The ultrastructure of periductal connective tissue and distinctive populations of collagen fibrils associated with ductal epithelia of exocrine glands.

The ultrastructure of the connective tissue around the intraglandular ducts was observed in rat exocrine glands. Connective tissue with a dense population of collagen fibrils was found either around the ducts and blood vessels (perivasculoductal connective tissue; PVDCT) as in the lacrimal and salivary glands and liver, or solely surrounding the ducts (periductal connective tissue; PDCT) as in the exocrine pancreas, whereas the interlobular and intralobular interstitium of the glands-except for the liver-contained substantially fluid-filled spaces without collagen fibrils. The PVDCT and PDCT of these glands contained two populations of collagen fibrils-fibroblast-associated and epithelium-associated-although the development and density of these fibrils varied considerably in individual glands. Both populations of collagen fibrils were most developed in the lacrimal glands, in which the basal aspects of the ductal epithelium and the basement membrane showed considerable undulation associated with a distinctive peribasement membrane zone with amorphous matter and a small population of the fibrils. In the parotid and submandibular glands, both populations were distinct, though poorly developed. In the exocrine pancreas and hepatic Glisson's sheath, the two populations of collagen fibrils were moderately developed, and the basal aspects of the ductal epithelium were characterized by prominent invaginations in which the multilaminar basement membranes and the epithelium-associated collagen fibrils were frequently engulfed. These observations provide evidence that the two populations of collagen fibrils around the ducts are found universally in exocrine glands, and support the hypothesis of the collagen fibril-synthesizing and -secreting ability of ductal epithelial cells.