  Collective cell migration is essential for a wide range of biological
processes such as: morphogenesis, wound healing, and cancer spreading. However,
it is well known that migrating epithelial collectives frequently undergo
jamming, stay trapped some period of time, and then start migration again.
Consequently, only a part of epithelial cells actively contributes to the
tissue development. In contrast to epithelial cells, migrating mesenchymal
collectives successfully avoid the jamming. It has been confirmed that the
epithelial unjamming cannot be treated as the epithelial-to-mesenchymal
transition. Some other mechanism is responsible for the epithelial
jamming/unjamming. Despite extensive research devoted to study the cell
jamming/unjamming, we still do not understand the origin of this phenomenon.
The origin is connected to physical factors such as: the cell compressive
residual stress accumulation and surface characteristics of migrating
(unjamming) and resting (jamming) epithelial clusters which depend primarily on
the strength of cell-cell adhesion contacts and cell contractility. The main
goal of this theoretical consideration is to clarify these cause-consequence
relations.
