Mouse submaxillary epithelium undergoes branching morphogenesis with increase in the curvature of its surface in vivo. Recombination experiments in vitro of the epithelium and mesenchyme between 13- and 14-day rudiments showed (1) that the 14-day mesenchyme more actively induced the epithelium to branch than the 13-day mesenchyme, (2) that the 14-day mesenchyme could produce clefts on smaller epithelial lobes than the 13-day mesenchyme, (3) that the 14-day mesenchyme produced lobes with similar diameter to lobes of the 14-day intact rudiment, and (4) that the lobular morphology of assembled 14-day lobes became obscure in recombinates with the 13-day mesenchyme while it was well maintained in recombinates with the 14-day mesenchyme. From these results it is concluded that the mesenchyme determines the curvature of epithelial surface, and that clefts are formed on the epithelial surface as a result of increase in the epithelial curvature.

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