Epithelial coating controls mesenchymal shape change through tissue-positioning effects and reduction of surface-minimizing tension

signalling between mesenchymal and epithelial cells has a profound influence on organ morphogenesis. However, less is known about the mechanical function of epithelial mesenchymal interactions. Here, we describe two principal effects by which epithelia can regulate shape changes in mesenchymal cell aggregates. We propose that during formation of the embryonic body axis, the epithelial layer relieves surface minimizing tensions that would force cell aggregates into a spherical shape, and controls the serial arrangement of cell populations along the axis. the combined effects permit the tissue to deviate from a spherical form and to elongate.