Ultrastructure and histochemistry of the submandibular gland of the European hedgehog, Erinaceous europaeus L. I. Acinar secretory cells

Scanning transmission electron microscopic analysis of nitrogen generated by 3, 3'-diaminobenzidine-besed peroxidase reaction with resin ultrathin sections of rhinoceros parotid gland acinar cells

A 3, 3'-diaminobenzidine (DAB)-based method was used to detect the localization of endogenous peroxidase activity in Indian rhinoceros (Rhinoceros unicornis) parotid gland acinar cells. The tissue had previously been resin-embedded in gelatin capsules for routine electron microscopic observations and thus pre-incubation for endogenous peroxidase analysis was not possible. We attempted to demonstrate the relationship between secretory granules (SGs) in resin ultrathin sections of Indian rhinoceros parotid gland acinar cells and endogenous peroxidase activity. A JEM 1400 Plus scanning transmission electron microscope (STEM) was used to conduct energy dispersive X-ray spectroscopy (EDS) analysis of the presence of nitrogen generated by the DAB reaction in bipartite structural SG consisting of a dense body (or core). The mapping patterns of nitrogen were restricted to the dense body. We observed nitrogen localized in the rough endoplasmic reticulum (ER), nuclear envelope (NE) and several components of the Golgi apparatus (G) of rhinoceros parotid gland acinar cells participating in the synthetic pathway of secretory proteins. Moreover, we established a nitrogen-detection method by EDS analysis of rhinoceros parotid gland. The reliability of the method was validated by comparison of the test group (peroxidase detection in ultrathin resin sections) and the control group (ordinary peroxidase detection in semi-thin sections following glutaraldehyde pre-fixation) of rat submandibular gland. The same mapping patterns of nitrogen were detected by DAB reaction in the SG, ER, NE and G in these two groups. Hence, EDS-STEM approaches for endogenous peroxidase post-incubation analysis will prove useful for advanced cytochemical analysis for the identification of any other resin sections.

Protein Production and Secretion in Exocrine Cells

Acinar cells of exocrine glands are highly specialized for producing, storing, and discharging secretory proteins for use on surfaces that represent interfaces between the organism and the surrounding environment. These functions are achieved through the secretory pathway that includes a series of functionally distinct intracellular compartments — the endoplasmic reticulum, subcompartments of the Golgi complex, and the secretion granule in which exportable macromolecules are stored at high concentrations. Most secretion occurs by granule exocytosis in response to external hormonal or neural stimuli. Although these processes have been traced in a variety of morphological and biochemical studies, very Utile is known about the mechanisms involved in facilitating and maintaining secretory storage, orchestrating discharge at the apical cell surface, and in ensuring conservation and re-internalization of the granule membrane. Recent studies initiated on cell fractions obtained from the rat parotid gland have provided significant insight into the protein storage conditions that prevail in the granule interior and the components of the granule membrane that are likely to be involved in general secretory function such as exocytosis.

Basic Biological Sciences

Cytomegaloviruses undergo the classical morphologic stages of replication in hedgehog submandibular gland duct cells. A profound cytomegaly is associated with the infection. These features indicate that the hedgehog may be an excellent model in which to study the biology of cytomegalovirus-salivary gland interactions.

Sorting of a HaloTag protein that has only a signal peptide sequence into exocrine secretory granules without protein aggregation

The mechanism involved in the sorting and accumulation of secretory cargo proteins, such as amylase, into secretory granules of exocrine cells remains to be solved. To clarify that sorting mechanism, we expressed a reporter protein HaloTag fused with partial sequences of salivary amylase protein in primary cultured parotid acinar cells. We found that a HaloTag protein fused with only the signal peptide sequence (Met(1)-Ala(25)) of amylase, termed SS25H, colocalized well with endogenous amylase, which was confirmed by immunofluorescence microscopy. Percoll-density gradient centrifugation of secretory granule fractions shows that the distributions of amylase and SS25H were similar. These results suggest that SS25H is transported to secretory granules and is not discriminated from endogenous amylase by the machinery that functions to remove proteins other than granule cargo from immature granules. Another reporter protein, DsRed2, that has the same signal peptide sequence also colocalized with amylase, suggesting that the sorting to secretory granules is not dependent on a characteristic of the HaloTag protein. Whereas Blue Native PAGE demonstrates that endogenous amylase forms a high-molecular-weight complex, SS25H does not participate in the complex and does not form self-aggregates. Nevertheless, SS25H was released from cells by the addition of a β-adrenergic agonist, isoproterenol, which also induces amylase secretion. These results indicate that addition of the signal peptide sequence, which is necessary for the translocation in the endoplasmic reticulum, is sufficient for the transportation and storage of cargo proteins in secretory granules of exocrine cells.

Ultrastructure of the platypus and echidna mandibular glands

The secretory units of the platypus and echidna mandibular glands consist of a single serous cell type. Secretory granules within the cells of the platypus mandibular gland stained intensely with the periodic acid-Schiff staining procedure but failed to stain with Alcian Blue, suggesting the granules contained neutral glycoproteins. Secretory granules within the mandibular glands of the echidna failed to stain with the methods used indicating little if any glycoprotein was associated with the secretory granules. Ultrastructurally, secretory granules of the platypus mandibular gland were electron dense with a central core of less electron-dense material and were membrane bound. In contrast, those of the echidna presented a lamellated appearance and also were limited by a membrane. These secretory granules appeared to form as a result of concentric layering of lamellae within cisternae of the Golgi membranes. The intralobular ductal system of the platypus was more extensively developed than that of the echidna. The striated ducts of both species were characterized by elaborate infoldings of the basolateral plasmalemma and an abundance of associated mitochondria.

The serous demilune of rat sublingual gland is an artificial structure produced by conventional fixation

The ultrastructure of the secretory end-piece of the rat sublingual gland was examined in samples prepared by rapid freezing and freeze-substitution method, and results were analyzed in combination with 3-D images reconstructed by computer graphics from light micrographs of serial sections. Fixation by rapid freezing followed by freeze-substitution preserved cellular ultrastructures, especially the membrane structure, in perfect condition, and demonstrated the terminal portion of the sublingual gland to be a compound branched tubulo-alveolar gland with serous cells distributed throughout the end-pieces. All the serous cells aligned with mucous cells to surround a common lumen, leaving no demilune structure. In contrast, samples fixed by the conventional immersion method showed distended mucous cells displacing the serous cells toward the basal portion of the acinus to form the demilune structure. The luminal space was also compressed and appeared disconnected from the serous cells. From these observations, the serous demilune that for more than 130 years has been believed to be an actual histological entity was proved to be an artificial structure produced through compression by the hydrated and expanded mucous cells during immersion fixation.

Intracellular transport and secretion of salivary proteins

Intracellular transport and secretion of salivary proteins are major activities of salivary acinar cells. While the major intracellular pathway followed by salivary proteins following their synthesis has been described previously, there is only limited understanding of how this process is regulated at the molecular level. Studies of salivary proteins, especially proline-rich proteins, expressed in an endocrine cell line have begun to provide insight regarding intermolecular interactions during transport and the role played by structural signals during intracellular sorting. Analysis of the secretion of newly synthesized salivary proteins in parotid tissue has shown that there are multiple pathways of discharge from acinar cells. While granule exocytosis is the major pathway, at least two other pathways that export salivary proteins have been found to originate from maturing secretion granules. These pathways may contribute to other acinar cell functions, including secretion of proteins in the absence of acute stimulation and support of the secretory process for fluid and electrolytes.

Differential distribution of salivary agglutinin and amylase in the Golgi apparatus and secretory granules of human salivary gland acinar cells

The secretory granules of salivary glands often display complex internal substructures, yet little is known of the molecular organization of their contents or the mechanisms involved in packaging of the secretory proteins. We used post-embedding immunogold labeling with antibodies to two secretory proteins, agglutinin and alpha-amylase, to determine their distribution in the Golgi apparatus and secretory granules of the human submandibular gland acinar cells. With monoclonal antibodies specific for carbohydrate epitopes of the agglutinin, reactivity was found in the trans Golgi saccules, trans Golgi network, and immature and mature secretory granules. In the granules, labeling was seen in regions of low and medium electron density, but not in the dense cores. Reactivity seen on the apical and basolateral membranes of acinar and duct cells was attributed to a shared epitope on a membrane glycoprotein. Labeling with a polyclonal antibody to amylase was found in the Golgi saccules, immature and mature secretory granules, but not in the trans Golgi network. In the granules, amylase was present in the dense cores and in areas of medium density, but not in the regions of low density. These results indicate that these two proteins are distributed differently within the secretory granules, and suggest that they follow separate pathways between the Golgi apparatus and forming secretory granules. Small vesicles and tubular structures that labeled only with the antibodies to the agglutinin were observed on both faces of the Golgi apparatus and in the vicinity of the cell membrane. These structures may represent constitutive secretion vesicles involved in transport of the putative membrane glycoprotein to the cell membrane.

Ultrastructure of the submandibular gland in the African multimammate rodent, Praomys natalensis

Praomys natalensis, an African rodent that is phenotypically and cytogenetically intermediate to rats and mice, possesses a submandibular gland that is histologically similar to that in both of these near relatives, but is ultrastructurally unique. Acinar cells, which are seromucous in nature, contain secretory granules that often contain a perfect "bull's eye" inclusion (or some variant of this configuration) suspended in a dense matrix. The Golgi apparatus in these cells has an unusual structure, with the Golgi saccules often being doubled over, so that the outermost saccule also is the innermost. This peculiar architecture apparently arises fairly late in the secretory process, i.e., a Golgi apparatus of conventional structure gives rise to a nascent granule (condensing vacuole), then its saccules secondarily fold over. Intercalated ducts are preceded by a ring of specialized cells that have a number of serous-type granules, the duct cells themselves being devoid of such granules. Granular convoluted tubules (GCT) contain large dense granules that appear to be spontaneously involved in chain exocytosis. These GCT granules probably are the repositories of nerve growth factor, which is particularly abundant in Praomys. Striated ducts for the most part are typical in appearance, but they and, to a lesser extent, GCTs contain prominent, membrane-bound crystalloids with a periodicity of about 15 nm.

Sorting and secretory pathways in exocrine cells

Exocrine secretory cells contain multiple post-Golgi pathways from protein secretion. The major pathway in pancreatic and parotid acinar cells involves protein sorting into storage granules that undergo exocytosis with or without stimulation by secretagogues. This route of release is paralleled by a minor nongranular (but vesicular) pathway that originates by budding from maturing secretory granules. The nongranular pathway carries the same polypeptides that undergo storage in the granules but in different relative amounts. These features indicate that sorting into the stimulus-regulated pathway reflects not only the deposition of secretory proteins into immature granules but may also involve selective aggregation of proteins along with exclusion and vesicle-mediated secretion of other polypeptides that are inefficiently retained. Storage granules represent a distinct compartment of the secretory pathway, as indicated by the specific composition of their limiting membranes. Little is known about processes that maintain the low content and limited diversity of integral proteins of the granule membrane as compared to the membranes with which it fuses during exocytosis and formation. Future studies will examine the role of the nongranular secretory pathway in acinar cells, the branchpoint of pathways that are directed to the apical or basolateral cell surfaces, the structural determinants of secretory sorting, and the distribution and function of specific granule membrane polypeptides.

Evidence for sequential deployment of secretory enzymes during the normal acrosome reaction of guinea pig sperm in vitro

Experiments were conducted to determine if acrosomal enzymes are released simultaneously or in sequence during the normal acrosome reaction. Epididymal guinea pig sperm were incubated in a chemically defined, calcium-containing medium which supports normal acrosome reactions within 4-5 hours at 37 degrees C. The sperm suspensions were monitored for motility, normal acrosome reactions, and false acrosome reactions during in vitro incubation. At specified time intervals, the sperm were separated from the incubation medium by centrifugation, and the distribution of dipeptidyl peptidase (DPP II) and acrosin activity was determined by biochemically assaying the hydrolysis of trialanine and N-benzoyl-L-arginine ethyl ester (BAEE), respectively. When calcium was present, there was a significant increase in DPP II activity in the supernatants by 1 hour of incubation and a slight decline at later time points. This release was not correlated with false or normal acrosome reactions (loss of the acrosomal cap) monitored by phase-contrast microscopy but probably represents a very early stage in the normal acrosome reaction. This early stage is difficult to detect at the light microscope level because sperm are still in rouleaux and because membrane fusion is not directly observable. In contrast, acrosin activity, which was assayed in the same supernatants, increased at later times when sperm were observed to have completed normal acrosome reactions. The ultrastructural distribution of DPP II was determined in sperm pellets collected during in vitro incubation by using the DPP II substrate lysyl-alanyl-4-methoxy-2-naphthyamide. In freshly isolated cauda epidiymal sperm, reaction product is confined to the light-staining area in the dorsal bulge of the acrosome. However, by 1 hour of incubation, the light-staining area of many sperm was partially or completely dispersed, while other regions of the acrosome were unchanged. Our data are consistent with the conclusions that DPP II is a highly soluble component of the guinea pig sperm acrosome and that its release occurs during the initial phase of the acrosome reaction while sperm are still in rouleaux. Structural changes in the acrosome associated with DPP II release were detectable by electron microscopy but not by light microscopy. Acrosin, which is less soluble than DPP II, is released at a later time during the acrosome reaction. Both DPP II and acrosin appear to be partially inhibited following their release from sperm. A complete understanding of the sequential release and extracellular activities of the acrosomal enzymes will be necessary to fully define their functions in fertilization.

Ultrastructural morphology of secretory granules of submandibular and parotid salivary glands of man

Morphological variation among secretory granules of the same type of cell was found in the acini, intercalary and striated ducts, and collecting-ducts, and was greatest in serous and intercalary-duct cells. Secretory granules of monopartite and bipartite structure were seen in all these cell types; tripartite forms were seen in serous and intercalary-duct cells. A possible explanation for the variation is that one type of cell may be able to produce a range of secretory products and package them variously into secretory granules, thus creating different appearances.

Substructure of granules from serous cells of porcine tracheal submucosal glands

The ultrastructure of serous cells from porcine tracheal submucosal glands was studied by conventional transmission electron microscopy (TEM), and by cytochemical methods to stain for complex carbohydrates. In tissue fixed and processed for TEM, and stained with uranyl acetate and lead citrate, the condensing granules of serous cells occasionally possessed a hexagonal and sometimes a lamellar substructure. Tissue fixed in paraformaldehyde-glutaraldehyde and stained with periodic acid-thiocarbohydrazide-silver proteinate (PTS) or with phosphotungstic acid (PTA) showed secretory granules stained for complex carbohydrates and revealed a substructure similar to that noted in the condensing granules. The dark staining substructure revealed by either the PTS or the PTA technique appeared to correspond to electron-lucent areas observed in the condensing granules by conventional TEM. The PTS staining probably demonstrated the presence of neutral glycoprotein, since the serous-cell granules did not react with a dialyzed iron stain for acidic glycoproteins. Treatment of periodic acid oxidized thin sections with pronase or pepsin prior to thiocarbohydrazide and silver proteinate treatment decreased the intensity of the PTS staining, but did not digest away any components of the granules. The substructure revealed by the carbohydrate stains may be a reflection of the mechanism of packaging or the macromolecular structure of the glycoproteins in the serous-cell granules.

Fine structure and function of the prosomal glands of the two-spotted spider mite, Tetranychus urticae (Acari, Tetranychidae)

The prosomal glands of Tetranychus urticae (Acari, Tetranychidae) were examined light and electron microscopically. Five paired and one unpaired gland are found both in females and males. The silk spinning apparatus consists of paired silk glands which extend laterally on both sides of he esophagus into the pedipalps. There, they enter the terminal silk gland bag which opens into a silk bristle at the apex of hte pedipalps. The salivary secretions are formed in three paired glands which have an interconnecting duct, the podocephalic canal. The dorsal podocephalic glands may produce a serous secretion, the anterior podocephalic glands a mucous secretion, and the coxal organ may add a liquid, ion-rich secretion. These secretions pass the podocephalic canal and reach the mouth at the apex of the gnathosome. The function of the paired tracheal organs and the unpaired tracheal gland is still unclear. The tracheal gland may produce a secretion which facilitates the movement of the fused chelicerae and the stylets.

Fine structural aspects of silk secretion in a spider (Araneus diadematus). I. Elaboration in the pyriform glands

Pyriform glands of Araneus diadematus which produce the silky material used for the attachment discs of the web, consist of two kinds of secretory cells. One, located in the distal half of the glands, elaborates finely fibrillar proteinic granules through an extensive rough endoplasmic reticulum; another, in the proximal half of the glands, secretes complex-structured granules in areas of the cell where Golgi and ergastoplasmic cisternae are equally developed. The opaque nascent granules of secretion appear in swollen Golgi saccules. These aggregate is superposed circular interconnected layers leaving an electron-lucent space between them; in the course of maturation the space is progressively filled with a fibrillar material. Histochemical tests suggest that the secretory product of the proximal half is mainly a protein rich in acidic groups and associated with a carbohydrate component. The two products, extruded by a merocrine process, form respectively the core and the envelope of the silk fibre. The dual composition of the pyriform gland silk, which did not appear from the results of chemical analyses, is compared to the association of fibroin and sericin in Lepidoptera silk and to certain double-layered Trichoptera silks.

Ultrastructure of the parotid gland of the nine-banded armadillo

The parotid gland of the nine-banded armadillo (Dasypus novemcinctus) was examined by electron microscopy. In general, the ultrastructural morphology of this gland appears similar to that described in other species. The most unusual feature of the gland is that the secretory granules of the acinar cells contain a wide variety of substructures. These substructures range from a single dense core within a homogeneous matrix to a more complex cord-like network extending throughout the granule. It is suggested that this diversity of substructures is indicative of a maturation process of the secretory granules. The intercalated ducts are secretory. The myoepithelial cells are numerous, extend throughout the gland, and are associated with the striated ducts.

Ultrastructure of the acinar cells in the submandibular gland of the nine-banded armadillo

There are two discrete lobes comprising the armadillo submandibular gland. These two lobes can be defined grossly, histochemically and morphologically with the light and electron microscope. The minor lobe stains more intensely with PAS and AB. When viewed in the electron microscope, the secretory granules of the acinar cells within this lobe appear mucous-like. The granules of the demilune cells are slightly different in appearance. The secretory granules of the acinar cells in the major lobe contain many dense foci embedded in a fibrillar matrix, a substructure not described previously. The demilune cells of this lobe contain secretory granules with a mucous-like structure which is consistent throughout the entire lobe. As in the minor lobe, these demilune cells stain very intensely with PAS and AB.

Some ultrastructural features of acinic cell carcinoma

The ultrastructure of an acinic cell carcinoma, occurring in the left parotid gland of a 52-year-old woman and causing a total facial nerve paralysis, is described. Histologically the tumour consisted of numerous granulated cells arranged around lumen-like openings and resembling a secretory system. Furthermore, areas with agranulated cells growing in a solid pattern were also encountered. In the electron microscope the cytoplasmic granules of the tumour cells displayed a varied appearance. Granules of a dense homogeneous type, as well as granules with a more electron lucid appearance were observed. Furthermore, numerous cytoplasmic granules displayed a bipartite structure with a dense central and a more electron lucid outer zone. In specimens primarily fixed in OSO4 or KMnO4 the granules displayed a 'leached out' appearance. The membrane-bounded of the tumour cells also showed a strong positive staining with the periodic acid-chromic silver technique of Rambourg et al. (1969). Other characteristic ultrastructural features of the tumour cells studied were: Smooth cell surfaces, the presence of subplasmalemmal bands of electron dense material, desmosome-like attachment areas between cells and grossly altered mitochondria.

Ultrastructural cytochemistry of the secretory granules of the hamster submandibular gland

The ultrastructure and cytochemistry of the secretory granules of the male hamster submandibular salivary gland were studied; After fixation in glutaraldehyde followed by osmium tetroxide the granules exhibit a characteristic bipartite substructure, with an electron lucid crescenteric rim and a more dense central core. A differentiation into two regions of the granules could also be visualized in specimens primarily fixed in Millonig's osmium tetroxide or in potassium permanganate. The electron lucid peripheral portion of the membrane bounded secretory granules further displays a strong positive reaction after staining of ultrathin sections with the periodic acid-chromic acid-(PA-CrA)-silver technique. The strong periodate reactivity of the rim relative to the core, suggests a difference in mucin composition of the two granule regions. With the PA-CrA-silver straining technique a positive reaction was also observed within the Golgi apparatus of the acinar cells.

Ultrastructure of the secretory cells of the submucosal glands in the human maxillary sinus

Tissue samples obtained from the lateral wall of the maxillary sinuses of five patients were examined by light microscopical, histochemical, and ultrastructural techniques. Submucosal glands were tubulo-alveolar mixed glands. The acini consisted of either all serous or all mucous cells, or a mixture of both. Serous granules were stained by toluidine blue, or by hematoxylin and eosin (H and E), but showed little or no reaction with periodic acid-Schiff (PAS) or Alcian blue. Mucous granules were pale in toluidine blue or H and E preparations, and consisted primarly of acid mucosubstances, as demonstrated by their staining reaction with PAS and Alcian blue. At the electron microscope level, the serous granules were either homogeneously dense, or showed a substructure consisting of at least two layers of distincly different electron-opacity. Typical mucous droplets consisted of a fibrillar network dispersed in a translucent matrix. A second secretory product was present in the mucous cells in the form of elongated, membrane-bounded structures containing numerous parallel filaments, which measured about 55 A in diameter. The mucous droplets and the filamentous bodies appear to arise from the opposite faces of the Golgi complex in the mucous cells. The filamentous bodies showed a pronounced tendency to fuse with the mucous droplets. All acini were surrounded by a well-defined myoepithelial layer and contained intercellular nerve terminals.

Morphogenesis and ultrastructure of the mouse embryonic salivary gland in tissue culture. Normal, and following exposure to trypsin

The primordial submandibular glands of 12-day-old mouse embryos were studied in tissue culture before and after treatment with trypsin under the electron microscope. In vitro differentiation proceeded normally and reached a high level of differentiation. Following a soak in trypsin for 15 or 30 minutes, considerable changes were noted in the basal lamina and in the mesenchymatous cells. There often occurred bizarre bullous protrusions of the cytoplasm through the apparently weakened basal lamina and the mesenchymatous cells were converted into so-called "ropalocytes". Subsequently the cells regained their normal appearance and the basement lamina was covered by a thick layer of amorphous electron-opaque basement membrane like material. It is concluded that the basal lamina (the basement membrane under the light microscope) might be the keystone in the differentiation of an organ and its maintenance in the adult. The development of innervation has also been studied and it was shown that the developing submandibular galnd is endowed with large bundles of nerve axons surrounded by Schwann cells lying in the epithelial-mesenchymal region. Intra-epithelial nerves were conspicuous and occasional synaptic bars or rings could be seen contributing to thedifferentiation of the secretory cell.