Mucous droplets with multiple membranes in the accessory submandibular glands of long-winged bats.

Background: Certain species of bats possess two sets of submandibular glands, namely, principal and accessory. The ultrastructure and histochemistry of the accessory submandibular gland was examined in three species of long-winged bats.
Methods: Specimens of Miniopterus schreibersi and M. magnator were live-trapped in Thailand, and of M. inflatus were live-trapped in Kenya. For electron microscopy, accessory submandibular lands were initially fixed in triple aldehyde-DMSO, postfixed in osmium tetroxide, and embedded in Epon-Maraglas. A portion of the glands collected in Thailand (M. schreibersi and M. magnator) was fixed in buffered formalin and embedded in paraffin. Sections of the latter material were subjected to a battery of histochemical tests for glycoconjugates.
Results: Although in all three species the accessory submandibular glands have normal histological structure, the glands in two, M. schreibersi and M. magnator, were distinguished by possessing mucous droplets of unusual morphology. These droplets, whose identity as mucous was confirmed by histochemical tests for glygoconjugates, are delimited by manifold membranes: up to 10 in M. schreibersi and fewer, but still multiple, in M. magnator. In both species, the entire array of surface membranes may fold inward in the fashion of mitochondrial cristae, forming packets of membranes, many of which have the spurious appearance of floating free in the droplet matrix. These multipartite limiting membranes appear to originate simply by Golgi saccules and moderately large, flattened Golgi vesicles repeatedly wrapping themselves around the surface of nascent mucous droplets. During exocytosis, the outermost membrane of each mucous droplet contacts the luminal membrane, this barrier ruptures, then the remainder of the droplet--multiple membranes and matrix--either flow into the lumen or are cast out in toto. In either case, a great deal of membrane phospholipid is added to the saliva. This salivary lipid may permit these bats to consume insects that normally are able to repel predators with chemical defenses that make them unpalatable. The third species that we studied, M. inflatus, has mucous droplets of normal appearance, i.e., they have only one limiting membrane.
Conclusions: The varying structure of mucous secretory products among the species of Miniopterus provides important clues as to the evolution of this genus as well as to the evolution of secretory cells in general.