THE ANATOMICAL RECORD 248:164–175 (1997) Ultrastructure of the Unusual Accessory Submandibular Gland in the Fringe-Lipped Bat, Trachops cirrhosus BERNARD TANDLER,1 TOSHIKAZU NAGATO,2 AND CARLETON J. PHILLIPS3* 1Department of Oral Anatomy II, Kyushu Dental College, Kitakyushu, Japan 2Second Department of Oral Anatomy, Fukuoka Dental College, Fukuoka, Japan 3Department of Biological Sciences, Illinois State University, Normal, Illinois ABSTRACT Background: The phyllostomid fringe-lipped bat, Trachops cirrhosus, is sui generis (in a family of ca. 138 species) in that it subsists in part on tropical frogs. These amphibians frequently possess highly toxic integument. We examined the salivary glands of this bat to determine if these glands could be the source of protective factors that permit consumption of seemingly unsavory prey. The parotid and principal salivary glands of this bat are similar to homologous glands in other phyllostomids, but the accessory submandibular gland is unique. Methods: The accessory submandibular glands of live-trapped T. cirrhosus were fixed and processed for transmission electron microscopy by conventional means. Results: The accessory submandibular gland consists of follicles and ducts. The principal cells of the follicular walls have an abundance of rough endoplasmic reticulum (RER), free ribosomes, and extensive Golgi apparatuses. Typically, these cells have relatively few serous secretory granules. The cells contain collections of peculiar lipid droplets, and some of their mitochondria have dense crystalloids within expanded cristae. A layer of irregular, moderately dense bodies lies immediately subjacent to the luminal plasmalemma; it is not clear if these structures are endocytotic or exocytotic. Clusters of mucous cells, some of which have a single, hugely distended RER cisterna, are ensconced in the follicular walls; mucus from these cells reaches the lumen via intercellular canaliculi. Ducts progress from simple cuboidal to simple columnar epithelium. They lack basal striations, and their constituent cells contain relatively few mitochondria. Follicles and ducts have numerous myoepithelial cells at their periphery, and both are heavily innervated by hypolemmal nerve terminals. Conclusions: The unusual accessory submandibular gland in T. cirrhosus documents the extreme modifications in gland histology and in cell ultrastructure that have occurred in mammalian families. The cells composing the follicle walls and ducts bear little similarity to typical acinar or duct cells. Duplication of the submandibular gland in some bat lineages might be the key innovation underlying such plasticity. The heavy innervation of both follicles and ducts also implies that these structures are sensitive to and capable of responding to various inputs, perhaps including dietary factors. Anat. Rec. 248:164–175, 1997. r 1997 Wiley-Liss, Inc. Key words: salivary glands; follicles; lipid droplets; mitochondrial crystalloids; hypolemmal nerves; frogs Salivary glands may be the most diversified organs in mammals. This diversification involves histological organization, cell structure, secretory and transport processes, and gene regulation (Phillips and Tandler, 1987; Phillips et al., 1993) and is important for two principal reasons. First, most organ systems and, more specifically, most cell types are relatively conservative. Conservatism in cell structure and cellular processes is parar 1997 WILEY-LISS, INC. doxical in view of the evolutionary plasticity seemingly required to achieve the diversity found within major groups of animals (Gerhart, 1995). An unraveling of *Correspondence to: Carleton J. Phillips, Department of Biological Sciences, Felmley Hall, Room 206, Illinois State University, Normal, IL 61761-6901. Received 23 October 1996; accepted 11 December 1996. BAT ACCESSORY SUBMANDIBULAR GLAND this paradox requires an appreciation of the balance between conservation and plasticity in cells, which can be gained through comparative data on the patterns and types of diversification that have occurred in evolutionary lineages. Second, the multifarious biological roles of mammalian salivary glands place them at the interface between organism and environment (Phillips and Tandler, 1996). Thus, it has been argued that an understanding of salivary glands in the context of diet, dentition, and behavior could lead to greater comprehension of adaptation (Phillips, 1996). Accessory submandibular glands are noteworthy because they are common in bats and because they have the potential for great plasticity. Previously, we found that the accessory submandibular glands of the phyllostomid fringe-lipped bat, Trachops cirrhosus, display extraordinary follicular histologic organization (Phillips et al., 1987; Tandler et al., 1996). The fringe-lipped bat is unusual because it is the only neotropical bat (even among the ca. 138 species in its family) that feeds almost exclusively on frogs. Because most tropical frogs have toxic integument, we hypothesized that the unusual accessory submandibular glands in T. cirrhosus might be associated with diet. To test this idea, we examined three additional species of Asian and African bats known to include frogs in their diet and found similar histological features in their accessory submandibular glands (Phillips et al., 1987; Phillips and Tandler, 1996a). Because T. cirrhosus originated and evolved independently of the Old World species with similar diets, we concluded that the follicular salivary glands are an example of convergent evolution associated with adaptation to a specialized carnivorous diet (Phillips et al., 1993). The accessory submandibular gland of T. cirrhosus thus appears to be a model that can be employed to delineate the kinds of ultrastructural diversification that can occur in salivary glands in correlation with diet. MATERIALS AND METHODS Three specimens, two males and one female, of adult fringe-lipped bats, T. cirrhosus, live-trapped in Suriname, were anesthetized with T-61 euthanasia solution and their salivary glands extirpated. Tissues were fixed in triple aldehyde-DMSO (Kalt and Tandler, 1971) as modified by Phillips (1985). After postfixation in osmium tetroxide, specimens were soaked overnight in acidified 0.25% uranyl acetate (Tandler, 1990). Thorough rinsing in distilled water was followed by dehydration in ethanol, passage through propylene oxide, and embedment in Epon-Maraglas (Tandler and Walter, 1977). Thin sections were stained with acidified methanolic uranyl acetate (Tandler, 1990) followed by lead tartrate (Millonig, 1961) or lead citrate (Venable and Coggeshall, 1965) and examined in a Siemens 101 or JEOL 1200EX electron microscope. Semithin sections were stained with methylene blue-azure II (Richardson et al., 1960) and examined in an Olympus Vanox. RESULTS The accessory submandibular gland of Trachops consists of large, folliclelike structures corresponding to the secretory endpieces of conventional mammalian salivary glands and of ducts. Both of these structures 165 are shown to advantage in Figure 1, a light micrograph of an epoxy-embedded gland. In a low magnification electron micrograph (Fig. 2), the secretory follicles are seen to consist largely of cells with a rich cytoplasm and which exhibit a band of moderately dense material in their subluminal zones. Occasional mucous cells are inserted in the follicle walls, and myoepithelial cells also may be present; both of these features aid in distinguishing the accessory submandibular gland from the look-alike thyroid gland. The principal cells of the follicular walls have a fairly high cytoplasmic density. They contain a few scattered secretory granules and have all of the cytoplasmic equipment of secretory cells in general, yet they do not at all look like typical protein-secreting cells, even at moderate magnifications (Fig. 3). Lipid droplets of peculiar appearance are abundant in the perinuclear cytoplasm (Figs. 3 & 4). These have a moderately dense, ragged inclusion amid a collection of irregular lamellae. The secretory cells have a modest number of mitochondria of typical structure. Some of these organelles contain what seem to be dense rods ensconced within expanded cristae (Fig. 5). At high magnification, these rods are seen to consist of crystalloids with a 6.2-nm periodicity (Fig. 6). When the secretory cells are viewed at higher power, they are seen to possess an extensive rough endoplasmic reticulum (RER) in the form of cisternal stacks or a network of tubules and numerous free ribosomes (Fig. 7). Because of the relatively high density of the cytoplasm resulting from the ubiquitous ER and free ribosomes, the Golgi apparatuses usually are difficult to detect unless their saccules are sectioned diagonally, in which case the intrasaccule spaces are exaggerated and the organelle stands out in a sort of ‘‘negative image’’ (Fig. 8). Some Golgi apparatuses are surrounded by small, dense granules (pro-granules?), presumably secretory in nature (Fig. 9). These apparently increase in size, perhaps by fusion, to form mature secretory granules of high density (Fig. 10) that measure about 0.9 µm in diameter. Immediately below the apical membrane are many irregular membrane-delimited bodies whose matrix exactly matches the luminal content in density (Fig. 11). These bodies may contain one or several inclusions in the form of a twisted, lucent ribbon edged by a linear density (Fig. 12) or else may contain a small lipid droplet. Such bodies sometimes are seen open to the lumen (Fig. 13). Many lucent lipidlike bodies are present in the follicular matrix (Fig. 14); these bodies may represent lipid of principal cell origin that was liberated into the lumen by an apocrine process. Because the luminal droplets are considerably larger than the cytoplasmic ones, the latter probably undergo a degree of fusion once in the lumen. Gap junctions are fairly common between adjacent principal cells. These tend to be much larger than run-of-the mill gap junctions, often measuring up to 3 µm in length and presumably in diameter (Fig. 15). The occasional mucous cells that make up a small part of the follicle walls are characterized by the presence of numerous mucous droplets, which display the familiar propensity of such secretory products to fuse with one another (Tandler, 1993b; Fig. 16). These droplets, which have a diameter of approximately 1 µm, 166 B. TANDLER ET AL. Fig. 1. Photomicrograph of an epoxy-embedded accessory submandibular gland in T. cirrhosus. Two conjoined follicles (F) contain lightly stained material in their lumina. Note the layer of light bodies just beneath the luminal border of the follicular cells. D, duct. The arrow points to a pair of mucous cells. Methylene blue-azure II. 3650. Fig. 2. Survey electron micrograph of a small follicle. The lucent bodies in both the follicular lumen and cells represent lipid droplets. At the lower right corner, a mucous cell is housed in the follicular wall. An extremely dense myoepithelial cell (MEC) is at the left of the follicle. A duct is at the upper left. 32,800. are of very low density and for the most part are structureless, but the few droplets that exhibit a compound substructure seem to consist of two hemispheres of slightly differing texture and density (Fig. 17). Many of the mucous cells contain a single infra- or para- nuclear, highly dilated RER cisterna filled with dense, structureless material (Figs. 18 & 19). Such an expanded cisterna may be larger than the cell nucleus. Almost no mucous cells reach the lumen of the follicles; instead, these cells are in direct contact with at least BAT ACCESSORY SUBMANDIBULAR GLAND Fig. 3. Survey electron micrograph of a portion of a follicular wall. Although the cells contain a good deal of RER, they do not resemble typical secretory cells; only a few dense secretory granules are present in the apical cytoplasm. Lipid droplets of peculiar morphology are abundant in the perinuclear cytoplasm. A dense myoepithelial cell is present at the lower border of the micrograph. 35,000. 167 Fig. 4. Cytoplasmic lipid droplets seen at a higher magnification. These consist of lamellae and a ragged inclusion of moderate density. 317,100. Fig. 5. A follicular cell mitochondrion that contains three dense intracristal crystalloids. 348,000. Fig. 6. Mitochondrial crystalloids at a high magnification. They consist of lamellar densities with regular spacing. 3200,000. 168 B. TANDLER ET AL. Fig. 7. The base of a follicular cell showing its well-organized and extensive RER. 314,200. Fig. 8. The Golgi region of a follicular cell. This organelle ordinarily is masked by the abundant ribosomes and small elements of RER that impart a high density to the cytoplasm. In this micrograph, however, the Golgi apparatus has been sectioned diagonally, an orientation that exaggerates its intrasaccular spaces, so that the obviously extensive organelle stands out in a sort of pseudonegatively stained background. A few dense secretory granules are seen in relation to the Golgi apparatus. 39,500. Fig. 9. A secretory granule in a follicular cell. It is homogeneously dense and is delimited by a single unit membrane. Based on its morphology, this is a serous granule, a designation confirmed by histochemistry. 384,000. Fig. 10. A maturing secretory granule associated with a Golgi apparatus. Many small dense bodies are in this region. It is not clear if these are pro-granules that will either coalesce to form further granules or be added to the prospective granule, or if they have some other fate. 329,300. Fig. 11. The apical cytoplasm of adjacent follicular cells illustrating the layer of irregular, moderately dense bodies that underlie the luminal plasmalemma. 36,400. Fig. 12. The apical irregular bodies at higher magnification. Two bodies contain a lucent, twisted ribbon that sporadically is edged with a linear density. 323,800. Fig. 13. The apex of a follicular cell. The direction of movement of organic material cannot be gauged with certainty. In the center is what appears to be an erumpent secretory granule. It is flanked by irregular apical bodies that are open to the lumen, but it is not clear whether the lucent ribbons are being released or being taken up by these bodies. 323,800. Fig. 14. A lipidlike body in the follicular matrix. It is edged in part by a thin layer of dense material. Such droplets probably gain access to the lumen by apocrine secretion. 319,600. Fig. 15. A gap junction of unusual length between two follicular secretory cells. 334,000. 170 B. TANDLER ET AL. one intercellular canaliculus, the latter structures apparently serving as conduits for mucus (Fig. 20). The assorted secretory materials (mucus, lipids, protein, lucent ribbons, and membranes) plus cell detritus that end up in the follicular lumina as a result of apocrine or merocrine secretion, or both, often undergo degenerative changes leading to the formation of large luminal agglomerations that consist of dense lamellae and various kinds of vesicles and vacuoles (Fig. 21). These inclusions apparently follow the saliva during its discharge from the gland. The walls of the secretory follicles are very highly innervated, with clusters of hypolemmal nerve terminals abounding (Fig. 22). Nerve clusters are composed of naked axons and varicosities laden with clear synaptic vesicles; a smaller number of larger, dense cored vesicles also may be present (Fig. 23). Myoepithelial cells are arranged around the periphery of each follicle, where they are easily distinguishable from secretory cells by virtue of their high cytoplasmic and nuclear density. These cells continue onto the ducts unchanged in morphology and are described in connection with those structures. Proximal ducts in the Trachops accessory submandibular gland consist of simple cuboidal epithelium (Fig. 24); more distal ducts consist of simple columnar epithelium (Fig. 26). In both, the duct cells have cytoplasm that is considerably less dense than that of the secretory cells. Duct cell cytoplasm contains scattered cisternae of RER, many polysomes, and some elliptical mitochondria. The apical cell surface, which generally is devoid of microvilli, frequently is underlaid by numerous small vesicles and tubular elements of ER. Hypolemmal nerve terminals are as abundant in the duct walls as they are in the secretory follicles. Where a duct cell overlies a myoepithelial cell or process, its basal surface may have a number of folds, but these are more slender than those in typical striated ducts. The ductular (and follicular) myoepithelial cells appear to be typical in shape (Fig. 25). Flattened in a vertical direction, they consist of a perikaryon that contains the applanate nucleus, the usual cytoplasmic organelles, and an array of myofilaments. These cells have long, tapering processes that clasp the contiguous parenchymal structure, be it duct or follicle; the ‘‘grip’’ of the myoepithelial cell is maintained by desmosomes that attach this cell to the adjacent epithelial cell. Nerve terminals frequently occur between a myoepithelial cell perikaryon or process and the neighboring duct or secretory cell. Such terminals also are abundant even when no myoepithelial component is present. We encountered one cell in a duct wall that contained a plenitude of small, dense granules, giving it the general appearance of an APUD cell. In addition, this cell had a few prominent microvilli with extremely long rootlets, a structural feature reminiscent of brush cells. This granular cell had an apical dome that bulged into the duct lumen. DISCUSSION That some bats possess a set of accessory submandibular glands has been known at least since the work of Robin (1881). Robin’s nomenclature was based on size and position of the respective organs, with the larger inferior one being called the principal gland and the smaller superior one being called the accessory gland. In our continuing large-scale study of the comparative ultrastructure of bat salivary glands (Tandler et al., 1990; Phillips et al., 1993), we have encountered many chiropteran species having two pairs of submandibular glands. Rather than basing our labels on gross anatomical properties of the organs, we have used microscopically based criteria to sort out principal from accessory gland. Our convention is to call that gland that exhibits typical submandibular gland morphology the principal gland, with the accessory being the gland that differs at least in some respects from typical submandibular glands. Although in a few cases the accessory gland structurally is virtually indistinguishable from the principal gland (even though it usually shows histochemically detectable differences in glycoconjugates; Pinkstaff et al., 1982), the usual case is that the accessory gland exhibits obvious structural (and presumably functional) differences from conventional glands. The accessory submandibular glands described here constitute the most extreme departure from the typical major salivary gland histology yet reported. The serous secretory cells of Trachops are extremely unusual. Although they have abundant RER and free ribosomes, they contain relatively few typical secretory granules. According to Pinkstaff (1993), these granules are more perfectly serous in nature than any other granules he had encountered; in other words, they contain no glycoconjugates whatsoever. The irregular bodies at the cell apex may be involved in the uptake of luminal material. This possibility would account for the presence within these structures of occasional lipid droplets and ribbonlike lucent laminae, which are quite abundant in the colloidlike material. Such activity is reminiscent of the action by thyroid gland cells that endocytose thryoglobulin for further processing. The possibility that these bodies may be involved in exocytosis of secretory material cannot be dismissed out of hand. If they do in fact release material into the lumen, they would constitute a novel form of secretory granule. Some cellular components, especially the unusual lipid droplets, appear to be released into the lumen by an apocrine process. The scattered mucous cells also discharge mucus into the follicles. Once liberated into the lumen, the secretory material forms a matrix in which are suspended spicular aggregates of laminae, membranes, and lipid droplets. The stored secretory material in the follicles probably is discharged into the duct system by contraction of the numerous myoepithelial cells positioned at the follicular periphery and then hastened on its way by contraction of the ductal myoepithelial cells. Striated ducts constitute a significant segment of the excurrent duct system in typical submandibular and parotid glands in almost all mammals (Tandler, 1993b). These ducts consist principally of tall columnar epithelial cells whose bases are interlocked in a highly complex fashion (Riva et al., 1993) to form a series of basal compartments in which elongated mitochondria reside, the folded plasma membranes and vertically oriented mitochondria being responsible for the ‘‘basal striations.’’ These ducts, working in concert with the excretory ducts (Schneyer et al., 1972), are thought to be major sites of electrolyte resorption from the isotonic BAT ACCESSORY SUBMANDIBULAR GLAND Fig. 16. A cluster of mucous cells in a follicular wall. These cells rest on the follicular basement membrane but almost never reach the follicular lumen. 35,900. Fig. 17. Mucous droplets at a higher magnification. Although most of the droplets have a farinaceous content, granules cut in the appropriate plane (asterisks) show two distinct hemispheres of slightly differing texture and density. 331,200. 171 Fig. 18. An RER cisterna in a follicular mucous cell. The cisterna is grossly distended with structureless dense material. 310,500. Fig. 19. The edge of a distended cisterna in a mucous cell showing the attached ribosomes that indelibly mark it as an element of the RER. 357,000. Fig. 20. Mucous cell bordering an intercellular canaliculus (ICC). Mucus reaches the follicular lumen via such pathways. 316,800. 172 B. TANDLER ET AL. Fig. 21. An aggregate of degenerated cellular material in the follicular lumen. It consists of dense lamellae (a form of myelin?) and clear and dense-cored vesicles. 313,200. Fig. 22. The periphery of a follicle showing several clusters of hypolemmal nerve terminals, including axons and varicosities. 311,500. primary saliva, so that the final saliva that reaches the mouth is hypotonic. Saliva from the human sublingual glands (Riva et al., 1988) or labial minor salivary glands (Tandler et al., 1970), both of which have a paucity of striated ducts, is more or less isotonic. Although the accessory submandibular gland in T. cirrhosus has a prominent duct system, striated ducts Fig. 23. Higher magnification of a cluster of follicular hypolemmal nerve terminals. These varicosities contain mostly clear synaptic vesicles, with just a few dense-cored vesicles apparent. 328,500. are noteworthy by their absence. The ducts that are present have no basal striations, and their complement of mitochondria seems rather skimpy. Moreover, the ducts lack any secretory activity (unless the apical, empty-appearing vesicles are involved in secretion), and there is no evidence of endocytotic activity at the luminal surface. At the same time, the ducts are heavily BAT ACCESSORY SUBMANDIBULAR GLAND 173 Fig. 24. Survey electron micrograph of a small duct. Mitochondria are randomly distributed in the duct cells. Hypolemmal nerve terminals are indicated by the arrows. The duct lumen contains lipid droplets and oddments of cell debris, perhaps the result of a form of apocrine secretion taking place in the follicles. 33,100. Fig. 25. The perikaryon of a myoepithelial cell in a small duct. The arrows indicate several hypolemmal nerve terminals in the duct wall. 311,500. innervated, suggesting that these ducts may play an as yet indecipherable role in modifying the saliva of the accessory glands. From an evolutionary perspective, the accessory submandibular gland in T. cirrhosus calls attention to three important issues: (a) the significance of duplication of organs, (b) the relationship between salivary gland structure and adaptation to specialized diets, and (c) the potential for rapid diversification in secretory cell structure and cellular processes. The origin of binary salivary glands is of some interest. Such twin organs are not necessarily the result of a long evolutionary descent but may arise quite suddenly in evolutionary terms. For example, the bilaterally paired submandibular glands in human beings normally are single organs. Recently, however, a case has been reported of a duplicated human submandibular gland, each moiety having it own main excretory duct (Codjambopoulo et al., 1992). Other instances of duplication of the main excretory duct of the same gland have been reported (Myerson et al., 1966; Towers, 1971). If such duplications are not simple developmental (epigenetic or somatic) anomalies but are genetically based in the germ line and are functionally innocuous, they could eventually spread throughout a population. In the case of bats, accessory submandibular glands are phylogenetically widespread in the suborder Microchiroptera. Thus, such glands seem to be symplesiomorphic, i.e., share ancestral features. Given the importance of diet and the great diversification of dietary habits in the microchiropteran bats (Phillips et al., 1993), these equally diversified accessory salivary 174 B. TANDLER ET AL. Fig. 26. A cell in a larger duct. It contains a modest number of mitochondria, which display no preferential orientation, and some scattered, small, dense granules. There is a network of smoothsurfaced tubular reticulum immediately below the luminal membrane. 39,700. glands might have been an innovative key to chiropteran adaptive radiation. In the case of T. cirrhosus, we think that the unique accessory submandibular gland is related to feeding on tropical frogs. Typically, the tropical anuran integument is a storehouse of amines, peptides, and alkaloids; this armamentarium of potentially toxic compounds provides a chemical defense against predation (Phillips et al., 1993). The fringe-lipped bat can learn to recognize different species of frogs on the basis of their vocalizations, thus permitting the bats to select the least toxic prey (Tuttle and Ryan, 1981). Nevertheless, the frogs that they end up eating still present a potential toxicological problem, so it would not be surprising to find either gastric or salivary modifications that permit the consumption of these normally inedible frogs. In terms of stomach morphology, T. cirrhosus is similar to other carnivorous phyllostomid bats in having a well-developed zone of mucous cells and numerous mucous neck cells in its gastric glands (Studholme et al., 1986). However, the absence of observable specialization of gastric morphology points the finger at the accessory submandibular gland as the more important organ in the evolution of frog-eating in these bats. Our theory of cells as the centerpiece of adaptation complexes (Phillips, 1996) calls attention to the importance of extracellular signaling, which provides the basis for cell interaction with the environment. The exceptional innervation of the accessory submandibular glands in T. cirrhosus provides morphological evidence of the potential responsiveness of these glands. At present, we do not know whether the remarkable number of nerve terminals is a reflection of multiple independent pathways, is related to a need for multiple impulses in short time frames, or both. The principal cells in the walls of the follicles probably were derived from ancestral acinar cells, possibly from intercalated duct cells, or from both. In any case, the ultrastructure of the principal follicular cells provides a measure of the extent to which ‘‘typical’’ cells can become diversified. For all practical purposes, the follicle wall cells are unrecognizable as acinar or ductal cells. Because they are unique to this species of bat, they possibly originated with this monotypic genus. LITERATURE CITED Codjambopoulo, P., I. Ender-Griepekoven, and H. Broy 1992 Bilaterale Doppelanlage der Glandula submandibularis und des Submandibularisganges. Fortschr. Röntgenstr., 157:185–186. Gerhart, J. 1995 Summing up: conservation and diversification in metazoan eukaryotic cells. Phil. Trans. R. Soc. Lond. B, 349:333– 336. Kalt, M.R., and B. Tandler 1971 A study of fixation of early amphibian embryos for electron microscopy. J. Ultrastruct. Res., 36:633–645. Millonig, G. 1961 A modified procedure for lead staining of thin sections. J. Biophys. Biochem. Cytol., 11:736–739. BAT ACCESSORY SUBMANDIBULAR GLAND Myerson, M., E.S. Crelin, and H.W. Smith 1966 Bilateral dupication of the submandibularis ducts in a patient with a sublingual dermoid cyst. Arch. Otolaryngol., 83:588–590. Phillips, C.J. 1985 Field fixation and storage of museum tissue collections suitable for electron microscopy. Acta Zool. Fennica, 170:87–90. Phillips, C.J. 1996 Cells, molecules, and adaptive radiation in mammals. In: Contributions in Mammalogy: A Memorial Volume Honoring Dr. J. Knox Jones, Jr. R.J. Baker, H.H. Genoways, eds. Museum Texas Tech University, Lubbock, pp. 1–24. Phillips, C.J., and B. Tandler 1987 Mammalian evolution at the cellular level. Curr. Mammal. 1:1–66. Phillips, C.J., and B. Tandler 1996 Salivary glands, cellular evolution, and adaptive radiation in mammals. Eur. J. Morphol., 34:155– 161. Phillips, C.J., and B. Tandler 1997 Follicular architecture of the accessory submandibular gland in the African false vampire bat, Cardioderma cor. J. Mammal., in press. Phillips, C.J., B. Tandler, and C.A. Pinkstaff 1987 Unique salivary glands in two genera of tropical microchiropteran bats: An example of evolutionary convergence in histology and histochemistry. J. Mammal. 68:235–242. Phillips, C.J., B. Tandler, and T. Nagato 1993 Evolutionary divergence of salivary gland acinar cells: A format for understanding molecular evolution. In: Biology of the Salivary Glands. K. DobrosielskiVergona, ed. CRC Press, Boca Raton, pp. 39–80. Pinkstaff, C.A. 1993 Serous, seromucous, and special serous cells in salivary glands. Microscopy Res. Tech., 26:21–31. Pinkstaff, C.A., B. Tandler, and R.P. Cohan 1982 Histology and histochemistry of the parotid and principal and accessory ubmandibular glands of the little brown bat. J. Morphol., 172:271–285. Richardson, K.C., L. Jarret, and E.M. Fink 1960 Embedding in epoxy resins for ultrathin sectioning in electron microscopy. Stain Technol., 35:313–323. Riva, A., B. Tandler, and F. Testa Riva 1988 Ultrastructural observations on human sublingual glands. Am. J. Anat., 181:385–392. 175 Riva, A., L. Valentino, M.S. Lantini, A. Flores, and F. Testa Riva 1993 3-D structure of cells of human salivary glands as seen by SEM. Microscopy Res. Tech., 26:5–20. Robin, H. 1881 Recherches anatomiques les mammiferes de l’Order des Chiroptéres. Ann. Sci. Natl. Zool., 12: 1–180. Schneyer, L.H., J.A. Young, and C.A. Schneyer 1972 Salivary secretion of electrolytes. Physiol. Rev., 52:720–777. Studholme, K.M., C.J. Phillips, and G.L. Forman 1986 Results of the Alcoa Foundation Suriname Expeditions. X. Patterns of cellular divergence and evolution in the gastric mucosa of two genera of phyllostomid bats, Trachops and Chiroderma. Ann. Carnegie Mus., 55:207–235. Tandler, B. 1990 Improved uranyl acetate staining for electron microscopy. J. Electron Microsc. Tech., 16:81–82. Tandler, B. 1993a Structure of mucous cells in salivary glands. Microsc. Res. Tech., 26:49–56. Tandler, B. 1993b Structure of the duct system in mammalian major salivary glands. Microsc. Res. Tech., 26:57–74. Tandler, B., and R.J. Walter 1977 Epon-Maraglas embedment for electron microscopy. Stain Technol., 52: 238–239. Tandler, B., C.R. Denning, I.D. Mandel, and A.H. Kutscher 1970 Ultrastructure of human labial salivary glands. III. Myoepithelium and ducts. J. Morphol., 130:227–246. Tandler, B., C.J. Phillips, T. Nagato, and K. Toyoshima 1990 Ultrastructural diversity in chiropteran salivary glands. In: Ultrastructure of the Extraparietal Glands of the Alimentary Tract. A. Riva and P. Motta, eds. Kluwer Academic Publishers, Boston, pp. 31–52. Tandler, B., C.J. Phillips, and T. Nagato 1996 Histological convergent evolution of the accessory submandibular gland in four species of frog-eating bats. Eur. J. Morphol., 34:163–168. Towers J.F. 1971 Duplication of the submandibular salivary duct. Oral Surg. Oral Med. Oral Pathol., 44:326. Tuttle, M.D., and M.J. Ryan 1981 Bat predation and the evolution of frog vocalization in the Neotropics. Science, 214:677–679. Venable, J.H., and R.Coggeshall 1965 A simplified lead citrate stain for use in electron microscopy. J. Cell Biol., 25:407–408.