Fig. 1.

A mechanism to translate tissue planar polarity into self-sustained CE. Signaling and communication pattern the tissue (A) and establish positional information domains that provide cellular identity (B). (C) If cell identity implies distinct mechanical properties that promote cell intermingling, it leads to an auto-catalytic intercalation mechanism (see “Methods”). As schematically represented by the green line, cellular intercalation challenges the smoothness of boundary lines until cell identities are reassigned (“Methods”). (D) Directional features of cellular processes, such as elongation or cleavage, indicate that cells elongate along an axis perpendicular to the domain boundary, but the resulting CE arising for intercalations extends the tissue along a direction parallel to the domain boundary.