Fine structure of excretory ducts of human salivary glands.

Excretory ducts of human major salivary glands are lined by an epithelium consisting of principal cells and by a discontinuous row of basal cells. The principal cells are tall and columnar with mitochondria, large lipofuscin granules and a central nucleus. Just beneath the plasmalemma bordering the lumen, their cytoplasm contains a number of small granules and vesicles similar to those observed in cells of striated ducts. Both in TEM and SEM, these cells also show large apical protrusions devoid of cytoplasmic organelles that may represent a kind of apocrine secretion. The cytoarchitecture of the principal cells seems to be at variance with that of cells of striated ducts. First, the cell body remains unique and does not split into major basal processes. Second, these cells usually lack the long laminated basal folds, housing vertically aligned mitochondria, that are typical of striated ducts. Instead, below the smooth area occupied by the junctional complexes, the lateral cell surfaces are completely covered by a great number of short irregular processes. These organelle-free folds are apparently involved in the mechanism of ion transport since, at their level, there is a strong reactivity for the transporting enzyme K(+)-pNPPase. The basal cells, which are small and cuboidal, have a dense and filamentous cytoplasm. Their functional role is still uncertain.