3D cell packing dynamics in converging and extending epithelial sheets

Epithelial sheets can realize diverse shapes by deforming and rearranging their constituent cells along specific motifs in space and time. During early embryonic development, major tissue shape changes often occur in bursts of activity, when endogenous and/or exogenous forces induce transient states of rapid internal reorganization, or fluidity. The “self-sculpting” motion of fluidizing tissues is facilitated by changes in tissue rheology that are linked to the transient local structure of cell packings. The 3D dynamics of cell packings in epithelial sheets and their contribution to global deformations remain to be fully understood. Here we study cell shape changes and rearrangements in two model tissues undergoing convergent extension flows: the mouse neural plate, a proliferating pseudostratified epithelium, and the fruit fly germband, a non-proliferating columnar epithelium. In both systems, we observe gradients in cell shapes and alignment along the apical-to-basal axis, indicative of distinct basal and apical tissue flows. Mutations that disrupt cytoskeletal machineries have distinct effects along the apical-basal axis and lead to measurable changes in global tissue flow and curvature. Coordination of apical and basal tissue flows via 3D cell shape changes and rearrangements appears to be critical for epithelial sheets to properly achieve their morphogenetic targets with implications for understanding developmental defects rooted in aberrant tissue mechanics during morphogenesis.