  A number of biological processes, such as embryo development, cancer
metastasis or wound healing, rely on cells moving in concert. The mechanisms
leading to the emergence of coordinated motion remain however largely
unexplored. Although biomolecular signalling is known to be involved in most
occurrences of collective migration, the role of physical and mechanical
interactions has only been recently investigated. In this paper, a versatile
framework for cell motility is implemented in-silico in order to study the
minimal requirements for the coordination of a group of epithelial cells. We
find that cell motility and cell-cell mechanical interactions are sufficient to
generate a broad array of behaviours commonly observed in vitro and in vivo.
Cell streaming, sheet migration and susceptibility to leader cells are examples
of behaviours spontaneously emerging from these simple assumptions, which might
explain why collective effects are so ubiquitous in nature. This analysis
provides also new insights into cancer metastasis and cell sorting, suggesting
in particular that collective invasion might result from an emerging
coordination in a system where single cells are mechanically unable to invade.
