Branching submandibular glands of 12-day mouse embryos and those cultured in the presence and absence of a collagenase inhibitor from the culture medium of bovine dental pulp or a Clostridial collagenase were examined with the scanning electron microscope. Fracturing of fixed and dried glands with the tip of a fine needle succeeded in exposing the surfaces of the lobules and of their mesenchymal replicas at different stages of branching. At the beginning of branching, corresponding parts of the mesenchyme formed ridges on or in which the fibrillar structures were often found. At the stage forming deeper clefts thicker fibres, 0·5–2·5 μm in diameter, were observed between two adjacent lobules. On the contrary, no apparent differences in the fibrillar structures on the epithelial surfaces were detected between the shallow cleft and noncleft regions at the initial phase of branching. These fibrillar structures were very abundant in glands cultured with collagenase inhibitor and were completely lost in glands cultured with bacterial collagenase, strongly indicating that these materials consisted of collagen. The possible involvement of mesenchyme in epithelial branching is discussed with special reference to mesenchymal traction forces that would be elicited by fibrillar collagens.

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