Fig. 3.

Branching via epithelial folding. The collective folding of cells in an epithelium can produce new branches. Changes in epithelial shape can occur by: (A) apical constriction of cells within a localized region (pink) of the epithelium; (B) differential growth, caused by increased cell division in the epithelium (pink) relative to an adjacent tissue (white); or (C) mechanical buckling, which casts the epithelium into a wave-like morphology when loaded with sufficient compressive force, possibly produced by adjacent tissues.