THE ANATOMICAL RECORD 229:482-488 (1991)
Ultrastructure of the Sublingual Gland in the African MuItimammate Rodent KUNIAKI TOYOSHIMA AND BERNARD TANDLER Department of Oral Biology, School of Dentistry, Case Western Reserve University, Cleveland, Ohio
ABSTRACT The sublingual gland of Praomys natalensis, a n African rodent that is phenotypically and cytogenetically intermediate to mice and rats, is a mixed gland, consisting of mucous acini that are capped by serous demilunes, of intercalated ducts, and of some short striated ducts that quickly become excretory ducts. The mucous cells are typical in appearance, with lucent granules that contain a n assortment of scattered vermiform or particulate densities. The serous cells display a n array of secretory granules with a highly unusual substructure. Rather than a pattern based on the manner in which light and dark regions are disposed in their matrix, these granules contain packets-some furled, some flat-of membranes that exhibit a pronounced axial periodicity of -5 nm. Intercalated ducts are simple in structure, with no obvious morphological specializations. Striated ducts resemble those in the salivary glands of less exotic rodents, but they and the excretory ducts often have clusters of cytoplasmic crystalloids consisting of linear densities that intersect at right angles and that have a periodicity in both directions of -12 nm. Unlike the parotid and submandibular salivary glands, the ultrastructure of sublingual glands has seldom been studied. The latter organs have been examined in detail in selected mammals: European hedgehogs (Tandler, 1986); mice (Laj et al., 1971; Guimaraes et al., 1979); rats (Leeson and Booth, 1961; Enomoto and Scott, 1971; Kim et al., 1972; Kostulak et al., 1976; Sat0 et al., 1983; Shimono e t al., 1984); gerbils (Ichikawa and Ichikawa, 1977, 1987); hares and rabbits (Bondi e t al., 1983; Bondi et al., 1985); cats (Tandler and Poulsen, 1977); rhesus monkeys (Ichikawa and Ichikawa, 1977); ferrets (Jacob and Podar (1989); and human beings (Riva et al., 1988). Because the predominant cell type in these glands is the mucous cell (Young and van Lennep, 1978) and because mucous secretory droplets show little of the substructural diversity that is so common in serous granules (Pinkstaff, 1980), the notion has arisen in some quarters that these organs have a rather mundane structure. We recently studied the submandibular gland in the African rodent, Praomys natalensis (Toyoshima and Tandler, 1991), which is intermediate to rats and mice in terms of size and bodily conformation. The acinar cells of this gland contain beautifully wrought secretory granules that consist, in most cases, of a dense matrix containing a target-like inclusion. The granular convoluted tubules have huge secretory granules, presumably the repositories of nerve growth factor, which is present in higher concentrations in Praomys submandibular glands than in the salivary glands of any other species (Aloe et al., 1981). These ducts and their successors, the striated ducts, contain abundant cytoplasmic crystalloids. Because the submandibular gland of Praomys exhibits so many differences from the same gland in common 0 1991 WILEY-LISS, INC
laboratory rodents, examination of the sublingual gland was considered warranted. This gland, like its submandibular partner, has many unusual features, including serous secretory granules of unique morphology. MATERIALS AND METHODS
Twelve adult Praomys were used in this study. Methods of specimen preparation and instrumentation were detailed in the previous report (Toyoshima and Tandler, 1991). RESULTS
The sublingual gland of P. natalensis is a mixed gland consisting of large mucous acini that sometimes are capped or rimmed by slender serous demilunes (Fig. l ) , inconspicuous intercalated ducts, and a few striated ducts that shade into excretory ducts. The mucous cells are large and surround a central lumen. The nucleus with its dense karyoplasm is basally displaced by the mucous droplets crowded into the apical cytoplasm. Individual mucous droplets are themselves quite large, and because they are plastic structures, vary in shape to accommodate their neighbors; the droplets display the familiar propensity for fusion that is often seen in mucous cells of whatever kind. The droplets consist of a lucent matrix in which
Received June 27, 1990; accepted August 15, 1990. Address reprint requests to Dr. Bernard Tandler, School of Dentistry, Case Western Reserve University, Cleveland, Ohio 44106. K. Toyoshima’s present address is: Department of Oral Anatomy 11, Kyushu Dental College, 2-6-1, Manazuru, Kokurakita-ku, Kitakyushu 803, Japan.
SALIVARY GLAND STRUCTURE
483
Fig. 1. Photomicrograph of a semithin section of a Pruomys sublingual gland showing the lightly stained mucous cells (MC) capped by serous demilune cells (SDC) that contain some densely stained secretory granules. An intralobular duct (D), probably a striated duct, is at the left. Toluidine blue. x 935.
Fig. 2. Survey electron micrograph of a serous cell (left) in close relation to two mucous cells (right). Even at this magnification, the peculiar substructure of the serous granules is evident. The mucous droplets have a light matrix in which are suspended many fine, short, threadlike structures. x 5.400.
there is an assortmcnt of randomly distributed vermiform or particulate densities (Figs. 2, 6 ) . The contents of the mucous droplets are released by exocytosis so that the particulate material often fills the acinar lumen and its tributaries, the intercellular canaliculi. The serous demilune cells are relatively flat, but they frequently send a long process between mucous cells to border on the acinar lumen. The most obvious feature of the serous cells is their array of secretory granules of highly unusual appearance (Fig. 3 ) . Often polygonal in outline, the granules at low magnification seem to consist of a dense cortex that is contorted into a variety of angular patterns. At higher magnification, the dense “cortical” material is seen to consist of packets of lamellae (Fig. 4)-it is the highly individual arrangements and packing of the lamellae within a somewhat lighter matrix that are responsible for the variegated substructure of the granules. Some idea of the almost infinite variety of designs generated by the random passage of the plane of section through the serous granules can be gained by inspection of Figure 3. In the most regular arrangements of lamellae, individual dense plies measure -12 nm in thickness and alternate with electron-lucent intervals of about the same thickness. The dense plies are not continuous with the limiting membrane of the granules, which is approximately 8 nm thick and has typical unit membrane structure. In some serous cells, the dense lamellae in the secretory granules roll up to form cylinders with walls consisting of two to six or more dense plies. Examination of the plies at very high magnification reveals that, whatever their mode of aggregation, they lack unit membrane structure and have a pronounced axial periodicity of -5 nm (Figs. 4,5). The Golgi complexes, of which there are several in each serous cell,
are extensive and tend to be arrayed around the perim-
eter of the accumulated serous granules. The Golgi saccules, which usually are five in number, show a marked polarity: the two cis saccules have a relatively light content, whereas the three trans saccules have a dense content. The stacked saccules are surrounded by myriad small vesicles. A few small bodies, probably condensing vacuoles, are on the trans side of the Golgi stack. Some of these putative condensing vacuoles already contain a hint of lamellar substructure. The area between the mass of serous granules and the cell base is occupied by the nucleus and an extensive rough endoplasmic reticulum. An occasional lipid droplet may be present. Myoepithelial cells of typical morphology are found between the secretory cells (either serous of mucous) and the basal lamina. The same myoepithelial process often spans both a portion of a demilune cell and a contiguous mucous cell. Hypolemmal nerve terminals occasionally are noted in their favored location between secretory and myoepithelial cells. Intercalated ducts in Pruomys sublingual gland consist of simple cuboidal epithelium (Fig. 6 ) . In some of these ducts, the lumen is quite circular or ovoid in cross section, whereas in others the apical surface of the duct cells bulges into the lumen, giving it an irregular outline. The duct cells have a simple cytology, with only a few cells showing any signs of secretory activity; such cells contain scattered serous-type secretory granules that are small and homogeneously dense. The ducts usually are surrounded by longitudinally oriented myoepithelial processes. Other than their position with respect to the lobules of the gland, there are no obvious structural differences between the striated and excretory ducts. Both of these ducts have basal striations (Fig. 71, but the interfolia-
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Fig. 3. Serous granules in a demilune cell. They appear to consist of packets of lamellae that have a haphazard orientation. A centriole is indicated by the arrow. X 40,000.
SALIVARY GLAND STRUCTURE
Fig. 4. A serous granule in which virtually all of the lamellae are arranged in a single stack. Some of the larnellae exhibit axial periodicity. x 105,000. Fig. 5. Some lamellae in a serous granule seen a t high magnification, showing their axial periodicity. x 132,000.
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K. TOYOSHIMA AND B. TANDLER
Fig. 6. Single intercalated duct cell adjoining two mucous cells (upper corners). The duct cell shows little cytological specialization. x 12,200.
tion of basal processes is less regular than that normally found in other salivary glands, including the submandibular gland of Praomys (Toyoshima and Tandler, 1991).The mitochondria appear to be shorter than those in the Praomys submandibular gland and lack the prominent matrix granules that are present in that gland. A striking feature of the sublingual striated and excretory ducts is the presence of clusters of cytoplasmic crystalloids (Fig. 7); these tend to be confined to the supranuclear cytoplasm, although an occasional crystal may be present below the nucleus. These crystalloids are identical in appearance to their counterparts in the submandibular glands of P. natalensis (Toyoshima and Tandler, 1991); they consist of linear densities that intersect at a 90" angle and have a periodicity in both directions of -12 nm (Fig. 8). At high magnification, the linear densities are seen to consist of rows of small particles, each measuring -4.8 nm in diameter. Many of the crystalline bodies show multiple foci of crystallization. The crystalloids frequently are found adjacent to small Golgi complexes; the saccules of these organelles sometimes contain particles identical to components of the crystalloids. The duct cells bear some stubby microvilli on their luminal surface, and there is a prominent terminal web just beneath the apical plasma membrane. DISCUSSION
The serous granules of the sublingual gland in Praomys, unlike those in laboratory rodent sublingual glands that have been examined by electron microscopy, are highly structured. The substructure exhibited by these granules is unique in the catalogue of secretory granules, most of which depend on the disposition of dense and lucid areas within the matrix to produce characteristic patterning (Phillips et al., 1987; Tandler et al., 1990). Rather than having a design resulting
from the arrangement of light and dark regions of varying shape, the Praomys sublingual serous granules contain serried lamellae that, at high magnification, have a striated appearance. The only matching structures that we are aware of are membranous bodies that have been reconstituted from the mitochondria1 enzyme, cytochrome oxidase (Wakabayashi et al., 1972; Vanderkooi et al., 1972). Such reconstituted membrane bodies consist of lamellae that have a paracrystalline structure. It is tempting to speculate that the striated lamellae in Praomys sublingual serous granules may represent arrays of enzymes, perhaps of a digestive nature. The precise process by which the sublingual granules form could not be determined by us, but it is clear that striated lamellae, albeit rudimentary and few in number, are present in some Golgi saccules. Thus, the cellular pathway by which these unusual granules are manufactured seems to be the garden variety one. It appears that, in nascent Praomys serous granules, proteins, if such they be, polymerize in sheets, which, viewed in profile, seem to be striated. Determination of the nature of the lamellar subunits awaits their isolation and chemical analysis. The ducts of the P. natalensis sublingual gland have the same general architecture as those in the submandibular gland of Praomys, but mitochondria in the striated ducts of the sublingual gland lack the matrix granules that are present in mitochondria of the latter organ (Toyoshima and Tandler, 1991). Since matrix granules are involved in electrolyte storage and release (Peachey, 1964), the absence of these structures from the sublingual gland mitochondria suggests that this organ, as in some other mammalian species (Riva et al., 19881, elaborates a final saliva that has plasmalike concentrations of electrolytes rather than being hypotonic. Crystalloids of the same sort as those found in the Praomys submandibular gland (Toyoshima and
SALIVARY GLAND STRUCTURE
Fig. 7. Intralobular duct that occupies a position normally maintained by a striated duct. Although the duct cells contain many mitochondria, these organelles are randomly disposed in the cytoplasm, rather than being concentrated at the cell base. The duct cells lack the characteristic basal infoldings of typical striated ducts. The dense, apparently structureless bodies scattered throughout the cytoplasm
487
are crystalloids; such inclusions are shown a t a considerably higher magnification in Fig. 8. x 8,700. Fig. 8. High magnification micrograph of several duct cell crystalloids showing their substructure. x 82,900.
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K. TOYOSHIMA AND B. TANDLER
Tandler, 1991) are present in duct cells of the sublingual gland, but here again they are of unknown nature. ACKNOWLEDGMENT
This work was supported in part by NIH grant DE07648. LITERATURE CITED Aloe, L., C. Cozzari, and R. Levi-Montalcini 1981 The submaxillary salivary glands of the African rodent Praomys (mastomys) natalensis as the richest available source of the nerve growth factor. Exp. Cell Res., 133:475-480. Bondi, A.M., G. Menghi, D. Accili, and G. Matterazzi 1985 Sublingual gland of the hare (Lepus europaeus): ultrastructural aspects and carbohydrate histochemistry. Cell. Mol. Biol., 31t397-405. Bondi, A.M., G. Menghi, and G . Materazzi 1983 Ultrastructural localization of complex carbohydrates present in the sublingual gland of rabbits. Acta Histochem., 72:187-193. Enomoto, S., and B.L. Scott 1971 Intracellular distribution of mucosubstances in the major sublingual gland of the rat. Anat. Rec., 1699-96. Guimaraes, J.P., J.B. Guedes e Silva, J.A.C. Lisboa, and K. Hirano 1979 Regeneration of the sublingual gland in the mouse: A light and electron microscopic study. Morf. Norm. Patol. Sect. B, 3: 193-208. Ichikawa, M., and A. Ichikawa 1977 Light and electron microscopic histochemistry of the serous secretory granules in the salivary glandular cells of the Mongolian gerbil (Meriones meridianus) and rhesus monkey (Macaca irus). Anat. Rec., 189t125-140. Ichikawa, M., and A. Ichikawa 1987 The fine structure of sublingual gland acinar cells of the Mongolian gerbil, Meriones unguiculatus, processed by rapid freezing followed by freeze-substitution fixation. Cell Tissue Res., 250.305-314. Jacob, S., and S. Poddar 1989 Ultrastructure of the ferret sublingual gland. Acta Anat., 135t344-346. Kim, S.K., C.E. Nasileti, and S.S. Han 1972 The secretion processes in mucous and serous secretory cells of the rat sublingual gland. J. Ultrastruct. Res.. 38r371-389. Kostulak, A,, K. Kozlowska, and A. Mysliwski 1976 Influence of cortisone on ultrastructure of the sublingual gland in rats. Folia Morphol. (Warsaw); 35:1-6. Laj, M., E. Borghese, and B. di Caterino 1971 Acinar ultrastructure of
the sublingual gland of Mus musculus during embryonic development. J . Submicrosc. Cytol., 3t139-151. Leeson, C.R., and W.G. Booth 1961 Histological, histochemical, and electron-microscopic observations on the postnatal development of the major sublingual gland of the rat. J. Dent. Res., 403338845. Peachey, L.D. 1964 Electron microscopic observations on the accumulation of divalent cations in intramitochondrial granules. J. Cell Biol., 20:95-111. Phillips, C.J., T. Nagato, and B. Tandler 1987 Comparative ultrastructure and evolutionary patterns of acinar secretory product of parotid salivary glands in Neotropical bats. In: Studies in Neotropical Mammalogy: Essays in Honor of Philip Hershkovitz. B.D. Patterson and R.M. Timm, eds. Field Museum of Natural History, Chicago, pp. 213-229. Phillips, C.J., and B. Tandler 1987 Mammalian evolution a t the cellular level. Curr. Mammal., 1:l-66. Pinkstaff, C.A. 1980 The cytology of salivary glands. Int. Rev. Cytol., 63.141-261. Riva, A,, B. Tandler, and F. Testa Riva 1988 Ultrastructural observations on human sublingual glands. Am. J. Anat., 181t385-392. Sato, A., J. Yahiro, and S. Miyoshi 1983 Fine structure of the duct systems of the rat major sublingual gland. J . Fukuoka Dent. Coll., 10:94-104. Tandler, B. 1986 Ultrastructure of the retrolingual salivary gland in the European hedgehog. J . Submicrosc. Cytol., 18r249-260. Tandler, B., C.J. Phillips, T. Nagato, and K. Toyoshima 1990 Comparative ultrastructure of 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., and J.H. Poulsen 1977 Ultrastructure of the cat sublingual gland. Anat. Rec., 187:153-172. Toyoshima, K., and B. Tandler 1991 Ultrastructure of the African multimammate rodent, Praomys natalensis Anat. Rec., 229:209218. Vanderkooi, G., A.E. Senior, R.A. Capaldi, and H. Hayashi 1972 Biological membrane structure. 111. The lattice structure of membranous cytochrome oxidase. Biochim. Biophys. Acta, 274:38-48. Wakabayashi, T., A.E. Senior, 0. Hatase, H. Hayashi, and D.E. Green 1972 Conformational changes in membranous preparations of cytochrome oxidase. Bioenereetics. 3t339-344. Young, J.A., and E.W. van Lelnep 1978 The Morphology of Salivary Glands. Academic Press, London, p. 46.
Ultrastructure of the Sublingual Gland in the African MuItimammate Rodent KUNIAKI TOYOSHIMA AND BERNARD TANDLER Department of Oral Biology, School of Dentistry, Case Western Reserve University, Cleveland, Ohio
ABSTRACT The sublingual gland of Praomys natalensis, a n African rodent that is phenotypically and cytogenetically intermediate to mice and rats, is a mixed gland, consisting of mucous acini that are capped by serous demilunes, of intercalated ducts, and of some short striated ducts that quickly become excretory ducts. The mucous cells are typical in appearance, with lucent granules that contain a n assortment of scattered vermiform or particulate densities. The serous cells display a n array of secretory granules with a highly unusual substructure. Rather than a pattern based on the manner in which light and dark regions are disposed in their matrix, these granules contain packets-some furled, some flat-of membranes that exhibit a pronounced axial periodicity of -5 nm. Intercalated ducts are simple in structure, with no obvious morphological specializations. Striated ducts resemble those in the salivary glands of less exotic rodents, but they and the excretory ducts often have clusters of cytoplasmic crystalloids consisting of linear densities that intersect at right angles and that have a periodicity in both directions of -12 nm. Unlike the parotid and submandibular salivary glands, the ultrastructure of sublingual glands has seldom been studied. The latter organs have been examined in detail in selected mammals: European hedgehogs (Tandler, 1986); mice (Laj et al., 1971; Guimaraes et al., 1979); rats (Leeson and Booth, 1961; Enomoto and Scott, 1971; Kim et al., 1972; Kostulak et al., 1976; Sat0 et al., 1983; Shimono e t al., 1984); gerbils (Ichikawa and Ichikawa, 1977, 1987); hares and rabbits (Bondi e t al., 1983; Bondi et al., 1985); cats (Tandler and Poulsen, 1977); rhesus monkeys (Ichikawa and Ichikawa, 1977); ferrets (Jacob and Podar (1989); and human beings (Riva et al., 1988). Because the predominant cell type in these glands is the mucous cell (Young and van Lennep, 1978) and because mucous secretory droplets show little of the substructural diversity that is so common in serous granules (Pinkstaff, 1980), the notion has arisen in some quarters that these organs have a rather mundane structure. We recently studied the submandibular gland in the African rodent, Praomys natalensis (Toyoshima and Tandler, 1991), which is intermediate to rats and mice in terms of size and bodily conformation. The acinar cells of this gland contain beautifully wrought secretory granules that consist, in most cases, of a dense matrix containing a target-like inclusion. The granular convoluted tubules have huge secretory granules, presumably the repositories of nerve growth factor, which is present in higher concentrations in Praomys submandibular glands than in the salivary glands of any other species (Aloe et al., 1981). These ducts and their successors, the striated ducts, contain abundant cytoplasmic crystalloids. Because the submandibular gland of Praomys exhibits so many differences from the same gland in common 0 1991 WILEY-LISS, INC
laboratory rodents, examination of the sublingual gland was considered warranted. This gland, like its submandibular partner, has many unusual features, including serous secretory granules of unique morphology. MATERIALS AND METHODS
Twelve adult Praomys were used in this study. Methods of specimen preparation and instrumentation were detailed in the previous report (Toyoshima and Tandler, 1991). RESULTS
The sublingual gland of P. natalensis is a mixed gland consisting of large mucous acini that sometimes are capped or rimmed by slender serous demilunes (Fig. l ) , inconspicuous intercalated ducts, and a few striated ducts that shade into excretory ducts. The mucous cells are large and surround a central lumen. The nucleus with its dense karyoplasm is basally displaced by the mucous droplets crowded into the apical cytoplasm. Individual mucous droplets are themselves quite large, and because they are plastic structures, vary in shape to accommodate their neighbors; the droplets display the familiar propensity for fusion that is often seen in mucous cells of whatever kind. The droplets consist of a lucent matrix in which
Received June 27, 1990; accepted August 15, 1990. Address reprint requests to Dr. Bernard Tandler, School of Dentistry, Case Western Reserve University, Cleveland, Ohio 44106. K. Toyoshima’s present address is: Department of Oral Anatomy 11, Kyushu Dental College, 2-6-1, Manazuru, Kokurakita-ku, Kitakyushu 803, Japan.
SALIVARY GLAND STRUCTURE
483
Fig. 1. Photomicrograph of a semithin section of a Pruomys sublingual gland showing the lightly stained mucous cells (MC) capped by serous demilune cells (SDC) that contain some densely stained secretory granules. An intralobular duct (D), probably a striated duct, is at the left. Toluidine blue. x 935.
Fig. 2. Survey electron micrograph of a serous cell (left) in close relation to two mucous cells (right). Even at this magnification, the peculiar substructure of the serous granules is evident. The mucous droplets have a light matrix in which are suspended many fine, short, threadlike structures. x 5.400.
there is an assortmcnt of randomly distributed vermiform or particulate densities (Figs. 2, 6 ) . The contents of the mucous droplets are released by exocytosis so that the particulate material often fills the acinar lumen and its tributaries, the intercellular canaliculi. The serous demilune cells are relatively flat, but they frequently send a long process between mucous cells to border on the acinar lumen. The most obvious feature of the serous cells is their array of secretory granules of highly unusual appearance (Fig. 3 ) . Often polygonal in outline, the granules at low magnification seem to consist of a dense cortex that is contorted into a variety of angular patterns. At higher magnification, the dense “cortical” material is seen to consist of packets of lamellae (Fig. 4)-it is the highly individual arrangements and packing of the lamellae within a somewhat lighter matrix that are responsible for the variegated substructure of the granules. Some idea of the almost infinite variety of designs generated by the random passage of the plane of section through the serous granules can be gained by inspection of Figure 3. In the most regular arrangements of lamellae, individual dense plies measure -12 nm in thickness and alternate with electron-lucent intervals of about the same thickness. The dense plies are not continuous with the limiting membrane of the granules, which is approximately 8 nm thick and has typical unit membrane structure. In some serous cells, the dense lamellae in the secretory granules roll up to form cylinders with walls consisting of two to six or more dense plies. Examination of the plies at very high magnification reveals that, whatever their mode of aggregation, they lack unit membrane structure and have a pronounced axial periodicity of -5 nm (Figs. 4,5). The Golgi complexes, of which there are several in each serous cell,
are extensive and tend to be arrayed around the perim-
eter of the accumulated serous granules. The Golgi saccules, which usually are five in number, show a marked polarity: the two cis saccules have a relatively light content, whereas the three trans saccules have a dense content. The stacked saccules are surrounded by myriad small vesicles. A few small bodies, probably condensing vacuoles, are on the trans side of the Golgi stack. Some of these putative condensing vacuoles already contain a hint of lamellar substructure. The area between the mass of serous granules and the cell base is occupied by the nucleus and an extensive rough endoplasmic reticulum. An occasional lipid droplet may be present. Myoepithelial cells of typical morphology are found between the secretory cells (either serous of mucous) and the basal lamina. The same myoepithelial process often spans both a portion of a demilune cell and a contiguous mucous cell. Hypolemmal nerve terminals occasionally are noted in their favored location between secretory and myoepithelial cells. Intercalated ducts in Pruomys sublingual gland consist of simple cuboidal epithelium (Fig. 6 ) . In some of these ducts, the lumen is quite circular or ovoid in cross section, whereas in others the apical surface of the duct cells bulges into the lumen, giving it an irregular outline. The duct cells have a simple cytology, with only a few cells showing any signs of secretory activity; such cells contain scattered serous-type secretory granules that are small and homogeneously dense. The ducts usually are surrounded by longitudinally oriented myoepithelial processes. Other than their position with respect to the lobules of the gland, there are no obvious structural differences between the striated and excretory ducts. Both of these ducts have basal striations (Fig. 71, but the interfolia-
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K. TOYOSHIMA AND B. TANDLER
Fig. 3. Serous granules in a demilune cell. They appear to consist of packets of lamellae that have a haphazard orientation. A centriole is indicated by the arrow. X 40,000.
SALIVARY GLAND STRUCTURE
Fig. 4. A serous granule in which virtually all of the lamellae are arranged in a single stack. Some of the larnellae exhibit axial periodicity. x 105,000. Fig. 5. Some lamellae in a serous granule seen a t high magnification, showing their axial periodicity. x 132,000.
485
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K. TOYOSHIMA AND B. TANDLER
Fig. 6. Single intercalated duct cell adjoining two mucous cells (upper corners). The duct cell shows little cytological specialization. x 12,200.
tion of basal processes is less regular than that normally found in other salivary glands, including the submandibular gland of Praomys (Toyoshima and Tandler, 1991).The mitochondria appear to be shorter than those in the Praomys submandibular gland and lack the prominent matrix granules that are present in that gland. A striking feature of the sublingual striated and excretory ducts is the presence of clusters of cytoplasmic crystalloids (Fig. 7); these tend to be confined to the supranuclear cytoplasm, although an occasional crystal may be present below the nucleus. These crystalloids are identical in appearance to their counterparts in the submandibular glands of P. natalensis (Toyoshima and Tandler, 1991); they consist of linear densities that intersect at a 90" angle and have a periodicity in both directions of -12 nm (Fig. 8). At high magnification, the linear densities are seen to consist of rows of small particles, each measuring -4.8 nm in diameter. Many of the crystalline bodies show multiple foci of crystallization. The crystalloids frequently are found adjacent to small Golgi complexes; the saccules of these organelles sometimes contain particles identical to components of the crystalloids. The duct cells bear some stubby microvilli on their luminal surface, and there is a prominent terminal web just beneath the apical plasma membrane. DISCUSSION
The serous granules of the sublingual gland in Praomys, unlike those in laboratory rodent sublingual glands that have been examined by electron microscopy, are highly structured. The substructure exhibited by these granules is unique in the catalogue of secretory granules, most of which depend on the disposition of dense and lucid areas within the matrix to produce characteristic patterning (Phillips et al., 1987; Tandler et al., 1990). Rather than having a design resulting
from the arrangement of light and dark regions of varying shape, the Praomys sublingual serous granules contain serried lamellae that, at high magnification, have a striated appearance. The only matching structures that we are aware of are membranous bodies that have been reconstituted from the mitochondria1 enzyme, cytochrome oxidase (Wakabayashi et al., 1972; Vanderkooi et al., 1972). Such reconstituted membrane bodies consist of lamellae that have a paracrystalline structure. It is tempting to speculate that the striated lamellae in Praomys sublingual serous granules may represent arrays of enzymes, perhaps of a digestive nature. The precise process by which the sublingual granules form could not be determined by us, but it is clear that striated lamellae, albeit rudimentary and few in number, are present in some Golgi saccules. Thus, the cellular pathway by which these unusual granules are manufactured seems to be the garden variety one. It appears that, in nascent Praomys serous granules, proteins, if such they be, polymerize in sheets, which, viewed in profile, seem to be striated. Determination of the nature of the lamellar subunits awaits their isolation and chemical analysis. The ducts of the P. natalensis sublingual gland have the same general architecture as those in the submandibular gland of Praomys, but mitochondria in the striated ducts of the sublingual gland lack the matrix granules that are present in mitochondria of the latter organ (Toyoshima and Tandler, 1991). Since matrix granules are involved in electrolyte storage and release (Peachey, 1964), the absence of these structures from the sublingual gland mitochondria suggests that this organ, as in some other mammalian species (Riva et al., 19881, elaborates a final saliva that has plasmalike concentrations of electrolytes rather than being hypotonic. Crystalloids of the same sort as those found in the Praomys submandibular gland (Toyoshima and
SALIVARY GLAND STRUCTURE
Fig. 7. Intralobular duct that occupies a position normally maintained by a striated duct. Although the duct cells contain many mitochondria, these organelles are randomly disposed in the cytoplasm, rather than being concentrated at the cell base. The duct cells lack the characteristic basal infoldings of typical striated ducts. The dense, apparently structureless bodies scattered throughout the cytoplasm
487
are crystalloids; such inclusions are shown a t a considerably higher magnification in Fig. 8. x 8,700. Fig. 8. High magnification micrograph of several duct cell crystalloids showing their substructure. x 82,900.
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K. TOYOSHIMA AND B. TANDLER
Tandler, 1991) are present in duct cells of the sublingual gland, but here again they are of unknown nature. ACKNOWLEDGMENT
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