predictions.