The proteoglycans and glycosaminoglycans synthesized by embryonic mouse salivary glands during normal morphogenesis and in the presence of fl-xyloside, an inhibitor of branching morphogenesis, have been partially characterized. Control and p-nitrophenyl-fl-Dxyloside-treated salivary rudiments synthesize proteoglycans that are qualitatively similar, based on mobility on Sepharose CL-4B under dissociative conditions and glycosaminoglycan composition. However, fl-xyloside inhibits total proteoglycan-associated glycosaminoglycan synthesis by 50%, and also stimulates synthesis of large amounts of free chondroitin (dermatan) sulfate. This free glycosaminoglycan accounts for the threefold stimulation of total glycosaminoglycan synthesis in fl-xyloside-treated cultures. Several observations suggest that the disruption of proteoglycan synthesis rather than the presence of large amounts of free glycosaminoglycan is responsible for the inhibition of branching morphogenesis. (a) We have been unable to inhibit branching activity by adding large amounts of chondroitin (dermatan) sulfate, extracted from fl-xyloside-treated cultures, to the medium of salivary rudiments undergoing morphogenesis. (b) In the range of 0.1-0.4 mM fl-xyloside, the dose-dependent inhibition of branching morphogenesis is directly correlated with the inhibition of proteoglycan synthesis. The stimulation of free glycosaminoglycan synthesis is independent of dose in this range, since stimulation is maximal even at the lowest concentration used, 0.1 mM. The data strongly suggest that the inhibition of branching morphogenesis is caused by the disruption of proteoglycan synthesis in fl-xyloside-treated salivary glands.Epithelial-mesenchymal interactions are required for the development of many diverse organ systems. The tissues are separated by an extraceUular matrix, suggesting that developmental events might be mediated by such extracellular macromolecules as collagens (7, 28), proteoglycans (12, 33), and glycoproteins (7, 32). Recently, attention has focused on the role of the extraceUular matrix and the basal lamina, a zone of extraceUular material having a characteristic ultrastructural organization, in tissue interactions.Development of the embryonic salivary gland provides a useful model system for studying the role of proteoglycans in epithelial-mesenchymal interactions. The multilobular morphology of salivary glands is established by branching morphogenesis, repetitive budding, and folding of the epithelium. Although these shape changes are generated by forces within the epithelial cells themselves (29), interaction with the mesenchyme is required for branching morphogenesis. Indeed, the pattern of epithelial morphogenesis appears to be regulated by the mesenchyme (21). The two tissues are separated by a basal lamina that contains abundant glycosaminoglycan (5), presumably as proteoglycan. Integrity of the basal lamina is required for maintenance of lobular morphology of the salivary rudiments (1). Furthermore, turnover of basal laminar gly...
