Structure of the duct system in mammalian major salivary glands

Organic secretion in ductal cells of the sublingual salivary gland of ferret: Histochemical observations

The presence of organic secretion in ductal cells of the sublingual salivary gland of ferret has been questioned, which prompted the present investigation. Paraffin or cryostat sections from aldehyde fixed or quenched sublingual glands of this species were tested for some amino acid residues, mucosubstances, oxidative and hydrolytic enzymes, and lectins. The glands showed inconspicuous ducts of simple appearances on routine histology. The histochemical procedures, however, revealed a granulated substance in the apical (periluminal) region of ductal cells, which contained tryptophan, disulphides, neutral mucosubstances, αFuc and GalNAc, and showed chloroacetate esterase activity. Occurrence of the substance varied between different ducts of the same gland and/or cells of the same duct. The ductal cells also showed diffuse peroxidase and acid phosphatase, and Golgi-like thiamine pyrophosphatase activities. Acetylcholinesterase-positive nerve fibres embraced the ducts. The results support a particular localisation of protein-bound amino acid residues and enzymatic catalytic activities indicative of organic secretion, possibly tissue kallikrein, in sublingual ductal cells of ferret.

Morphological alterations in salivary glands of mice (Mus musculus) submitted to tannin enriched diets: comparison with sialotrophic effects of sympathetic agonists stimulation

In order to study the effects of tannins at histomorphological level, mice were either fed with three structurally different types of tannins (tannic acid, chestnut, and quebracho) or treated with isoproterenol, during 10 days. Acini of parotid and submandibular glands increased significantly, being the increase higher for parotid compared to submandibular glands, and higher in the quebracho compared with the other tannin groups. Sublingual acinar size also increased after tannin consumption, by opposition to isoproterenol-treated animals. The results present evidences that the effects produced by tannins are dependent on their structure.

Tight junctions in salivary epithelium

Epithelial cell tight junctions (TJs) consist of a narrow belt-like structure in the apical region of the lateral plasma membrane that circumferentially binds each cell to its neighbor. TJs are found in tissues that are involved in polarized secretions, absorption functions, and maintaining barriers between blood and interstitial fluids. The morphology, permeability, and ion selectivity of TJ vary among different types of tissues and species. TJs are very dynamic structures that assemble, grow, reorganize, and disassemble during physiological or pathological events. Several studies have indicated the active role of TJ in intestinal, renal, and airway epithelial function; however, the functional significance of TJ in salivary gland epithelium is poorly understood. Interactions between different combinations of the TJ family (each with their own unique regulatory proteins) define tissue specificity and functions during physiopathological processes; however, these interaction patterns have not been studied in salivary glands. The purpose of this review is to analyze some of the current data regarding the regulatory components of the TJ that could potentially affect cellular functions of the salivary epithelium.

On approaches to the functional restoration of salivary glands damaged by radiation therapy for head and neck cancer, with a review of related aspects of salivary gland morphology and development

Radiation therapy for cancer of the head and neck can devastate the salivary glands and partially devitalize the mandible and maxilla. As a result, saliva production is drastically reduced and its quality adversely altered. Without diligent home and professional care, the teeth are subject to rapid destruction by caries, necessitating extractions with attendant high risk of necrosis of the supporting bone. Innovative techniques in delivery of radiation therapy and administration of drugs that selectively protect normal tissues can reduce significantly the radiation effects on salivary glands. Nonetheless, many patients still suffer severe oral dryness. I review here the functional morphology and development of salivary glands as these relate to approaches to preventing and restoring radiation-induced loss of salivary function. The acinar cells are responsible for most of the fluid and organic material in saliva, while the larger ducts influence the inorganic content. A central theme of this review is the extent to which the several types of epithelial cells in salivary glands may be pluripotential and the circumstances that may influence their ability to replace cells that have been lost or functionally inactivated due to the effects of radiation. The evidence suggests that the highly differentiated cells of the acini and large ducts of mature glands can replace themselves except when the respective pools of available cells are greatly diminished via apoptosis or necrosis owing to severely stressful events. Under the latter circumstances, relatively undifferentiated cells in the intercalated ducts proliferate and redifferentiate as may be required to replenish the depleted pools. It is likely that some, if not many, acinar cells may de-differentiate into intercalated duct-like cells and thus add to the pool of progenitor cells in such situations. If the stress is heavy doses of radiation, however, the result is not only the death of acinar cells, but also a marked decline in functional differentiation and proliferative capacity of all of the surviving cells, including those with progenitor capability. Restoration of gland function, therefore, seems to require increasing the secretory capacity of the surviving cells, or replacing the acinar cells and their progenitors either in the existing gland remnants or with artificial glands.

Early markers of regeneration following ductal ligation in rat submandibular gland

Rat submandibular glands can recover their function and secretory protein content following ductal ligation-induced atrophy. Morphological studies have established that following ligation, deligation of the gland allows the regeneration of new salivary gland tissue. However, little is known about changes happening during early regeneration following intra-oral duct ligation, which does not damage the parasympathetic nerves. Glands that had been 2 weeks ligated or 2 weeks ligated + 3 days deligated were compared. Tissue was prepared for histological, immunohistochemical (SMG-B and Ki-67) and immunocytochemical analyses (smooth muscle actin, aquaporin 5). Haematoxylin and eosin staining of deligated glands showed that some acini regained their cytoplasmic volume; moreover, the loss of Alcian blue/periodic acid-Schiff's staining from the lumen of ducts suggested successful deligation. The deligated gland was characterized by atypical acinar-ductal branched structures, which were less frequent in the ligated gland and rarely seen in normal unoperated tissue. Myoepithelial cells were also investigated since changes in their morphology reflected changes in the acini morphology not readily detected by conventional staining. Actin staining revealed the presence of some shrunken acini in the atrophic tissue, whereas they had regained their normal morphology in the deligated gland suggesting that the acini were recovering. Some acini during deligation regained aquaporin 5 expression, which had decreased during atrophy. SMG-B protein, located in the pro-acinar cell during gland development and usually found in the intercalated duct cells in the adult, was detected in the newly formed acini of the deligated gland. This study suggests that morphological markers of regeneration appear as early as 3 days following ligation removal.

Differential expression of Prominin-1 (CD133) and Prominin-2 in major cephalic exocrine glands of adult mice

The major cephalic exocrine glands share many morphological and functional features and so can be simultaneously affected in certain autoimmune- and inherited disorders leading to glandular hypofunction. Phenotypic characterization of these exocrine glands is not only an interesting biological issue, but might also be of considerable clinical relevance. The major salivary and lacrimal glands might therefore be potential subjects of future cell-based regenerative/tissue engineering therapeutic approaches. In the present study, we described the expression of the stem and progenitor cell marker Prominin-1 and those of its paralogue, Prominin-2, in the three pairs of major salivary glands, i.e., submandibular-, major sublingual-, and parotid glands in adult mice. We have also documented their expression in the extraorbital lacrimal and meibomian glands (Glandulae tarsales) of the eyelid (Palpebra). Our analysis revealed that murine Prominin-1 and Prominin-2 were differentially expressed in these major cephalic exocrine organs. Expression of Prominin-1 was found to be associated with the duct system, while Prominin-2 expression was mostly, but not exclusively, found in the acinar compartment of these organs with marked differences among the various glands. Finally, we report that Prominin-2, like Prominin-1, is released into the human saliva associated with small membrane particles holding the potential for future diagnostic applications.

Microenvironmental adaptations in the parotid of ferret investigated by electron microscopy

Previous histochemical investigations suggested that the microenvironment in the parotid of ferret would foster microlithiasis because of a combination of pockets of inefficient secretion, brisk phagy and secretory material rich in calcium. We have undertaken the present ultrastructural investigation in an attempt to provide corroborative evidence. Parotids from four normal ferrets were examined by electron microscopy. Phagosomes in which there was cellular debris and occasionally secretory granules were present in acinar cells, which indicates crinophagy, and residual bodies were present in ductal cells. Atrophic parenchymal cells, degenerate parenchymal cells and apoptotic bodies were present. Cellular debris and secretory material were present in lumina, which indicates stagnation. The results indicate that removal of redundant secretory material and cells and low flow of saliva are features of the parotid of ferret and support the concept of pockets of inefficient secretory activity.

Grainyhead-related transcription factor is required for duct maturation in the salivary gland and the kidney of the mouse

Duct epithelial structure is an essential feature of many internal organs, including exocrine glands and the kidney. The ducts not only mediate fluid transfer but also help to maintain homeostasis. For instance, fluids and solutes are resorbed from or secreted into the primary fluid flowing through the lumen of the ducts in the exocrine glands and kidneys. The molecular mechanism underlying the functional maturation of these ducts remains largely unknown. Here, we show that a grainyhead-related transcription factor, CP2-like 1 (CP2L1), is required for the maturation of the ducts of the salivary gland and kidney. In the mouse, Cp2l1 is specifically expressed in the developing ducts of a number of exocrine glands, including the salivary gland, as well as in those of the kidney. In Cp2l1-deficient mice, the expression of genes directly involved in functional maturation of the ducts was specifically reduced in both the salivary gland and kidney, indicating that Cp2l1 is required for the differentiation of duct cells. Furthermore, the composition of saliva and urine was abnormal in these mice. These results indicate that Cp2l1 expression is required for normal duct development in both the salivary gland and kidney.

Subcellular localization of epidermal growth factor receptor in human submandibular gland

The subcellular distribution of the epidermal growth factor receptor (EGFr) was demonstrated in the normal human submandibular gland by means of immunogold cytochemistry. EGFr labelling appeared in both acinar and ductal cells, where strong immunoreactivity was associated with a tubulovesicular system near the basolateral surfaces. In addition, groups of reactive vesicles were highlighted among secretory granules of both serous and mucous cells and at the apex of ductal cells. Basolateral vesicles were interpreted as being a result of EGFr internalization after activation by an exogenous ligand, although the functional meaning of those located apically remains unclear.

Interlobular excretory ducts of mammalian salivary glands: structural and histochemical review

In the major salivary glands of mammals, excretory ducts (EDs) succeed striated ducts. They are for the most part interlobular in position, although their proximal portions sometimes are on the periphery of a lobule, where they occasionally retain some of the structural features of striated ducts. Based on a survey of a broad range of mammalian species and glands, the predominant tissue type that composes EDs is pseudostratified epithelium. In some species, there is a progression of epithelial types: the proximal EDs are composed of simple cuboidal or columnar epithelium that, in the excurrent direction, usually gives way to the pseudostratified variety. Secretory granules are visible in the apical cytoplasm of the principal cells of the EDs of only a few species, but histochemistry has shown the presence of a variety of glycoproteins in these cells in a spectrum of species. Moreover, the latter methodology has revealed the presence of a variety of oxidative, acid hydrolytic, and transport enzymes in the EDs, showing that, rather than simply acting as a conduit for saliva, these ducts play a metabolically active role in gland function. It is difficult to describe a "typical" mammalian ED because it can vary along its length and interspecific variation does not follow a phylogenetic pattern. Moreover, in contrast to intercalated and striated ducts, ED cellular features do not exhibit a relationship to diet.

Histological and histochemical observations on salivary microliths in ferret

The fortuitous observation of salivary microliths in ferret was pursued in the present investigation. Major salivary glands obtained post-mortem from mature ferrets of either sex were examined with the use of histology and light microscopical histochemistry for calcium, protein, amino acids, mucosubstances and hydrolytic enzymes. Microliths were detected in most parotids, but were absent from submandibular and sublingual glands. The microliths were usually seen in lumens, and occasionally in parenchyma and interstices. They were variably stained for calcium, tryptophan, and neutral and acidic mucosubstances, similarly to acinar or ductal secretory granules. Unlike secretory granules, microliths showed autofluorescence, high levels of tyrosine and a low concentration of -SS- groups. Acid phosphatase and beta-glucuronidase reaction surrounded non-luminal microliths. The present data establish ferret as a new model for the investigation of salivary microliths and do not support the notion of microliths being almost absent from the parotid. Probably there is secretory inactivity in ferret parotid and this fosters the formation and accumulation of microliths containing calcium and disintegrated secretory material.

Immunohistochemical identification of antigen presenting cells in rat salivary glands

Mucosal dendritic cells affect immune responses through secretion of cytokines and exposure of naïve B- and T-lymphocytes to foreign matter as antigen presenting cells (APCs). APC in oral tissues may play a role in the development of local and secretory immune responses [Crit. Rev. Oral Biol. Med. 7 (1996) 36]. Previous studies have shown that APC are present in the interstitial tissues of rat salivary glands [Arch. Oral Biol. 40 (1995) 1015]. This study sought to further define the distribution of APC in salivary glands. The major glands and ducts of male Sprague-Dawley and Wistar rats were fixed with 4% paraformaldehyde and prepared for immunofluorescence and pre- and post-embedding immunoelectron microscopy. Monoclonal antibodies to the dendritic cell marker Ia antigen (OX-6 antibody), monocyte lineage cytoplasmic antigen (ED-1), and resident tissue macrophage antigen (ED-2) were visualized with FITC-conjugated secondary antibodies for light microscopy and HRP- and gold-labelled secondary antibodies for electron microscopy. Light microscopy revealed numerous OX-6-positive cells with branching processes in the epithelium of striated and excretory ducts of both rat strains, as well as in the connective tissue stroma. ED-1-positive cells had a similar distribution but exhibited a more compact shape with fewer processes. ED-2-positive cells were found only in the connective tissue. Acinar and duct epithelial cells were unreactive. Electron microscopy confirmed that both OX-6-positive and ED-1-positive, non-epithelial cells were present within the duct epithelium. The presence of APC in the duct epithelium suggests that these ducts may be exposed to antigens, possibly by retrograde access from the oral cavity, and that APC located in the salivary gland epithelium may participate in local immune responses.

Immunocytochemistry of myoepithelial cells in the salivary glands

MECs are distributed on the basal aspect of the intercalated duct and acinus of human and rat salivary glands. However, they do not occur in the acinus of rat parotid glands, and sometimes occur in the striated duct of human salivary glands. MECs, as the name implies, have structural features of both epithelial and smooth muscle cells. They contract by autonomic nervous stimulation, and are thought to assist the secretion by compressing and/or reinforcing the underlying parenchyma. MECs can be best observed by immunocytochemistry. There are three types of immunocytochemical markers of MECs in salivary glands. The first type includes smooth muscle protein markers such as alpha-SMA, SMMHC, h-caldesmon and basic calponin, and these are expressed by MECs and the mesenchymal vasculature. The second type is expressed by MECs and the duct cells and includes keratins 14, 5 and 17, alpha 1 beta 1 integrin, and metallothionein. Vimentin is the third type and, in addition to MECs, is expressed by the mesenchymal cells and some duct cells. The same three types of markers are used for studying the developing gland. Development of MECs starts after the establishment of an extensively branched system of cellular cords each of which terminates as a spherical cell mass, a terminal bud. The pluripotent stem cell generates the acinar progenitor in the terminal bud and the ductal progenitor in the cellular cord. The acinar progenitor differentiates into MECs, acinar cells and intercalated duct cells, whereas the ductal progenitor differentiates into the striated and excretory duct cells. Both in the terminal bud and in the cellular cord, the immediate precursors of all types of the epithelial cells appear to express vimentin. The first identifiable MECs are seen at the periphery of the terminal bud or the immature acinus (the direct progeny of the terminal bud) as somewhat flattened cells with a single cilium projecting toward them. They express vimentin and later alpha-SMA and basic calponin. At the next developmental stage, MECs acquire cytoplasmic microfilaments and plasmalemmal caveolae but not as much as in the mature cell. They express SMMHC and, inconsistently, K14. This protein is consistently expressed in the mature cell. K14 is expressed by duct cells, and vimentin is expressed by both mesenchymal and epithelial cells. After development, the acinar progenitor and the ductal progenitor appear to reside in the acinus/intercalated duct and the larger ducts, respectively, and to contribute to the tissue homeostasis. Under unusual conditions such as massive parenchymal destruction, the acinar progenitor contributes to the maintenance of the larger ducts that result in the occurrence of striated ducts with MECs. The acinar progenitor is the origin of salivary gland tumors containing MECs. MECs in salivary gland tumors are best identified by immunocytochemistry for alpha-SMA. There are significant numbers of cells related to luminal tumor cells in the non-luminal tumor cells that have been believed to be neoplastic MECs.

Secretion by striated ducts of mammalian major salivary glands: review from an ultrastructural, functional, and evolutionary perspective

In addition to their role in electrolyte homeostasis, striated ducts (SDs) in the major salivary glands of many mammalian species engage in secretion of organic products. This phenomenon usually is manifested as the presence of small serous-like secretory granules in the apical cytoplasm of SD cells. The composition of these granules is largely unknown, except in the case of the cat and rat submandibular gland, where the granules have unequivocally been shown to contain kallikrein. In some species, the apical cytoplasm of SD cells contains variable numbers of vesicles, both spherical and elongated, that vary in appearance from 'empty' to moderately dense. In the rat parotid gland, lucent vesicles transport glycoproteins to the luminal surface where they are incorporated into the apical plasmalemma and the glycocalyx. There is a strong possibility that in various species some of these vesicles are involved in transcytosis of antibodies to the saliva from their source (plasma cells) in the surrounding connective tissue. In addition, vesicles may engage in transfer of growth factors from the saliva to the interstitium. In a few species, conventional SDs have been replaced by ducts that are wholly given over to secretion, i.e., they entirely lack basal striations; although such ducts occupy the histological position of conventional SDs, it is not clear whether they represent a new type of duct or merely are modifications of SDs. Broad-based comparisons of ultrastructural and other data about SDs offer some insight into evolutionary history of salivary glands and their role in the adaptive radiation of mammals. Evolutionary patterns emerged when we made interspecific comparisons across mammalian orders. Among the bats, there is a clear relationship between SD secretion and general categories of diet.

Subcellular localization of epidermal growth factor in human parotid gland

The intracellular distribution of epidermal growth factor was investigated in human parotid gland by immunogold cytochemistry at the electron-microscopy level. Epidermal growth factor immunoreactivity was demonstrated in both acini and ducts. In acinar cells, secretory granules appeared moderately stained, clearly indicating that parotid gland contributes to salivary epidermal growth factor through granule exocytosis. In ductal cells, gold particles were found to decorate numerous cytoplasmic vesicles, particularly abundant in striated duct cells. Since epidermal growth factor reactive vesicles were seen not only at the cellular apex, but nearby lateral plasma membranes as well, it leads to the hypothesis that epidermal growth factor may be discharged both apically into the saliva, and basally into the interstitium.

Ultrastructure of submucosal glands in human anterior middle nasal turbinates

The abundant glands situated in the lamina propria of the human anterior middle nasal turbinate were complex tubules that consist of serous, seromucous, and mucous cells, either singly or in combination. Serous granules were homogeneously dense, but could have a small lighter core. Seromucous granules had a dense rim and a large compartment of appreciably lighter density. Gradation between serous and seromucous granules made precise identification of these secretory cell types difficult. Mucous cells were of conventional morphology. The secretory tubules, which possessed a complement of myoepithelial cells, gradually transformed into ducts or the changeover was relatively sudden. The ductular portions of the tubules consisted either of tall prismatic cells or of shorter columnar cells, both of which lacked secretory granules, but had many mitochondria in their supranuclear cytoplasm. In many cases the ducts, for most of their length, consisted of secretory cells. These glands clearly participate in the elaboration of the glycoconjugate coat that serves to protect the nasal mucosa and keeps it from drying out.

Subcellular localization of epidermal growth factor in human submandibular gland

Epidermal growth factor in human submandibular gland was localized at the subcellular level by means of an immunogold staining method. Labelling was observed in serous acini and ducts. In the acini, gold particles were found within secretory granules, indicating that the growth factor is released into the saliva through granule exocytosis. In the ductal system, the most intense reactivity was revealed in the principal cells of striated ducts. In these cells, an abundant population of small cytoplasmic vesicles was specifically stained. Immunoreactive vesicles were found both apically and basally, suggesting that ductal cells can release their products not only into the saliva but also into the interstitium.

Complex carbohydrate histochemistry and ultracytochemistry of the sheep lacrimal gland

The chemical content of the secretion of the sheep lacrimal gland was analysed at the light and electron microscope levels by applying histochemical techniques and an ultrastructural histochemical method (periodic acid, thiocarbohydrazide and silver proteinate). Mucosubstance histochemistry demonstrated acidic glycoconjugates, mainly sulphated, in the mucous and seromucous glandular cells and in the apical portion of the cells lining the terminal ducts. Moreover, secretory granules, stained with PA-TCH-SP, showed a different localization of the reaction product. The presence of lysozyme was also found in the glandular serous cells. These histochemical studies demonstrate that the secretion of sheep lacrimal glands is mixed, having serous, mucous and seromucous components, and that an excellent correlation exists between the secretory granule substructure and glycoprotein localization.

Ultrastructure of the parotid gland in two species of naked-backed bats

Naked-backed bats of the genus Pteronotus (family Mormoopidae) occur in the Neotropics from Mexico through northern South America. These are relatively small-sized insectivorous species that frequently roost in caves. Eight specimens of naked-backed bats (Pteronotus parnellii) were live-trapped in Suriname and one in Cuba (P. quadridens). Their parotid glands were fixed in an aldehyde mixture designed for field work and postfixed in the laboratory with osmium tetroxide. Tissues were further prepared for electron microscopy by conventional means. The parotid glands of the two species of Pteronotus closely resemble each other except for the substructure of their serous secretory granules. Serous granules in P. parnellii are bizonal, with a moderately dense inner matrix and an outer, denser corona or crescent. The matrix is occupied by laminae, flakes, and filaments in random array. In contrast, serous granules in P. quadridens consist of a uniform matrix that contains dense, usually stacked toroids or tubules either in random array or packed in bundles. A parotid gland from one specimen of P. parnellii contained an endpiece that consisted of cells that contained giant (up to 9 pm in diameter) serous granules. Serous cells in both species contain aggregates of small, uniformly dense, rod-like, membrane-delimited organelles as well as occasional bundles of cytofilaments. The endpieces are separated from intercalated ducts by a ring of granulated cells that contain secretory granules that often have a bull's-eye configuration. Intercalated and striated ducts are typical in appearance, except that many of the cells in the latter contain small, dense secretory granules in their apical cytoplasm. The parotid glands in the two species of naked-baked bats differ slightly in terms of acinar secretory granule ultrastructure, but otherwise are fairly conservative. It is thought that the glands in these particular bats might represent the "basal" condition of the salivary glands of insectivorous bats and thus can serve as a reference point for making comparisons to the highly diversified (in terms of diet) phyllostomid bats.

Simple method to isolate the intact duct system of the rat submandibular gland

The excurrent duct system of the rat submandibular gland consists of a number of distinct segments. Using the direction of salivary flow as a reference point, these segments are, in order, intercalated duct, granular convoluted tubule, striated duct, excretory duct, main excretory duct (MED), and salivary bladder (which is an expanded portion of the MED). Because these ducts (with the exception of the MED and the salivary bladder) are encased in secretory endpieces, they are difficult to locate and to observe by scanning electron microscopy. A simple method has been devised to rid the gland of these obscuring endpieces so that the detailed architecture of the duct system can be examined. Rat submandibular glands were fixed initially by vascular perfusion with half-strength Karnovsky's fixative. The connective tissue capsule was removed from extirpated glands and the glands remained in fixative for varying lengths of time. For our purposes, a 30-minute immersion in the aldehyde mixture was optimum. After the sublingual gland was removed, the submandibular gland was softly struck with forceps having rounded tips, then shaken in fixative or buffer. The tissue that remained was postfixed in osmium tetroxide. This method results in the complete divestment of nonductular parenchyma from the rat submandibular gland, leaving the duct system clean and ready for microscopic examination.

An unusual parotid gland in the tent-building bat, Uroderma bilobatum: possible correlation of interspecific ultrastructural differences with differences in salivary pH and buffering capacity

The tent-building bat, Uroderma bilobatum, is a small, frugivorous phyllostomid bat with a broad neotropical distribution. Generally found in humid forest, this bat lives in small groups that create daytime "roosts" from large leaves of a variety of tropical plants. Fruit eating engenders a variety of ecological and physiological challenges for bats, some of which could require adaptive features in their salivary glands. The parotid salivary glands of Uroderma bilobatum were prepared for transmission electron microscopy by using methods that have become standard for field work. The parotid gland is extremely unusual in structure. Although the secretory endpieces still produce serous granules with a complex substructure, they are modified into quasi striated ducts. Their basal folds, which are extensive, occasionally harbor some vertically oriented mitochondria, imparting a resemblance to striated ducts. Other evidence for the endpiece origin of these parenchymal components is a well-developed system of intercellular canaliculi, structures that never occur in bona fide striated ducts. The long but sparse intercalated ducts consist of two types of cells, each of which elaborates a modest number of secretory granules of differing substructure. Striated ducts are of conventional morphology, except that a few dark cells shaped like wine glasses are present in their walls. The striated duct cells produce no secretory granules, but their apical cytoplasm may contain some small, empty vesicles. Capillaries lie in longitudinal grooves in the base of the duct cells, an arrangement that might enhance electrolyte exchange. Excretory ducts consist of simple cuboidal epithelium composed of cytologically unspecialized cells that sometimes includes a dark cell. It was concluded that salivary glands could have a major role in adapting species to acquire nutrients from marginal sources, such as tropical fruits, which have a low protein and sodium content. The unusual parotid acinar cells in Uroderma bilobatum are discussed in the context of salivary pH and buffering capacity. Comparisons are made with four other bat species, including an insectivorous species with a salivary pH > 8.0 and a very high buffering capacity, an intermediate species, and a fruit bat with acidic-stimulated saliva and very low buffering capability. Such interspecific comparisons provide a foundation for hypothesizing that ultrastructural features of the acinar cell basolateral membranes and intercellular canaliculi correlate with differences involving Na/H+ exchangers and release of HCO3- and, thus, are associated with the species differences that are important to diet and nutrient acquisition.

Topographical distribution of cells in the rat submandibular gland duct system with special reference to dark cells and tuft cells

The duct system of the rat submandibular gland consists of the intercalated duct, the granular convoluted tubule, the striated duct, the excretory duct, the main excretory duct, and the salivary bladder. The duct system contains special cell types, such as dark cells and tuft cells, in addition to principal cells. However, little is known about cell distribution in the duct system. The purpose of the present study was to examine cell distribution and to perform a morphometric analysis of the duct system. Transmission and scanning electron microscopy were used to examine the duct system of the rat submandibular gland. Six regions in the duct system, the striated duct, the interlobular excretory duct, the 5-mm proximal excretory duct from the hilus, the main excretory duct at the hilus, the 10-mm distal main excretory duct from the hilus, and the salivary bladder, were investigated. Morphometric and statistical analyses of the data were then performed. The epithelium of the duct system consisted of a heterogeneous cell population. Dark cells and tuft cells were present throughout the duct system. The principal, dark, and tuft cells were distinguished by their different microvilli by using a scanning electron microscope. The frequency of these cells in the total epithelial cell population was as follows: The percentage of principal cells in the six regions of the duct system varied from 87.5% to 94.4%, that of dark cells varied from 4.1% to 7.2%, and that of tuft cells varied from 1.8% to 7.2%. The number of principal and tuft cells was significantly different between the striated duct and the main excretory duct at the hilus (P < 0.01). However, no significant difference in number of dark cells throughout the duct system was observed (P > 0.05). The abundance of the principal, dark, and tuft cells in the duct system of the rat submandibular gland was determined. Few tuft cells were distributed in the striated duct, and most were found at the hilus. Dark cells were distributed equally throughout the duct system.

Ultrastructure of the binary parotid glands in the free-tailed bat, Tadarida thersites. I. Principal parotid gland

BACKGROUND

Many species of bats have two sets of submandibular glands, principal and accessory. The accessory gland may resemble the principal one but more often shows wide morphological divergence. The free-tailed bat, Tadarida thersites, is very unusual in that it has two sets of parotid glands rather than binary submandibular glands. We studied the ultrastructure of the principal parotid gland to establish a baseline for comparison with the accessory parotid.

METHODS

Two specimens of adult free-tailed bats, one male and one female, were live-trapped in western Kenya. Parotid glands were fixed for electron microscopy using a protocol expressly designed for field fixation and then embedded by conventional means.

RESULTS

Histologically, the principal parotid is a typical serous gland. The secretory granules of the endpiece cells have an unusual substructure in that they contain variable numbers of lucent halos and one or several spherules. Intercalated duct cells contain a significant number of dense, serous-like granules. Striated ducts have the usual basal configuration of mitochondria and folded plasma membranes, but the supranuclear cytoplasm contains many small, dense granules, so that these ducts resemble the granular convoluted tubules found in the submandibular glands of many families of rodents. The apices of the duct cells have a peculiar contour--the luminal surfaces obliquely invaginate into the apical cytoplasm, so that in thin section the luminal membranes appear to be underlaid by a layer of vacuoles.

CONCLUSION

Although the principal parotid gland of the free-tailed bat shows some distinctive, species-specific ultrastructural features, it basically is similar to the parotid gland in two other molossid bats, Tadarida brasiliensis and Molossus molossus. The distinctive features in the principal parotid gland of T. thersites might relate to its feeding on hard-bodied insects and perhaps to the production of lysozyme.

Ultrastructure of the binary parotid glands in the free-tailed bat, Tadarida thersites. II. Accessory parotid gland

BACKGROUND

Many bat species have an extra set of major salivary glands. In some species, the accessory glands are quite similar to the principal one, but in others they may be radically different. Accessory glands usually are associated with the submandibular gland, but the free-tailed bat, Tadarida thersites, also has an accessory parotid gland. In the present study, we compared the accessory parotid gland with its principal counterpart.

METHODS

Salivary glands were removed from two specimens of free-tailed bats, one of each sex, that had been live-trapped in western Kenya and immersion-fixed in a specially formulated mixture designed for field fixation. Once back in the laboratory, the tissue was further prepared for electron microscopy by conventional means.

RESULTS

The secretory endpieces consist of serous tubules composed of typical serous cells that contain numerous dense granules. In contrast, the intralobular duct system shows a radical departure from normal. These ducts are enormous in caliber, their lumina measuring greater than 100 microm in diameter. They appear to arise by amalgamation of the homologues of intercalated and striated ducts into macroducts. The walls of the macroducts consist of intermingled patches of simple cuboidal and simple columnar epithelia that occasionally include a tuft cell and are underlaid by an almost continuous layer of myoepithelium. A few cells have some modified basal striations, but most cells display a cytological organization that differs radically from either of their two putative ancestral cell types. Both tall and short epithelial macroduct cells have a paranuclear collection of ovate mitochondria and aggregates of what presumably are peroxisomes. Macroduct cells in both the female and male are pervaded by a system of tubular smooth endoplasmic reticulum (SER). In the female, the SER gives rise to membranous whorls that consist of numerous plies. As the macroducts approach their termini, a single row of small dense secretory granules appears just beneath their luminal surface. At the lobular periphery, the ducts taper down to become excretory ducts of normal dimensions.

CONCLUSIONS

An accessory parotid gland occurs in T. thersites, but apparently is absent in the related species, T. brasiliensis. The ultrastructural data are consistent with a possible steroidogenic function, although other features of the gland might relate to the elaboration of a secretory product associated with feeding on chitinous beetles. The macroducts conceivably function as reservoirs of preformed saliva.

A unique parotid gland in Hart's little fruit bat, Enchisthenes hartii

BACKGROUND

Hart's little fruit bat, Enchisthenes hartii, is uncommon and, although it has been the subject of recent molecular genetic studies, is little known biologically. Because chiropteran salivary glands vary interspecifically in ways that reflect evolutionary history and ecology, we examined the parotid gland in E. hartii to ascertain the extent to which it resembles homologous glands in species to which this bat presumably is related.

METHODS

The parotid glands were prepared for electron microscopic examination by conventional means.

RESULTS

The parotid gland of E. hartii is structurally unique among all previously studied species of bats (> 230 species examined) and other mammals. In contrast to the same gland in other mammals, the parotid gland in E. hartii lacks secretory endpieces. In their place, there is a type of striated duct. Thus, in this species single secretory elements consist of (proceeding in the direction of salivary flow): striated duct--intercalated duct--and a conventionally located striated duct. The proximal ducts possess microvillus-lined intercellular canaliculi, whereas the walls of the distal ducts include occasional dark cells. Some small serous-like granules may be present in the intercalated duct cells.

CONCLUSIONS

The function(s) and biological role of the unique parotid gland in E. hartii are unknown. Nevertheless, the presence of two sets of striated ducts provides two separate glandular components seemingly capable of electrolyte transport. This might be of adaptive significance in enabling this species to make use of tropical nutrient resources that otherwise would be unavailable. The uniqueness of its parotid glands lends support to the current hypothesis that E. hartii should be classified as a monotypic genus rather than as a species of Artibeus, whose members it resembles morphometrically.

Ultrastructure of the salivary glands in the midtongue of the common vampire bat, Desmodus rotundus

BACKGROUND

All examined mammals have at least two sets of lingual salivary glands: von Ebner's glands and Weber's glands. A third set, the glands of Blandin and Nuhn, is present in the tongues of some but not all mammals. Vampire bats, Desmodus rotundus, are unusual in that they possess another set of lingual glands, these being in the midtongue region.

METHODS

The anterior half of the tongue was extirpated from several adult vampire bats, dissected, and tissue blocks derived from the midregions of the body of the tongue prepared for transmission electron microscopy by conventional means.

RESULTS

The midlingual glands are in the form of long, tubular secretory endpieces that are succeeded by ducts of simple morphology. In general, the secretory portions consist of two cell types, which may be intermingled in the same tubule or may form tubules that consist wholly of one cell type or the other. Seromucous cells usually have one or several rough endoplasmic reticulum cisternae that are hugely distended by a homogeneously dense material. Their granules have a bizonal substructure: one or several dense bands are embedded in a lighter matrix. Mucous cells are rather typical in structure, but their secretory product is different from run-of-the-mill mucous droplets. These droplets vary in density from cell to cell. In some cells, these droplets have a relatively light matrix; in other cells, the droplet is unusually dense, consisting mainly of a dark, structureless matrix with marginal lenticular lacunae of low density in which some short, irregular filaments are scattered. A rare finding is the presence of ciliated cells intermingled with secretory endpiece cells. The cilia are of conventional morphology. Secretory tubules are succeeded by ducts that resemble intercalated ducts; the epithelium of these ducts gradually increases in height to form a kind of excretory duct, without the intervention of striated ducts. As the ducts approach the lingual surface, the epithelium changes to stratified squamous.

CONCLUSIONS

Saliva produced by the midlingual glands may be an aid in the reciprocal grooming behavior of vampire bats. Based on their morphology, the excurrent ducts may not modify the initial saliva elaborated by these glands and might act simply as pipelines by which the saliva reaches the mouth.

Ultrastructural aspects of human gallbladder epithelial cells in cholelithiasis: production of anionic mucus

The surface epithelium of 28 gallbladders removed during elective cholecystectomies and pathology collection was studied ultrastructurally. Focusing on 10 of the 28 cases that were diagnosed as cholecystitis, we found that the epithelium displayed numerous apical mucous granules and bulging apical apices. Mucous granule changes included 1) hyperproduction of secretory granules of neutral type containing an electron-dense proteinaceous spherule, similar to that described in other mucus-producing glands of the digestive system, and 2) production of anionic, osmiophilic secretory mucus. Other alterations of the surface epithelial cells included the production of bizarre surface appendages resembling primitive cilia without axoneme and epithelial excrescences.

Crystalloids in the excretory ducts of the accessory submandibular gland of the long-winged bat, Miniopterus magnator

Cytoplasmic crystalloids are abundant in the excretory ducts of the accessory submandibular gland of the long-winged bat, Miniopterus magnator. The crystalloids, which always lack a membranous enclosure, may have an intricate silhouette. They consist of parallel linear densities with a 12.5 nm periodicity. These densities actually may be thin-walled tubules. In some crystalloids, intersecting subcrystalloids produce a complex pattern of decussate densities. In a few rare instances, continuities were detected between a crystalloid and a smooth-surfaced cisternal element. In other mammalian species, similar crystalloids connected to smooth endoplasmic reticulum play a role in steroid metabolism. We postulate that the ductular crystalloids in M. magnator might be involved in production of a factor that influences mating behavior.

Ultrastructure of the unusual accessory submandibular gland in the fringe-lipped bat, Trachops cirrhosus

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 ells 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.

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

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.

Special relationship between mitochondria and hypolemmal nerve terminals in salivary glands of some bats

BACKGROUND

Hypolemmal nerve terminals, which are abundant in many types of salivary glands, are naked axons that have penetrated the basement membrane to take up a position between adjacent parenchymal cells. Although they may form vesicle-filled varicosities, there usually are no obvious morphological indications as to which cell actually is being innervated.

METHODS

Specimens of over 200 species of bats were live-trapped and their major salivary glands extirpated and prepared for electron microscopic examination.

RESULTS

In 13 species of bats out of the more than 200, mitochondria-hypolemmal nerve terminal complexes were observed in different sites in different salivary glands. In these 13 species, mitochondria in epithelial cells that abut nerve varicosities are closely applied to the plasma membranes of their cells to follow the contours of these nerve elements. The complexes so formed often consist of a varicosity flanked by two mitochondria in separate cells. In intensely innervated glandular segments, mitochondria may be opposed not only to varicosities, but to the axonal portions of these nervous elements as well.

CONCLUSIONS

The physiological significance of such complexes is unknown; it is conceivable that the mitochondria facilitate signal transmission or play a role in local calcium homeostasis related to nerve function.