Figure 1. Live-Cell Extrusion Maintains Cell Density during Tissue Development and Homeostasis.

(A) Cell proliferation can generate overcrowding in an epithelium. Extrusion of cells in the overcrowded area relieves cell compression and returns the tissue to equilibrium. In cultured MDCK cells and the Drosophila notum, extruded cells are viable for a few hours before they undergo cell death, likely as a result of loss of adhesion to other cells or to the basement membrane.

(B) In one model, deformation of the cell membrane due to shape changes or overcrowding could open stretch-activated channels that promote extrusion through an unknown mechanism. Increased calcium influx, for example, could stimulate myosin light-chain kinase, activating myosin and promoting the assembly of a contractile ring around extruding cells.