An unusual parotid gland in the tent-building bat, Uroderma bilobatum: possible correlation of interspecific ultrastructural differences with differences in salivary pH and buffering capacity.
The tent-building bat, Uroderma bilobatum, is a small, frugivorous phyllostomid bat with a broad neotropical distribution. Generally found in humid forest, this bat lives in small groups that create daytime "roosts" from large leaves of a variety of tropical plants. Fruit eating engenders a variety of ecological and physiological challenges for bats, some of which could require adaptive features in their salivary glands. The parotid salivary glands of Uroderma bilobatum were prepared for transmission electron microscopy by using methods that have become standard for field work. The parotid gland is extremely unusual in structure. Although the secretory endpieces still produce serous granules with a complex substructure, they are modified into quasi striated ducts. Their basal folds, which are extensive, occasionally harbor some vertically oriented mitochondria, imparting a resemblance to striated ducts. Other evidence for the endpiece origin of these parenchymal components is a well-developed system of intercellular canaliculi, structures that never occur in bona fide striated ducts. The long but sparse intercalated ducts consist of two types of cells, each of which elaborates a modest number of secretory granules of differing substructure. Striated ducts are of conventional morphology, except that a few dark cells shaped like wine glasses are present in their walls. The striated duct cells produce no secretory granules, but their apical cytoplasm may contain some small, empty vesicles. Capillaries lie in longitudinal grooves in the base of the duct cells, an arrangement that might enhance electrolyte exchange. Excretory ducts consist of simple cuboidal epithelium composed of cytologically unspecialized cells that sometimes includes a dark cell. It was concluded that salivary glands could have a major role in adapting species to acquire nutrients from marginal sources, such as tropical fruits, which have a low protein and sodium content. The unusual parotid acinar cells in Uroderma bilobatum are discussed in the context of salivary pH and buffering capacity. Comparisons are made with four other bat species, including an insectivorous species with a salivary pH > 8.0 and a very high buffering capacity, an intermediate species, and a fruit bat with acidic-stimulated saliva and very low buffering capability. Such interspecific comparisons provide a foundation for hypothesizing that ultrastructural features of the acinar cell basolateral membranes and intercellular canaliculi correlate with differences involving Na/H+ exchangers and release of HCO3- and, thus, are associated with the species differences that are important to diet and nutrient acquisition.