Figure 2. Observed 'rules' of cell intercalation.

Four rules of cell intercalation have been observed during CE movements. A) Planar polarity involves the progressive orientation of non-polarized cells. Cells must first localize factors at mediolateral surfaces (m/l; asterisks) or alternatively their anterior-posterior surfaces (not shown). Polarized factors then manifest by polarized protrusions and polarized cell shape change in intercalating cells (right-most panel). B) The remain-in-plane rule increases the efficiency of CE by keeping all three cells within the same plane. Converging cells crawling on top of or below their neighbors would reduce the efficiency of CE by thickening rather than elongating the tissue. C) The irreversibility rule stems from the need to lock in mechanical "gains" of intercalation. If the direction of cell intercalation is reversed in the next round the contribution to elongation of the tissue is lost. D) The cell shape constraint is also needed to "lock in" the mechanical gains of elongation. If cells merely change shape after intercalation, accommodating to their former space (grey rectangle), there will be no CE and no change in overall tissue shape.