A morphogenetic concept of salivary duct regeneration and metaplasia.

The exact mechanisms of physiological regeneration and of metaplastic processes of the salivary duct have not been definitely established, although regeneration from a putative uncommitted stem cell population has long been favored. In the present study, double immunohistochemical labeling for Ki 67 and alpha-actin or different cytokeratin subtypes, respectively, made possible an exact localization and quantification of cellular proliferation in the regular salivary duct and in different types of metaplasia. Our data demonstrate a baseline proliferative capacity in all five cell types of the salivary duct. Luminal secretory cells of the acinus and intercalated duct regenerate independently from myoepithelial or basal cells. In contrast, the renewal of oxyphilic cells in the striated and excretory duct is maintained by proliferation and differentiation of basal cells. The great majority of metaplasias develops from uncommitted, Bcl-2 positive basal cells of striated/excretory ducts which possess an enormous capacity for pluridirectional morphogenetic differentiation. Despite this important role of basal cells, our findings demonstrate that all cell types principally have to be considered as potential progenitor cells for salivary gland tumors. The improved insight into regenerative and metaplastic processes of the salivary duct may contribute to a better understanding of the complex formal carcinogenesis.