The epithelial to mesenchymal transition (EMT) is an enticingly simple mechanism that converts stationary epithelial cells into migratory mesenchymal cells. EMT is meant to provide a unified explanation for phenomena as complex as gastrulation and metastasis. However, cell movements turn out to be diverse, and many are collective. Cells commonly migrate in clusters, strands, sheets, elongating tubes, or in fluid-like masses. Moreover, plenty of cells move without activating the EMT program. Here I propose that EMT can be understood as one of many types of transitions in a broader landscape-or phase space-of cell morphologies and behaviors. Throughout biology, and at multiple scales, complexity arises from the combinatorial deployment of simple, modular components. I propose that diversity of cell shapes and behaviors similarly arises from combinatorial use of modular biomechanical properties.
Keywords: Cell forces; Cell shape and behavior; Collective cell migration; Epithelial to mesenchymal transition; Mechanical properties.