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The unjamming transition is distinct from the epithelial-to-mesenchymal transition

Every organ surface and body cavity is lined by a confluent collective of epithelial cells. In homeostatic circumstances the epithelial collective remains effectively solid-like and sedentary. But during morphogenesis, remodeling or repair, as well as during malignant invasion or metastasis, the epithelial collective becomes fluid-like and migratory1–4. This conversion from sedentary to migratory behavior has traditionally been understood as a manifestation of the epithelial-to-mesenchymal transition (EMT) or the partial EMT (pEMT)5–8. However, in certain contexts this conversion has been attributed to the recently discovered unjamming transition (UJT), in which epithelial cells move collectively and cooperatively9–11. UJT and pEMT share certain aspects of collective cellular migration, but the extent to which these processes are distinct, overlapping or perhaps even identical has remained undefined. Using the confluent layer of well-differentiated primary human bronchial epithelial (HBE) cells, here we triggered UJT by exposing the sedentary layer to mechanical compression9–12. Cells thereafter migrated cooperatively, aligned into packs locally, and elongated systematically. Nevertheless, cell-cell junctions, apico-basal polarity, and barrier function remained intact in response, and mesenchymal markers remained unapparent. As such, pEMT was not evident. When we triggered pEMT and associated cellular migration by exposing the sedentary layer to TGF-β1, metrics of UJT versus pEMT diverged. To account for these striking physical observations a new mathematical model attributes the effects of pEMT mainly to diminished junctional tension but attributes those of UJT mainly to augmented cellular propulsion. Together, these findings establish that UJT is sufficient to account for vigorous epithelial layer migration even in the absence of pEMT. Distinct gateways to cellular migration therefore become apparent – UJT as it might apply to migration of epithelial sheets, and EMT/pEMT as it might apply to migration of mesenchymal cells on a solitary or collective basis, activated during development, remodeling, repair or tumor invasion. Through the actions of UJT and pEMT working independently, sequentially, or interactively, living tissue is therefore seen to comprise an active engineering material whose modules for plasticity, self-repair and regeneration, are far richer than had been previously appreciated.