Contributions of intercalated duct cells to the normal parenchyma of submandibular glands of adult rats

The parenchyma of the submandibular gland in the adult male rat is self-ren ewing, with most newly formed acinar and granular duct cells believed to di fferentiate from the rapidly proliferating intercalated duct (ID) compartme nt. Since the LD cells are phenotypically diverse, based on their different expression of perinatal secretory proteins, we systemically injected triti ated thymidine for 24 hours, and followed the pattern of thymidine distribu tion in cells by autoradiography and immunocytochemistry of defined cellula r phenotypes over a 1-month chase period. Proliferating cells were found wi thin all parenchymal cell compartments; they were most numerous in ID, and primarily in those cells lacking immunoreactivity for the perinatal protein s SMG-B1, -C, and -D. The labeling index (LI) of the ID cells reached a pea k at 7 days postinjection, and then decreased over the next 3 weeks. Concur rently, the LI increased significantly in those cells at the junctions of I D with both acini and granular ducts, and also within these larger parenchy mal elements. We conclude that the ID cells not reactive for perinatal prot eins proliferate to expand the ID compartment, and that LD cells at the end s of the ducts differentiate into both acinar and granular duct cells. Our data provide no evidence for the di differentiation of ID cells into cells of striated ducts (SD); however, the small number of excretory duct (ED) pr ofiles seen in our preparations showed extremely high LI (>25%), suggesting that more extensive data might reveal a precursor role for the ED in repla cement of SD cells. In addition to the stepwise passage of cells from ID to other parenchymal elements at their junctions, the reported occurrence of occasional clusters of B1-positive acini (BAC) among the typical BI-negativ e acini had suggested an alternate pathway, in which entire segments of new ly expanded ID might develop directly into a recapitulated perinatal stage of BI-reactive cell, pursuant to becoming mature acinar cells. Consistent w ith this suggestion, the BAC had a fourfold greater LI than typical adult a cini; moreover, when analyzed by electron microscopic immunocytochemistry, they appeared similar to the novel perinatal Type III cells both ultrastruc turally and in their pattern of B1-immunogold labeling. In contrast, the le ss common acini showing a sublingual gland phenotype had no significant dif ference in LI from typical acinar cells. Overall, our results emphasize the importance of the nonimmunoreactive ID cells in normal cellular replacemen t, and the possibility that ID can undergo en bloc differentiation into rep lacement acini as well as incremental addition of single cells at the bound aries of ID with acini and with granular ducts. Anat Rec 263:202-214, 2001. (C) 2001 Wiley-Liss, Inc.