It is now increasingly recognised that cell collectives undergo a transition to a glassy or jammed state

that is akin to the one observed in molecular and colloidal matter [1,2]. In mature epithelial tissues,
such transition may ensure the proper development of elasticity but, whenever a tissue must adapt to
changes or perturbations of its physiological state, unjamming might be advantageous. By combining
experiments and simulations [3,4], we study the dramatic reawakening of cell motility that occurs at
constant cell density when a jammed mammary epithelial cell monolayer is perturbed by
overexpression of one single protein. Directed cell migration emerges when cell-cell interactions
promote coherence of local cell polarization, which is reminiscent of flocking in birds and other
animal species. We discuss also the relevance of these flocking states for tumour dissemination.

[1] Angelini, T., et al., Glass-like dynamics of collective cell migration, PNAS. 108, 4714-4719
[2] Park J.A., et al., Unjamming and cell shape in the asthmatic airway epithelium, Nature Materials.
14, 1040-1048 (2015)
[3] Malinverno, C., Corallino, S., et al, Endocytic reawakening of motility in jammed epithelia, Nature
Materials 16, 587 (2017)
[4] Giavazzi, F., Paoluzzi, M., et al., submitted (