Spatial pattern of smooth muscle differentiation is specified by the epithelium in the stomach of mouse embryo

Smooth muscle induction in the stomach of mouse embryo was analyzed experimentally by tissue recombination and organ culture technique. When observed immunohistochemically with anti-alpha-smooth muscle actin antibody, smooth muscle myoblasts first differentiated and formed the circular muscle layer at 13 days of gestation. The longitudinal muscle layer appeared at 15 days in the forestomach and at neonatal period in the glandular stomach. The lamina muscularis mucosae was observed by the neonatal period. When the undifferentiated 11-day stomach was cultivated in vitro for 3 days, an alpha-smooth muscle actin-positive cell layer was formed and the explant exhibited rhythmical contraction. The mesenchyme alone remained as an undifferentiated cell mass. When the mesenchyme was recombined with the epithelium, a distinct smooth muscle layer was formed. These demonstrated that the alpha-smooth muscle actin expression was induced by the epithelium. Next, we examined by heterologous recombination the roles of the epithelium in formation of the organ-specific spatial pattern of smooth muscle differentiation. The stomach mesenchyme recombined with the tracheal epithelium exhibited a partially tracheal type of alpha-smooth muscle actin expression. The uterine epithelium induced only a small number of smooth muscle myoblasts and the spatial organization was disordered. Finally, we assayed if the mode of smooth muscle induction by the stomach epithelium changes during development. Heterochronic recombination proved that the 14-day forestomach epithelium induced a muscularis mucosae-like smooth muscle layer that was contiguous to the epithelium, whereas the 11-day stomach and the 13-day forestomach epithelium induced a circular muscle-like smooth muscle layer that was distant from the epithelium. As a whole, our data suggest that the spatial pattern of smooth muscle differentiation is regulated by organ-specific and stage-specific influences from the epithelium.