Tissue narrowing and elongation via planar polarized cell intercalation
Convergent extension is a morphogenetic movement in which a tissue simultaneously narrows (converges) along one axis while lengthening (extends) along the perpendicular axis, without net cell division or growth. It is driven by mediolateral intercalation: cells use planar-polarized protrusions (enriched with Rac1 at mediolateral ends, inhibited laterally by Pk/Vangl2) to migrate between their anterior-posterior neighbors, executing T1 transitions — neighbor exchanges where a shared vertex is exchanged. This mechanism elongates the body axis during gastrulation (Xenopus notochord), neurulation, and gut morphogenesis. Wnt/PCP pathway coordinates the polarization; loss of Dishevelled produces randomized protrusions and failure to extend.