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 histoc hemistry of the accessory submandibular gland was examined in three sp ecies of long-winged bats. Methods: Specimens of Miniopterus schreiber si and M. magnator were live-trapped in Thailand, and of M. inflatus w ere live-trapped in Kenya. For electron microscopy, accessory submandi bular lands were initially fixed in triple aldehyde-DMSO, postfixed in osmium tetroxide, and embedded in Epon-Maraglas. A portion of the gla nds collected in Thailand (M. schreibersi and M magnator) was fixed in buffered formalin and embedded in paraffin. Sections of the latter ma terial were subjected to a battery of histochemical tests for glycocon jugates. Results: Although in all three species the accessory submandi bular glands have normal histological structure, the glands in two, M. schreibersi and M magnator, were distinguished by possessing mucous d roplets of unusual morphology. These droplets, whose identity as mucou s was confirmed by histochemical tests for glygoconjugates, are delimi ted by manifold membranes: up to 10 in M. schreibersi and fewer, but s till multiple, in M. magnator. In both species, the entire array of su rface membranes may fold inward in the fashion of mitochondrial crista e, forming packets of membranes, many of which have the spurious appea rance of floating free in the droplet matrix. These multipartite limit ing membranes appear to originate simply by Golgi saccules and moderat ely large, flattened Golgi vesicles repeatedly wrapping themselves aro und the surface of nascent mucous droplets. During exocytosis, the out ermost membrane of each mucous droplet contacts the luminal membrane, this barrier ruptures, then the remainder of the droplet-multiple memb ranes and matrix-either flow into the lumen or are cast out in tote. I n either case, a great deal of membrane phospholipid is added to the s aliva. This salivary lipid may permit these bats to consume insects th at normally are able to repel predators with chemical defenses that ma ke them unpalatable. The third species that we studied, M. inflatus, h as mucous droplets of normal appearance, i.e., they have only one limi ting 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 secretor y cells in general. (C) 1994 Wiley-Liss, Inc.