Basal lamina of embryonic salivary epithelia. Nature of glycosaminoglycan and organization of extracellular materials

Abstract

The ultrastructure and the composition of newly synthesized glycosaminoglycans (GAG) in the epithelial basement membrane(s) (BM) of mouse embryo submandibular glands were assessed. The labeled GAG accumulating in these membranes are distinct from those in the tissue of origin, the epithelium, or from those in the surrounding mesenchyme. In the BM, hyaluronic acid accounts for ∞50% of the labeled GAG, chondroitin 4 sulfate is twice the chondroitin 6 sulfate, and there is a low proportion of chondroitin. This composition is constant regardless of whether the BM is labeled by whole glands or, in the absence of mesenchyme, by isolated epithelia retaining a BM and by isolated epithelia generating a BM de novo. These results indicate that the labeled GAG are bona fide components of the BM, and suggest that BM GAG is deposited in units of constant composition. Ultrastructural observations following ruthenium red staining or tannic acid fixation establish that the BM is a highly ordered specialization of the basal cell surface. Discrete structures in macroperiodic arrays apparently attached to the plasmalemma could be visualized. This organization is seen in intact glands and in the BM produced by epithelia in the absence of mesenchyme or biologic substrata. The data are interpreted as indicating that the BM contains supramolecular complexes of hyaluronic acid and proteoglycan which are organized into an extracellular scaffolding which plays a role in the structure of the epithelium.