  Cells collectively migrate as a cluster in three-dimensional (3D) tissues,
such as in embryogenesis and cancer invasion. Here, numerical simulations using
a 3D vertex model show that polarized interfacial tension, expressing cell
adhesion and cortex contractility, induces the cluster migration in the 3D
space. The mechanism is that polarized interfacial tension induced a
directional flow of cell-cell interfaces from the front to rear within the
whole cluster, producing a driving force, i.e., cells move forward as a cluster
by simply expanding and contracting cell-cell boundaries.
