The Main Excretory Duct (Stensen's) of the Human Parotid Gland: A Transmission and Scanning Electron Microscope Study

The epithelial cells of the human parotid main excretory duct (Stensen) were studied by transmission (TEM) and scanning (SEM) electron microscopy through a variety of procedures that allowed the visualization of their three-dimensional microanatomy. Stensen's duct in humans is lined, in its distal portion, with a pseudostratified epithelium with tall principal cells and smaller basal cells, while the epithelium becomes progressively stratified cylindrically toward the oral stoma. Goblet cells are scattered among the other epithelial cells. The principal cells exhibit, on their lateral surfaces, numerous flattened laminar folds probably involved in transporting processes. A welldeveloped smooth endoplasmic reticulum intermingled with mitochondria occupies the cellular apices. Some vesicles are recognized on the cytoplasmic surfaces off the apical and lateral plasmalemma when cytoplasmic organelles are removed. All these features are interpreted as being involved in the process of endocytosis. In both TEM and SEM, the principal cells show a relevant number of irregular apical protrusions that may represent a kind of apocrine secretion. Thus, with regard to function, the human Stensen's duct seems to modify the composition of saliva by processes of resorption and secretion, the latter coming from goblet cells as well. The basal cells have a surface microanatomy completely different from that of principal cells. They exhibit, in fact, only sparse microvillosities and smooth areas on their lateral aspect, while their stromal surface is greatly augmented by irregular thin ramified processes. The role of basal cells is also discussed.