Ultrastructure of the parotid salivary glands in seven species of fruit bats in the genus Artibeus

Abstract

Background: In previous studies, we determined that the submandibular glands of five species of Neotropical fruit bats in the genus Artibeus had seromucous granules in their demilune cells with substructures that varied interspecifically in accordance with systematic relationships. Moreover, the striated ducts in these frugivores exhibited structural modifications that apparently are related to the consumption of a diet rich in potassium, but deficient in sodium. We now turn our attention to the parotid gland in a large number of species in this genus to determine if it follows the same structural pattern as does the submandibular gland.

Methods: Members of seven different species of Artibeus were live-trapped in various Neotropical locations. The parotid glands were extirpated from euthanized bats, fixed in the field, and prepared for electron microscopic examination by conventional means. The parotid glands in all seven species were virtually identical in morphology. The acinar cells (determined to be seromucous on the basis of ultrastructural criteria) contain large numbers of what appear to be vacuoles, but which are a type of secretory granule. These granules have an electron-lucent matrix and may contain one or several circular membranous profiles arranged either concentrically or in a random array. These granules appear to form by progressive dilatation of the termini of Golgi saccules, with the nascent granules finally severing their connection with the Golgi apparatus. Many of the internal membranous profiles are formed simply by invaginations of the limiting membrane of the granule; others may result from indentation of the limiting membrane by protrusions from adjacent granules; the source of multiple internal membranes in certain granules is unclear. The exocytosis of these granules results in the acinar and intercalated duct lumina being filled with an abundance of membranous material. Such extruded membranes are present in some striated ducts, but not in others, suggesting that they are degraded during passage through the duct system. The striated ducts are of conventional appearance, lacking the frondose processes that are prominent in the submandibular glands of Artibeus.

Conclusions: The parotid gland in Artibeus shows none of the interspecific ultrastructural variability that characterizes the submandibular gland in bats of this genus. The seromucous acinar cells secrete granules that release phospholipids as well as glycoconjugates into the saliva. Based on the lack of frondose processes with their sodium-transporting portasomes, the striated ducts of the parotid gland are less concerned with electrolyte homeostasis than are those in the submandibular gland.