Embryonic mesenchymal cells share the potential for smooth muscle differentiation: myogenesis is controlled by the cell's shape

Undifferentiated embryonic mesenchymal cells are round/cuboidal in shape. D uring development, visceral myogenesis is shortly preceded by mesenchymal c ell elongation. To determine the role of the cell's shape on smooth muscle development, undifferentiated embryonic mesenchymal cells from intestine (a bundant visceral muscle), lung (some visceral muscle) or kidney (no viscera l muscle) were plated under conditions that maintained cell rounding or pro moted elongation. Regardless of their fate in vivo, all the cells different iated into smooth muscle upon elongation as indicated by the expression of smooth muscle-specific proteins and the development of membrane potentials of -60 mV and voltage-dependent Ca2+ currents, characteristic of excitable cells. Smooth muscle differentiation occurred within 24 hours and was indep endent of cell proliferation. Regardless of their fate in vivo, all the rou nd cells remained negative for smooth muscle markers, had membrane potentia ls of -30 mV and showed no voltage-activated current, These cells, however, differentiated into smooth muscle upon elongation. The role of the cell's shape in controlling smooth muscle differentiation was not overcome by trea tment with retinoic acid, TGF-beta 1, PDGF BE or epithelial-conditioned med ium (all modulators of smooth muscle differentiation). These studies sugges t that the mesenchymal cell shape plays a main role in visceral myogenesis.