Ultrastructure of the angularis oris salivary gland in the house sparrow

The angularis oris gland, an avian salivary gland that lies beneath the zygomatic arch, was studied in the house sparrow by means of light and electron microscopy. The gland consists of lobules composed of epithelial tubules and folds on a delicate connective tissue framework. The lobules are drained by large channels to form an intraglandular duct, which becomes the main extraglandular duct. The tubules, folds, and all ducts are covered by pseudostratified epithelium that shows progressive changes from the blind ends of tubules to the duct system. At the closed end of the tubules, the tall cells contain an aggregation of closely packed mucous droplets; their mitochondria are long and slender. As the epithelium nears the duct system, the mucus content of the tall cells is reduced and the mitochondria, which are increased in number, are ovoid. The lateral borders of the cells are complexly interlocked. In the ducts, mitochondria-rich cells with extensive lateral interfoliation comprise the major cell type; some of these cells contain a few mucous droplets at their apices. The basal cells of the pseudostratified epithelium have the same basic structure regardless of their location. These pyramidal cells have abundant cytofilaments and are joined to the basal lamina by many hemidesmosomes. Despite the absence of clear-cut morphological zonation, the angularis oris may function in a manner that is analogous to mammalian salivary glands: secretory cells produce the organic component of the saliva, mucus, while the mitochondria-rich cells with their extensive lateral membranes probably resorb electrolytes to render the final saliva hypotonic.