Development of the hamster submandibular gland. II. The ductal system.

The major secretory duct is differentiated from the SMG anlage in early embryogenesis and undergoes minor morphologic changes from its inception until full maturation. Structurally, the ducts appear to be suited for a conduit function. At birth, the extralobular and intralobular ducts arise directly from the major duct or indirectly from the primary and secondary branches, respectively. They are distinguishable from each other by their topography only. However, developmentally, intralobular ducts give rise to 'terminal tubule' complexes but no such function is performed by the extralobular ducts. Both duct types first show distinct evidence of striation of their cells at one week after birth. Their lateral and basal infoldings, interdigitations and close association with mitochondria provide them with increased surface area and a source of energy for exchange of ions and fluids. The differentiation of convoluted granular ducts begins at 2 weeks of age with the appearance of membrane-bound and dense secretory granules in the apical cytoplasm of the luminal cells. The changes start in the proximal segment of the intralobular striated duct, and extend to occupy a large part of it in a mature 6-week old animal. These ducts comprise the bulk of the ductal system. The distribution of the granules is size-gradient dependent, the small granules being near the lumen and the large ones being close to the nucleus. The morphologic features of CGD are in keeping with absorptive and secretory functions.