Ultrastructure and microvasculature of the parotid duct in the cat

Ultrastructure and microvascular pattern of the parotid duct (main excretory duct) in the cat were investigated by TEM and SEM. The duct was lined by a pseudostratified epithelium which was surrounded by a double-layered vascular sheath. Three kinds of principal cells were observed in this epithelium--tall columnar, light and basal cells. The tall columnar cells did not bear basement membrane specialization (infoldings) but well-developed lateral interdigitations. Microvilli were present on their luminal surface. Basal infoldings and lateral interdigitations did not develop in the light cells. Basal cells were located on the basement membrane of the epithelium. The basal surface of the duct epithelium was studded with half-desmosomes. The external sheath of the double-layered vascular sheath was composed of arterioles and venules, and the inner sheath was formed by a capillary network immediately beneath the epithelium. Meshes of the capillary network in the collecting ducts and the region of origin appeared round and in the superficial and deep regions became elongated along the duct axis. It can be concluded that the parotid duct in the cat not only transports the saliva into the oral cavity, but also actively builds up the primary saliva to the final saliva, owing to absorptive and secretory functions which were made by an important contribution of the double-layered vascular sheath surrounding the whole length of the duct.