Figure 1. The direction in which a cell extrudes has important consequences for its fate.

Extrusion removes either live or dying epithelial cells in response to crowding during homeostasis or apoptotic stimuli, respectively. Typically, live cells extrude apically from the epithelium into the lumen by contracting an intercellular actomyosin band basolaterally to squeeze the cell out (left). Apically extruded cells generally die in the lumen by the loss of survival signals from the matrix — a process called anoikis (top). Tumour cells with upregulated survival signalling could still be eliminated through the lumen by apical extrusion, which could function like a tumour-suppressor mechanism (top). Less frequently, cells extrude basally by apical contraction (right), back into the tissue that the epithelium encases. Several oncogenic mutations disrupt apical extrusion, thereby driving cells to extrude basally instead. Because basally extruded cells with upregulated survival signals can bypass anoikis, this may provide a novel mechanism to enable oncogenic cells with upregulated survival signalling to initiate invasion (bottom).