Convergence and extrusion driven by non‐muscle myosin II are required for normal fusion of the mammalian secondary palate.

Abstract

Fusion of two distinct embryonic prominences to form a single continuous structure occurs in numerous developmental contexts and typically requires integration of two epithelia and subsequent removal of that intervening epithelium. During fusion of the mammalian secondary palatal shelves, the medial epithelial seam (MES) is removed to form a continuous secondary palate with mesenchymal confluence, but the cellular mechanisms remain incompletely understood. Three main hypotheses for how this occurs have been proposed. Whereas considerable investigation has been made into epithelial to mesenchymal transition (EMT) and programmed cell death of the intervening epithelium as potential cellular mechanisms, the contribution of other cellular dynamics of the intervening epithelium has not been directly evaluated. Using confocal live imaging, we have now directly observed the cellular processes underlying tissue fusion in the palatal shelves. We find that convergence of a multi‐layered epithelium into a single‐layer epithelium is an essential early step driven by cell intercalation and is concurrent to orthogonal displacement of epithelial cells. Surprisingly, we find that epithelial cell extrusion, a process in which cells are surrounded and squeezed from an epithelium in an actomyosin contractility‐dependent manner functions in the removal of the MES during palate fusion. Functional studies in mice indicate that these processes require an actomyosin contractility pathway involving Rho kinase (ROCK) and myosin light chain kinase (MLCK), culminating in the activation of non‐muscle myosin IIA (NMIIA). Together, these data indicate that actomyosin contractility drives cell intercalation and cell extrusion during palate fusion and suggest a general mechanism for tissue fusion in development.