Ultrastructural ontogeny of the labial gland apparatus in termite Prorhinotermes simplex (Isoptera, Rhinotermitidae)

The labial glands in Prorhinotermes simplex consist of secretory cells organized into acini, water sacs and the ducts connecting the gland parts to the basis of the labium. Acini are composed of central and parietal cells. Central cells type I contain predominantly lucent vacuoles and are involved probably in hydroquinone production. They are lacking in soldiers. Type II central cells produce vacuoles of proteinaceous content which are of the same electron density (type IIa) or present in more shades (type IIb). Type IIa cells are present in all older individuals, whereas type IIb are lacking in soldiers and neotenics. Type III cells represent a specific stage of type I cells development, but they are definite functional secretory cells in soldiers. Acini of first instar larvae contain undifferentiated cells which differentiate into type I cells during the second instar. Specific larval central cells start to change into type II cells during first instar. The central cells of presoldiers show a transition from the pseudergate into the soldier situation. The parietal cells keep a uniform structure throughout the whole ontogeny. Only one type of cells form the water sacs in all castes. The cells are very flat with scarce organelles. The water sac cells produce lipid-like secretion, small lucent vacuoles and bunches of angulated vacuoles. The water sac probably functions as water storage organ only. Ontogenetical changes in water sac development are small. The acinar ducts originate inside the acinus where they are formed by flat cells with rare organelles. At the acinus border, cells equipped with mitochondria, microvilli and basal invaginations appear. The water sac ducts are formed by flat cells with rare organelles. Acinar ducts outside the acinus and water sac ducts are equipped with taenidiuam.     

The fine structure of the salivary glands of the desert locust Schistocerca gregaria Forskål

Summary This paper describes the structure of the salivary glands of Schistocerca gregaria as seen under the electron microscope and the light microscope. The salivary glands consist of a number of acini located on both sides of the pro-, meso-, and metathoracic segments of the locust. Each acinus is drained by a duct which unites with others from the same side to form a lateral collecting duct. The ducts from the two sides join in the head capsule and open into a salivary cup on the labium. Each acinus consists of parietal cells, zymogenic cells, duct cells, tracheoblasts, sheath cells and pigment cells. The parietal and zymogenic cells are the main sites for the production of the salivary gland secretions, which pass through microvilli from the zymogenic cells to the lumen of the ducts within the acinus. Outside the acinus each duct is composed of highly specialized cells with infolded basement membranes extending about a third of the way across the cell. The cytoplasm between the membranes contains elongated mitochondria and glycogen granules. The apical border of the cell is thrown into microvilli which are closely aggregated under the cuticle lining the duct. These cells have all the features of cells previously described in vertebrates and invertebrates which are known to absorb water and/or ions. Absorption of water from the gut could allow the excretion of hypertonic saliva by the locust.     

Ultrastructure of bovine parotid glands

Bovine parotid glands exhibit outstanding structural differences when compared with those of non-ruminant mammals. The acini are tortuous, branched and lined with cells of different heights, imparting a scalloped appearance to acinar lumina. Numerous microvilli, ca. 1.5 μ in length, extend into the lumina and intercellular canaliculi. Intercellular canaliculi measure ca. 3 μ in diameter and interweave in close association with intercellular tissue spaces. Intercellular tissue spaces are separated from the extraacinar spaces across a basal lamina only, whereas junctional complexes guard canaliculi from direct continuity with tissue spaces and/or extraacinar spaces. Flattened cytoplasmic lamellae extend from adjacent acinar cells and loosely interdigitate with one another across the tissue spaces. Acinar cells contain more mitochondria and less granular endoplasmic reticulum than parotid glands of non-ruminant mammals. Two types of secretory material, in the form of inclusions which vary in size and electron density, are present in the acinar cells. Intercalated ducts connect acini with striated ducts which in turn, empty into collecting ducts located between gland lobules. In terms of frequency of “basal infoldings” and numbers of mitochondria, striated ducts of calf parotid glands are not as well developed as those of certain other salivary glands. Myoepithelial cells are most often present at junctions of acini and intercalated ducts where they may attach to both acinar and ductal epithelium. Nerve “terminals” were not observed on the epithelial side of basement membranes in relation to the secretory cells.     

Ultrastructural aspects of cat submandibular glands

Submandibular glands of five adult female cats were examined by conventional electron microscopic techniques. All gland acini are mucous secreting and each acinus is capped with mucous secreting demilunar cells. Secretory product of demilunar cells is more electron lucent than that of acinar cells. The demilunes show intercellular tissue spaces and intercellular canaliculi whereas similar specializations are absent between acinar cells. Mitochondria and arrays of granular endoplasmic reticulum are more numerous in demilunar cells than in acinar cells. In acinar and demilunar cells secretory droplets first appear as enlarged Golgi saccules which subsequently become closely related to cisternae of the granular endoplasmic reticulum. Filamentous structures, interpreted as mucin molecules, are present in secretory droplets of acinar cells. Intercalated ducts are short, consisting of several junctional cells between acini and striated ducts. Striated ducts are long and tortuous and contain light cells, dark cells and basal cells. Light cells contain numerous membrane bound granules in their distal ends whereas dark cells show electron lucent vesicles in the same position. Basal cells contain a paucity of organelles and membrane plications but exhibit hemidesmosomes along their basal plasma membranes. Myoepithelial cells are abundant in relation to acinar and demilunar cells. Nerve terminals are present in some instances between acinar cells or between acinar and myoepithelial cells.     

Amblyomma cajennense (Acari: Ixodidae): salivary gland cells of partially engorged females ticks and the production of lipid by their mitochondria

Morphologically, the salivary glands of ticks are paired structures consisting of a secretory and an excretory portion, lacking a reservoir for the storage of the secretion. The secretory portion is composed in females by cells that form acini classified into the types I, II, and III. The excretory possess a major duct, from which arise several intermediate ducts that then subdivide to form the canaliculi or acinal tubules, which end at the acini from where they collect the secretion. The present study describes the ultrastructural changes that occur in the mitochondria of cells of the acini I, II, and III in the salivary glands of partially engorged females of the Cayenne tick Amblyomma cajennense. The results show that this organelle exhibits completely disarrayed crests due to the presence of lipidic material inside the matrix and between the crests, thus demonstrating their participation in the production of the lipids that would be used structurally by the cells. These organelles with ultrastructural changes were denominated derived mitochondria.     

Mandibular glands of male European beewolves, Philanthus triangulum (Hymenoptera, Crabronidae)

Males of a solitary digger wasp, the European beewolf, Philanthus triangulum, possess large mandibular glands that have been reported to produce a scent marking pheromone. We analysed the morphology and ultrastructure of these glands using light microscopy as well as scanning and transmission electron microscopy. The paired glands are located laterally in the head and each side consists of a larger and a smaller part. Both parts possess a collecting duct each with distinct openings at the mandible base. However, the collecting duct of the larger part is additionally connected to the pharynx through a lateral extension. The collecting ducts are bordered by a monolayered epithelium lined with cuticle that exhibits conspicuous ramified protuberances. About 1400 acini consisting of class 3 gland cells surround the ducts and are connected to them through conducting canals. The main components in the cytoplasm of these gland cells are mitochondria, well-developed smooth endoplasmatic reticulum, and electron lucent vesicles suggesting a high secretory activity. The connection between the large gland parts and the pharynx suggests that the secretion of the mandibular glands might not only be delivered directly onto the mandibles but might also be transported to and stored in the postpharyngeal gland.     

Fine reconstruction of the pancreatic ductular system at the onset of pancreatitis

The three-dimensional structure of the pancreatic ductular system (from the intercalated duct to the intercellular secretory canaliculus) is controversial and unclear. The aim of this study is to reveal the three-dimensional structure of the pancreatic ductular sysytem at the onset of pancreatitis. One day following rat pancreatic duct ligation, dilated lumina from the pancreatic ductular system were reconstructed by light microscopic and scanning electron microscopic examination of pancreatic tissue serial sections. The existence of the intra-acinar duct, which is formed only by centroacinar cells and interconnects the adjacent central lumina in an acinus, was demonstrated. The intercellular secretory canaliculi, which are the terminal parts of the pancreatic ductular system, anastomose and end blindly in the intercellular space located between adjacent lateral surfaces of the acinar cells. The intercalated ducts, the intra-acinar ducts, the central lumina, and the intercellular secretory canaliculi are arranged together in a complex connecting and branching system. However, there were no anastomoses found among the central lumina or acini.     

Expression and localization of immunoreactive-sialomucin complex (Muc4) in salivary glands

Sialomucin Complex (SMC; Muc4) is a heterodimeric glycoprotein consisting of two subunits, the mucin component ASGP-1 and the transmembrane subunit ASGP-2. Northern blot and immunoblot analyses demonstrated the presence of SMC/Muc4 in submaxillary, sublingual and parotid salivary glands of the rat. Immunocytochemical staining of SMC using monoclonal antisera raised against ASGP-2 and glycosylated ASGP-1 on paraffin-embedded sections of parotid, submaxillary and sublingual tissues was performed to examine the localization of the mucin in the major rat salivary glands. Histological and immunocytochemical staining of cell markers showed that the salivary glands consisted of varying numbers of serous and mucous acini which are drained by ducts. Parotid glands were composed almost entirely of serous acini, sublingual glands were mainly mucous in composition and a mixture of serous and mucous acini were present in submaxillary glands. Since immunoreactive (ir)-SMC was specifically localized to the serous cells, staining was most abundant in parotid glands, intermediate levels in submaxillary glands and least in sublingual glands. Ir-SMC in sublingual glands was localized to caps of cells around mucous acini, known as serous demilunes, which are also present in submaxillary glands. Immunocytochemical staining of SMC in human parotid glands was localized to epithelial cells of serous acini and ducts. However, the staining pattern of epithelial cells was heterogeneous, with ir-SMC present in some acinar and ductal epithelial cells but not in others. This report provides a map of normal ir-SMC/Muc4 distribution in parotid, submaxillary and sublingual glands which can be used for the study of SMC/Muc4 expression in salivary gland tumors.     

Ultrastructural investigation of a salivary gland in a centipede: structure and origin of the maxilla I-gland of Scutigera coleoptrata (Chilopoda, Notostigmophora)

The maxilla I-gland of Scutigera coleoptrata was investigated using light and electron microscopy methods. This is the first ultrastructural investigation of a salivary gland in Chilopoda. The paired gland opens via the hypopharynx into the foregut and extends up to the third trunk segment. The gland is of irregular shape and consists of numerous acini consisting of several gland units. The secretion is released into an arborescent duct system. Each acinus consists of multiple of glandular units. The units are composed of three cell types: secretory cells, a single intermediary cell, and canal cells. The pear-shaped secretory cell is invaginated distally, forming an extracellular reservoir lined with microvilli, into which the secretion is released. The intermediary cell forms a conducting canal and connects the secretory cell with the canal cell. Proximally, the intermediary cell bears microvilli, whereas the distal part is covered with a distinct cuticle. The cuticle is a continuation of the cuticle of the canal cells. This investigation shows that the structure of the glandular units of the salivary maxilla I-gland is comparable to that of the glandular units of epidermal glands. Thus, it is likely that in Chilopoda salivary glands and epidermal glands share the same ground pattern. It is likely that in compound acinar glands a multiplication of secretory and duct cells has taken place, whereas the number of intermediary cells remains constant. The increase in the number of salivary acini leads to a shifting of the secretory elements away from the epidermis, deep into the head. Comparative investigations of the different head glands provide important characters for the reconstruction of myriapod phylogeny and the relationships of Myriapoda and Hexapoda.     

Active participation of apoptosis and mitosis in sublingual gland regeneration of the rat following release from duct ligation

Summary This study was designed to establish how mitotic cell proliferation and apoptotic cell death participate in the regeneration of atrophied rat sublingual glands. To induce atrophy to the sublingual gland of rats, the excretory duct was ligated unilaterally near the hilum, and after 1 week of ligation (day 0) the duct ligation was released to enable gland regeneration. The regenerating glands were examined with routine histology, immunohistochemistry for proliferating cell nuclear antigen (PCNA) as a marker of proliferating cells, terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling (TUNEL) as a marker of apoptotic cells, and transmission electron microscopy. At day 0, a few acini and many ducts remained in the atrophic sublingual glands, and newly formed immature acini were observed at day 3. Thereafter acinar cells progressively matured and increased in number, although the number of ducts decreased. Many PCNA- and some TUNEL-positive cells were seen in acini and ducts during regeneration. The labeling indices for both cell types were statistically significantly different from that of the control at several time points of the regeneration. Apoptotic and mitotic cells were also confirmed to be present in the experimental sublingual glands by electron microscopy. These observations suggest that apoptosis as well as mitosis of duct and acinar cells actively participate in and play important roles in sublingual gland regeneration.     

Structural and histochemical changes in the salivary glands of Rhipicephalus appendiculatus during feeding

The salivary glands of the brown ear tick of cattle, R. appendiculatus, from both sexes and at all stages of feeding, were examined as whole glands and as sections for ultrastructural and histochemical changes. The type 1 acinus consists of a basal labyrinth formed by the interdigitations of a central cell and four peripheral cells. These cells form a specialized border with a central constrictor cell which surrounds the acinar duct. The plasma membrane of the central cell is exposed to the duct. The type 1 acini do not appear to secrete active saliva components involved in feeding. The type 2 acini undergo a great increase in synthetic and secretory activity during feeding in both sexes and secrete a lipoprotein probably to form part of the attachment cone and also glycoproteins and esterases of unknown functions. The type 3 acini of both sexes also secrete a lipoprotein probably to form part of the attachment cone. The f cells of these acini in the females transiently secrete a glycoprotein of unknown function and then transform to become part of a water excreting unit. In the males the secretory activity of the granular cells of the type 2 and 3 acini is maintained for further attachments. The type 4 acini of the males accumulate masses of proteinaceous granules. The system of interstitial cells and intercellular spaces in types 2, 3 and 4 acini is large and increasingly active during feeding.     

Morphology of the salivary glands of Mastotermes darwiniensis Froggatt (Isoptera : Mastotermitidae)

The salivary glands of Mastotermes darwiniensis (Isoptera : Mastotermitidae) workers consist of acini made up of peripherally and centrally placed cells with storage cells. A network of small efferent ductules is associated with each acinus. Intracellular ductules of the central and peripheral cells are filled with protruding microvilli. Central cells contain an abundance of rough endoplasmic reticulum and vacuoles. Continua of rough endoplasmic reticulum and vacuoles are present. Pinocytosis does not appear to be involved in secretion by the central cells. Peripheral cells are compartmental, having electron-lucent vesicle accumulations away from intracellular ductules and the basal plasmalemma invaginations. Peripheral cell morphology is similar to cells which are associated with fluid movement. Storage cells contain a large vacuole and exhibit micropinocytosis. Two axon profile types are present near and on the acini and salivary ducts, one of which appears associated with neuronal end plates.     

Fine structure of the mandibular gland of the Djungarian hamster (Phodopus sungarus)

The mandibular gland of the Djungarian hamster was examined by light microscopy, and transmission and scanning electron microscopies. Its acinar cells reacted with periodic acid-Schiff (PAS) and were weakly stained with alcian blue (AB). There were intercellular canaliculi between the acinar cells. These cells therefore appeared to be seromucous. The acinar epithelium was composed of light cells containing various spherical secretory granules. The granular cells of the mandibular gland possessed many acidophilic granules exhibiting a positive reaction to PAS stain. They were frequently observed at the junction of the acini and intercalated ducts in all mandibular glands examined. All of these cells were light and contained secretory granules of varying size and density. The intercalated ducts consisted exclusively of light cells possessing a few round granules of high density in the apical region. The striated ducts were comprised of two portions--a secretory portion and a typical striated portion without secretory granules. The secretory portion consisted of light, dark and specifically light epithelial cells containing acidophilic granules, which exhibited a strongly positive PAS reaction. The epithelium of typically striated portions was composed of light and dark cells containing fine vacuoles in the apical region. The mandibular gland of the Djungarian hamster revealed no histological differences between sexes.     

Prenatal development of the palatine gland of rats

The present study aimed to elucidate the prenatal development of the rat palatine gland. Parasagittal 5 microm thick serial sections made from Wistar rats at embryonic days (E) 17 to 22 were stained with haematoxylin-eosin (HE), Alcian blue-Kernechtrot or immunohistochemistry for 5-bromo-2'-deoxyuridine (BrdU) as a marker of proliferating cells. Additionally, three-dimensional images of developing glandular parenchyma were reconstructed from serial HE sections with a personal computer. At E 17, several thickenings of the palatal epithelium had appeared which thereafter became the epithelial cords. Branching and lumenization commenced at E 20, and immature acini were observed at E 21. Three-dimensional reconstruction showed that the proximal part of the epithelial cord differentiated into the duct, and the distal part of the epithelial cord differentiated into the acinus. In immunohistochemical staining, there were many BrdU-positive cells in the epithelial cords including thickenings of the palatal epithelium, ducts, and acini. The BrdU labeling index of the cells of the epithelial cord was the highest (statistically significant) of the three in the primitive palatine gland. In conclusion, during the development of the rat palatine gland, epithelial cords with very high proliferative activity arise from the palatal epithelium, and then the proximal part of the epithelial cord differentiates into the duct, and the distal part of the epithelial cord differentiates into the acinus. Proliferation of these glandular parenchyma contributes to the growth of the developing palatine gland.     

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 -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, 1β1 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 -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 -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.     

Chimeric analysis of EGFP and DsRed2 transgenic mice demonstrates polyclonal maintenance of pancreatic acini

The pancreatic islet is an assembly of specific endocrine cells. There are many conflicting reports regarding whether the acinus develops from single or multiple progenitor cells. This study investigated the development and maintenance clonality of the pancreatic acinus and duct using a chimeric analysis with EGFP and DsRed2 transgenic mice. Chimeric mice (G-R mice) were obtained by the aggregation method, using 8-cell stage embryos from EGFP and DsRed2 transgenic mice. The islets from the G-R mice were chimeric and mosaic, consisting of either EGFP- or DsRed2-positive populations, as in previous reports. On the other hand, most acini developed from either EGFP or DsRed2 origin, but some were chimeric. Interestingly, these chimeric acini were clearly separated into two-color regions and were not mosaic. Some large intralobular pancreatic ducts consisting of more than 10 cells were found to be chimeric, but no small ducts made up of less than 9 cells were chimeric. Our histological observations suggest that the pancreatic acinus polyclonally and directionally is maintained by multiple progenitor cells. Pancreatic large ducts also seem to develop polyclonally and might result from the assembly of small ducts that develop from a single origin. These findings provide useful information for further understanding pancreatic maintenance.     

The roles of apoptosis and mitosis in atrophy of the rat sublingual gland

The roles of apoptosis and mitosis of acinar and duct cells in the atrophy of the sublingual gland of rat induced by double duct ligation was investigated using immunohistochemistry for proliferating cell nuclear antigen (PCNA), terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-digoxigenin nick end labeling (TUNEL), and transmission electron microscopy (TEM). Many PCNA-positive duct cells were observed 3 days after duct ligation, and the numbers decreased thereafter. At 3 and 5 days, several TUNEL-positive acinar cells were observed and typical apoptotic acinar cells were identified by TEM. Necrotic acinar cells were also observed ultrastructurally. After 7 days, there were few acini but many ducts, as well as many structures representing transition from acinus to duct. These observations demonstrate that acinar cell loss by apoptosis and duct cell proliferation by mitosis occur in atrophic sublingual glands as well as in other atrophic salivary glands. In addition, it appears that the transition from acinar to duct cell and the necrosis of acinar cells play important roles in the atrophy of the sublingual gland.     

Microstereological and histochemical studies of the salivary glands of the giant rat (Cricetomys gambianus, waterhouse)

The histology and histochemistry of the parotid, submandibular and sublingual glands were studied. The submandibular gland contained only serous acini as in the guinea pig, but unlike in many other mammals. The parotid gland contained only serous acini while the sublingual gland was mixed, mucous acini being the predominant secretory tissue interspersed by a few serous acini. Serous demilunes also commonly formed caps on the mucous acini. The ducts of the gland contributed over 30% of the volume of the submandibular gland, while those of the parotid and sublingual glands formed about 12 and 10% of the gland, respectively. The secretions of the parotid gland, as judged by histochemical methods, contained neutral mucins and some sialomucins. Neutral mucins, sulphomucins and sialomucins were detected in both the submandibular gland and sublingual gland.     

A Computational Study of the Development of Epithelial Acini: I. Sufficient Conditions for the Formation of a Hollow Structure

Normal hollow epithelial acini are 3-dimensional culture structures that resemble the architecture and functions of normal breast glands and lobules. This experimental model enables in vitro investigations of genotypic and molecular abnormalities associated with epithelial cancers. However, the way in which the acinar structure is formed is not yet completely understood. Gaining more information about consecutive stages of acini development—starting from a single cell that gives rise to a cluster of randomly oriented cells, followed by cell differentiation that leads to a layer of polarised cells enclosing the hollow lumen—will provide insight into the transformations of eukaryotic cells that are necessary for their successful arrangement into an epithelium. In this paper, we introduce a two-dimensional single-cell-based model representing the cross section of a typical acinus. Using this model, we investigate mechanisms that lead to the unpolarised cell growth, cell polarisation, stabilisation of the acinar structure and maintenance of the hollow lumen and discuss the sufficient conditions for each stage of acinar formation. In the follow-up paper (Rejniak and Anderson, A computational study of the development of epithelial acini. II. Necessary conditions for structure and lumen stability), we investigate what morphological changes are observable in the growing acini when some assumptions of this model are relaxed.