The secretory cells of the submaxillary gland in the perinatal period of development in the rat

The lack of terminal tubule cells in the submandibular gland of mice deficient in submandibular gland protein C

The submandibular gland (SMG) of newborn mice has no mature acini but has the rudiments of acini called terminal tubules (TT). The TT are composed of TT cells with dark secretory granules and proacinar cells with lighter secretory granules, the latter being considered the immediate precursor of mature acinar cells. TT cells contain a specific secretory protein, submandibular gland protein C (SMGC) and they decrease in number postnatally at a higher rate in males than in females. In the present study, in order to clarify the biological roles of TT cells and their secretory product SMGC, we generated a knockout (KO) mouse strain deficient in SMGC. The KO mice of both sexes grew normally, had normal reproductive capacity and had normal acinar and duct systems in the SMG in adult ages. However, through the neonatal and early postnatal stages, the KO mice were deficient not only in the production of SMGC but also in TT cells. With electron microscopy of the SMG of newborn KO mice, TT cells with characteristic granules were absent and replaced by undifferentiated ductal cells, whereas proacinar cells were normal. These results suggested that the absence of SMGC inhibits the development of TT cells and that the absence of SMGC and TT cells has no notable influence on the postnatal development of the acinar and duct systems in the SMG.

Immunohistochemical localization of keratin 5 in the submandibular gland in adult and postnatal developing mice

Keratin 5 (K5) is a marker of basal progenitor cells in the epithelia of a number of organs. During prenatal development of the submandibular gland (SMG) in mice, K5(+) progenitor cells in the developing epithelia play important roles in its organogenesis. Although K5(+) cells are also present in the adult mouse SMG and may function in tissue regeneration, their histological localization has not yet investigated in detail. In the present study, we examined the immunohistochemical localization of K5 in the SMG in adult and postnatal developing mice. At birth, K5 immunoreactivity was detected in the entire duct system, in which it was localized in the basal cells of a double-layered epithelium, but was not detected in the terminal tubule or myoepithelial cells. At postnatal weeks 1-3, with the development of intercalated ducts (ID), striated ducts (SD), and excretory ducts (ED), K5-immunoreactive basal cells were gradually restricted to the ED and the proximal double-layered portions of the ID connecting to the SD. At the same time, K5 immunoreactivity appeared in myoepithelial cells, in which its positive ratio gradually increased. In adults, K5 immunoreactivity was localized to most myoepithelial cells, most basal cells in the ED, and a small number of ID cells at the boundary between the ID and SD in the female SMG or between the ID and granular convoluted tubules in the male SMG. These results suggest that K5 is a marker of differentiated myoepithelial cells and duct progenitor cells in the mouse SMG.

Selective localization of diacylglycerol kinase (DGK)ζ in the terminal tubule cells in the submandibular glands of early postnatal mice

The present immunohistochemical study was attempted to localize in the submandibular glands of mice at various postnatal stages a diacylglycerol kinase (DGK) isoform termed DGKζ which is characterized by a nuclear localization signal and a nuclear export signal. This attempt was based on following facts: the continuous postnatal differentiation of glandular cells in the rodent submandibular gland, the regulatory role of DGK in the activity of protein kinase C (PKC) through attenuation of diacylglycerol (DAG), and the possible involvement of PKC in various cellular activities including the saliva secretion as well as the cell differentiation. As a result, a selective localization of immunoreactivity for DGKζ was detected in terminal tubule (TT) cells which comprise a majority of the newborn acinar structure and differentiate into the intercalated duct cells and/or the acinar cells. The immunoreactivity was deposited in portions of the cytoplasm lateral and basal to the nucleus, but not in the nuclei themselves. Although the immunoreactive TT cells remained until later stages in female specimen than in male, they eventually disappeared in both sexes by young adult stages. The present finding suggests that the regulatory involvement of DGKζ in PKC functions via control of DAG is exerted in the differentiation of the TT cells. In addition, another possible involvement of DGKζ in the regulation of secretion of the TT cells as well as its functional significance of its nuclear localization in the submandibular ganglion cells was also discussed.

Expression of heat shock protein 27 with the transition from proliferation to differentiation of acinar precursor cell in regenerating submandibular gland of rats

Heat shock protein 27 (Hsp27) has been suggested to participate in the cell proliferation and differentiation during tissue development. In fact, we have demonstrated the transient occurrence of Hsp27 during the differentiation of salivary gland acinar cells in postnatal rats. The purpose of the present study is to explore the potential role of Hsp27 in the proliferation and differentiation of the acinar cells during regeneration of the salivary gland. Using the experimental regeneration model of the rat submandibular gland after the release of duct ligation, the spatio-temporal localization of Hsp27 was investigated in immunohistochemistry in regenerating acini. No epithelial cells were immunoreactive for Hsp27 immediately after unligation, but Hsp27-immunoreactive cells were observed in regenerating acini located at the end portion of survived ductal tissues on the third day after unligation. The number of Hsp27-immunoreactive cells in regenerating acini reached its peak on the 5th day after unligation, and started to decline on the 7th day. They were undetectable on the 14th day. Importantly, the increase in the number of Hsp27-immunoreactive cells was preceded by the decline in the cell proliferative activity, and Hsp27-immunoreactivity declined and disappeared in conjunction with the progression of acinar cell differentiation, as judged by the double-immunostaining for Hsp27 and proliferating cell nuclear antigen, a cell proliferation marker, or glycine-rich protein-alpha, a specific marker of differentiated acinar cells. All the findings suggest that Hsp27 is expressed with the transition from the cell proliferation to differentiation of the acinar precursor cells during the regenerating process.

Effect of glucocorticoid on the differentiation and development of terminal tubules in the fetal rat submandibular gland

Glucocorticoids (CORT) are known to promote branching of the epithelial cords during the development of the rat submandibular gland. The aim of this study was to examine the effect of CORT (triamcinolone) on the differentiation of cells forming the terminal tubules in the developing fetal rat submandibular gland and the properties of the secretory granules. Light and electron microscopy showed that the terminal tubules of the glands in the experimental group contained more type III cells, which have been identified as proacinar cells, than those in the control group, whereas the relative number of type I cells, which have been identified as terminal tubule cells, was reduced. Immunoelectron microscopy using an antibody against neonatal submandibular gland secretory protein B (SMGB) revealed the presence of more gold particles over type III cell granules in the experimental group than in the control group. Lectin histochemistry demonstrated more wheat germ agglutinin (WGA)-labeled gold particles over type III cell granules in the experimental group than in the control group. These findings suggest that CORT promote the differentiation of type III cells, and moreover stimulate the production of secretory granules reactive for SMGB and WGA by acting on the terminal tubules of the developing rat submandibular gland.

Changes in the distribution and fine structure of the intralobular blood vessels of the submandibular gland in the postnatally developing mouse

Previous studies have shown that the blood vessels supplying the endocrine organs and the mucosa of the intestinal canals change in terms of not only their distribution but also their structure with the development and growth of each organ. We examined changes in the distribution and structure of intralobular blood vessels, including capillaries, throughout the postnatal development of the submandibular gland, an exocrine organ. The mouse submandibular gland from days 0 (birth) to 49 was investigated chronologically and ultrastructurally. The capillaries changed from continuous to fenestrated on day 10, coincident with an increase in the number of acini to more than the number of terminal tubules. The number of sections of intralobular blood vessels per unit area gradually decreased with increasing acinar size and was lowest on day 21 when pups were weaned; the same number was maintained from then on. In contrast with the reduction in the number of intralobular blood vessels, the number of capillary pores appeared to increase gradually. Acinar size increased further till day 28. Capillary pore number also increased further, till day 35, apparently in relation to the increasing acinar size. These findings suggest that the changes in distribution and structure of the intralobular blood vessels in the submandibular gland of the postnatally developing mouse are closely related to the development of the parenchymal cells in preparation for weaning and sexual maturity.

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.

Developmental changes of sugar residues and secretory protein in mucous cells of the early postnatal rat parotid gland

Mucous cells have been identified in the terminal portions of the early postnatal parotid gland in human and rat, although mature parotid gland acini are composed of serous cells or seromucous cells. Previously, Ikeda et al. demonstrated that mucous cells are present in the rat parotid gland on days 1 to 8 after birth and that the secretory granules within these mucous cells share some histochemical characteristics with mature serous cells. However, it is still not clear whether the mucous cells change into serous cells as the gland develops. The purpose of this study was to determine whether the mucous cells that appear in the early postnatal rat parotid gland change into serous cells. Parotid glands were obtained from male or female Wistar rats (aged 0-14 days and adults). Fixed tissue sections were reacted with soybean agglutinin (SBA) and wheat germ agglutinin (WGA) to detect glycoconjugates, or were stained using an anti-neonatal submandibular gland protein B1 (SMG-B1) antibody to identify serous acinar cells. The sections were observed by transmission electron microscopy. Electron microscopy revealed that cells with characteristics intermediate between those of mucous and serous cells (transitional cells) appeared around day 8 and that the nuclei of these cells did not show chromatin condensation, a characteristic of apoptotic cells. Lectin histochemistry showed that the mucous cells had the same sugar residues as the serous cells, which appeared after day 10. Immunohistochemistry with an anti-SMG-B1 antibody gave a positive reaction not only in the cells with highly electron-dense granules but also in the electron-dense cores of bipartite or tripartite granules in the transitional cells. Cells with morphological characteristics intermediate between those of mucous and serous cells (transitional cells) appearing in the early postnatal rat parotid gland begin to produce B1-immunoreactive protein common to serous acinar cells during development of the gland.

[Prenatal development of the mandibular gland and parotid gland in cats]

The prenatal development of the cat's mandibular and parotid gland was examined by means of serial section of 36 cat embryos at 14-62 days of development. Both glands were excised from the epithelium of the primitive oral cavity and branched up to day 36 of the branching phase into a specific connective tissue. This tissue contained besides fine collagenous fibres, a high amount of proteoglycans. In the subsequent separation phase, ducts and acini differentiated themselves in primitive lobules which were separated by connective tissue. In the prenatal differentiation phase, from about day 50 up to birth, intercalated ducts and striated ducts were formed. In the acini, mucous and serous cells contained different amounts of complex carbohydrates. This secretory component changed shortly before birth.

Lipid droplet accumulation and lipoprotein lipase activity in the rat salivary gland during the perinatal period

The submandibular and sublingual glands of foetal and newborn rats aged 21 days in utero to 7 days after birth were examined morphologically and biochemically. Lipid droplets tended to be localized in secretory cells, especially in their basal cytoplasm. The degree of droplet accumulation varied with the age of the rat. No droplets were observed before and immediately after birth. The number of accumulated droplets peaked 24-48 h after birth, then gradually decreased and reached normal levels by 5 days. In the salivary glands of fasted newborn rats, no lipid droplets were observed throughout the experiment. The amount of triacylglycerol reached its maximum level 1 day after birth; it then decreased gradually until 5 days and after that did not change. The amount of cholesterol did not change during postnatal development. Lipase activity attained its maximum level in the salivary glands immediately after birth and then decreased rapidly. It was higher in the glands of fasted than fed 1-day-old rats. Antiserum against lipoprotein lipase inhibited the salivary gland lipase activity in a dose-dependent manner, with 5 microliters of antiserum producing 60-70% inhibition. Non-immune serum had little effect. It was concluded that (1) accumulated lipid in the secretory cell cytoplasm of the salivary glands originates from ingested milk; (2) the principal component of accumulated lipid droplets is triacylglycerol; (3) 60-70% of the total lipase activity represents lipoprotein lipase; (4) an increase of lipoprotein lipase activity is recognizable before the accumulation of triacylglycerol.

Occurrence of a third type of secretory cell in the acinus of the rat submandibular gland

A new, third type of secretory cell was found in the acini of the submandibular gland in rat. In paraffin sections stained with hematoxylin-eosin or reacted with periodic acid-Schiff, acini containing the third type of secretory cells were found as a mass of darkly stained secretory endpieces at the periphery of the lobule. In semi-thin sections stained with toluidine blue, these cells showed numerous dense secretory granules in their cytoplasm. Some acini consisted entirely of these cells, whereas other acini consisted of an admixture of such cells and typical seromucous cells. Acini containing the third type of secretory cell tended to form clusters. Electron microscopically, the cells had well-developed rough endoplasmic reticulum and Golgi apparatus. The secretory granules showed a wide range of variation in substructure, even within the same cell. Well-developed intercellular interdigitation and intercellular canaliculi also were observed between adjacent third type of secretory cells. Although little is known about the biological significance of these cells and the properties of their secretory granules, the present study shows that the rat submandibular gland possesses three types of secretory cells in their secretory endpieces.

Accumulation and localization of two adult acinar cell secretory proteins during development of the rat submandibular gland

The seromucous acinar cells of the adult rat submandibular gland secrete a characteristic mucin glycoprotein and a family of unusual glutamine/glutamic acid-rich proteins (GRP). Monoclonal antibodies to the mucin and GRP localized in a very few Type III cells in glands of newborn and 1 day-old rats, using light and electron microscopic immunocytochemistry. Both mucin and GRP reactivities were present in the polymorphic Type IIIP granules during the 1st postnatal week. By 9 days after birth, the granules contained both mucin and GRP and were mucous-like in appearance. At earlier stages, however, cells containing only GRP or mucin could be found, indicating that the initiation of GRP and mucin biosynthesis may not be coordinately regulated. No reactivity was seen in the neonatal Type I cells or in duct cells at any age. Northern and Western blot analysis showed GRP mRNA and protein levels to be barely detectable at birth, with marked increases during the first 2 postnatal weeks. In contrast, Western blots of B1-immunoreactive proteins (B1-IP) showed levels highest in the 1st week and markedly decreased in the adult. Immunocytochemical colocalization, using gold particles of different sizes, showed that the B1-IP, mucin, and GRP colocalized in the granules. These results strengthen the hypothesis that the adult acinar cells develop from the neonatal Type III cells. No evidence was obtained for the involvement of Type I cells in the pathway of acinar cell development.

Epithelial-mesenchymal interactions in the development of salivary glands

The full expression of both morphogenesis and secretory cell differentiation in salivary glands is modulated or controlled, at least in part, by interactions between the salivary epithelium and the surrounding mesenchyme. Salivary gland morphogenesis and cytodifferentiation are partially linked but independently regulated processes. This presentation reviews the information that establishes the role of various extracellular matrix molecules and direct epithelial-mesenchymal interactions in the induction, control, and maintenance of morphogenesis and secretory cell differentiation in salivary glands.

A secretory protein restricted to type I cells in neonatal rat submandibular glands

The perinatal submandibular gland of the rat contains an 89-kDa secretory protein (Protein C) that is released upon cholinergic stimulation. Polyclonal antibodies raised against Protein C show that this protein is localized in the Type I cells and is not found in typical Type III cells. However, morphological variants of Type III cells (Type IIIP) contain material that is cross-reactive with antibodies to Protein C. Cross-reactive components also are found in mucous cells of the neonatal sublingual glands, parotid and minor sublingual glands, and adult submandibular and sublingual glands. Immunoblots of electrophoretically separated proteins show a distinct Protein C band at 89 kDa only in neonatal submandibular glands; neonatal sublingual and minor sublingual glands show some diffuse reactivity over a range of mobilities encompassing that of Protein C. We propose that the cross-reactive components of mucous cells and Type IIIP cells are not Protein C, but different proteins associated with mucous differentiation, and that the Type IIIP cells of the neonatal submandibular gland are in transition from Type III to mature mucous cells.

Secretory proteins as markers for cellular phenotypes in rat salivary glands

The neonatal submandibular glands (SMG) of the rat contain two types of cells: Type III cells secrete a group of proteins in response to beta-adrenergic stimulation, and Type I cells secrete a different protein, called Protein C (89 kDa), in response to cholinergic stimuli (Ball and Redman, 1984). Polyclonal antibodies raised to Protein B1 (26 kDa) showed that the several proteins in the B1-Immunoreactive Protein (B1-IP) group are localized exclusively to Type III cells. Although we expected that antibodies to Protein B1 would label only the submandibular gland, we found instead that the serous demilunes of the sublingual gland (SLG) and the acinar cells and intercalated ducts of the parotid gland (PRG) were strongly reactive in both the neonate and the adult. Immunoelectrophoretic analysis of gland extracts showed the major reactive species in the sublingual gland to have different mobilities than the B1-IP. On the other hand, reactive species in the parotid gland had mobilities identical to those of two SMG proteins. In the adult SMG, the neonatal Type I and Type III cells are not present, and the acinar cells are devoid of B1-IP reactivity; however, the cells of the intercalated ducts have components reactive with anti-B1 antibodies, and these do not appear to be identical to any neonatal bands. In contrast to the submandibular gland, the adult parotid and sublingual glands retain the localization of B1-IP reactivity in PRG acinar and intercalated duct cells and in SLG demilunes, and they show the neonatal immunoelectrophoretic pattern. This raises the possibility that the major B1-IP species in the adult PRG may be identical to transient proteins of the neonatal SMG.

Quantitation and localization of acinar cell-specific mucin in submandibular glands of mice during postnatal development

In this study, antiserum to acinar cell-specific mucin was utilized to determine whether mucin could be detected in the mouse submandibular gland prior to cytodifferentiation of acinar cells. Results from radioimmunoassay indicated that mucin occurs in submandibular glands from newborn mice, i.e., before the appearance of mature acinar cells. Additionally, mucin quantitated in various stages of development was found to be antigenically identical to adult mucin. After sections of glands were treated with immunohistochemical reagents, we observed that the mature acinar cell-specific mucin was present in secretory terminal-tubule cells and in proacinar cells of newborn animals. The present findings suggest that in young animals, the proacinar cells are an immediate precursor of acinar cells and that the secretory terminal-tubule cells may represent an earlier stage in development of acinar cells. In adult female glands, mucin was also detected in the granular intercalating-duct cells. This latter observation is consistent with the hypothesis that these cells are an intermediate in the acinar cell replacement process.

Differential binding sites of peroxidase-labelled lectins in the submandibular gland of sucking and adult cats

The glycosidic residues in the submandibular gland of cat at different ages were studied using a battery of 6 lectins conjugated with horseradish peroxidase as histochemical reagents. The submandibular glands of sucking and adult cats showed diversities in the reaction intensity or in the localization of binding sites. The morphologically distinct secretory tracts appear to produce different types of oligosaccharides in sucking and adult subjects. The sugars that can be defined as growth markers in the cat submandibular gland seem to be alpha-L-fucose, N-acetyl-D-glycosamine and beta-galactose. The data originated from this research have been compared with biochemical data previously obtained by the authors on the cat submandibular gland during growth.

Acinar cell proliferation in the parotid and submandibular salivary glands of the neonatal rat

Although the rat salivary glands are deficient in acini at birth, acinar cells proliferate rapidly during the early post-natal period. The pattern of acinar cell proliferation was analysed in the parotid and submandibular glands of neonatal rats from day to birth until day 34. Mitotic and [3H]thymidine ([3H]TdR) labelling indices of the two glands show distinctly different patterns. Analysis of cell division in the rat parotid gland demonstrated a peak of mitotic index at 14 days (2.9 +/- 0.4%) and labelling index at 16 days (25.2 +/- 2.1%). Submandibular gland acinar cell proliferation reaches a zenith between 7-8 days; labelling index (14.2 +/- 1.1%) and mitotic index (2.3 +/- 0.3%). Cell proliferation decreases rapidly in both glands after reaching a peak in activity. Gland size increases more rapidly in both glands after gland which correlates with the earlier shift from cell proliferation to differentiation which occurs in this organ. Circadian rhythms of [3H]TdR incorporation were also investigated in this study. A circadian rhythm of [3H]TdR incorporation into DNA occurs at 15 days after birth with a peak at 06.00 hours in both glands and a trough occurring at 15.00 hours in parotid gland and 18.00 hours in the submandibular gland. Determination of specific activity of DNA (ct/min per microgram DNA) on days 8, 10, 12, 13, 14, 15, and 16 after birth at 06.00 and 15.00 hours indicated that a circadian rhythm in [3H]TdR incorporation into DNA began on day 14. The developmental switch from suckling to solid food may be an initiating factor in the sychronization of the circadian rhythm in cell proliferation.

Immunohistochemical localization of dipeptidyl aminopeptidase (DAP) IV in the rat submandibular gland during postnatal development

The localization of dipeptidyl aminopeptidase (DAP) IV in the rat submandibular gland during postnatal development was studied immunohistochemically using the unlabeled antibody enzyme method. Cryostat sections of rat submandibular glands were examined in animals 1 to 50 days old. In the first postnatal week, faint immunoreaction was detected in the apical region of the cytoplasm of terminal tubules. During the second postnatal week the reaction was gradually restricted to the luminal region of terminal tubules or developing acinar cells, and its intensity increased. The most striking change of the enzyme localization was observed in 15-day-old rats. At this time, DAP IV was confined to the luminal and lateral membranes of differentiated acinar cells and to the luminal membranes of intercalated and striated duct cells. This localization pattern was similar to that of the adult animal. DAP IV activity in the gland was also measured biochemically during the postnatal development. The results were in agreement with those of the immunohistochemical study. These results suggest that the localization of DAP IV is closely related to the postnatal differentiation and proliferation of acinar cells.

Morphometric and fine structural study of experimental autoallergic sialadenitis of rat submandibular glands

To further our understanding of the immunopathologic mechanisms involved in experimental autoallergic sialadenitis of rat submandibular gland (EAS), histometric and fine structural studies were undertaken. Rats were immunized with allogeneic submandibular glands (SMG) emulsified in complete Freund's adjuvant. Control rats were not treated (C) or adjuvant treated (At). The rats were sacrificed 7, 14, 21 and 28 days after immunization and their SMG were processed for light and electron microscopy. Groups "C" and "at" showed normal acini and ducts. The SMG at 14 days showed significant loss of acini and granular ducts, severe lymphocytic infiltration and the appearance of undifferentiated ducts. The cells of the latter showed abundant free ribosomes, few profiles of rer, no secretory granules and in some cells autophagic vacuoles. Pseudopods of many lymphocytes were found in juxtaposition to degenerating parenchymal cells, mast cells and eosinophils. The extralobular ducts were significantly increased at 7, 14, and 21 days. The immunized glands showed evidence of regeneration at 21 and 28 days. Terminal tubule cells, proacinar cells and acinar cells, at various stages of maturation, were found in the regenerating glands.

A stage-restricted secretory system in the submandibular gland of the neonatal rat

Ribonuclease and amylase have been shown to undergo a transient increase in activity in the submandibular gland of the perinatal rat [5]. We report here that brief stimulation with isoproterenol in vitro selectively releases these enzymes. In addition, analysis by polyacrylamide gel electrophoresis showed that the 4-day old submandibular gland contains three other major protein species that are not present in the sublingual gland and two of which are not evident in the adult submandibular gland. These are also selectively released by the drug. Examination of the glands by light and electron microscopy showed that concurrent with release of these protein products, extensive degranulation occurs in the immature acini (terminal tubules). Our experiments suggest that the 4-day glands show a marked incorporation of 3H-leucine into the submandibular-specific, secreted protein species. This indicates that these proteins will provide convenient molecualr markers of early submandibular differentiation.

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.

The effects of 5-bromodeoxyuridine and isoproterenol on the postnatal differentiation of rat submandibular gland

The effects of 5-bromodeoxyuridine (BrdU) on the postnatal differentiation of rat submandibular gland and on the isoproterenol-induced changes of differentiation were studied. The rats were injected with BrdU for three consecutive days, beginning at two days of age. The total dose of BrdU was 0.9 mg/g body weight. BrdU caused a severe retardation of growth up to 15 days of age. A rapid growth of the animals between 15 and 22 days indicated a recovery from the effect of BrdU. The growth of the submandibular gland was retarded similarly with a corresponding decrease in DNA, RNA and protein content. Incorporation of tritiated thymidine into the submandibular gland was not altered in the BrdU-treated animals at one and three days after the last injection of the analog. At days 15 and 22 the rate of thymidine incorporation was greater in the submandibular gland of BrdU-treated rats as compared to littermate controls. Isoproterenol stimulated thymidine incorporation into the submandibular glands of two-week-old rats. This stimulation was not observed in rats which received BrdU at age 7-9 days, prior to the administration of isoproterenol. Election microscopic observations, including a quantitative analysis of the frequency distribution of the various cell types in the terminal tubules and developing acini, indicated a retardation of acinar cell differentiation in the glands of BrdU-treated rats. In addition, there was an increase in the number and size of the secretory granules in the terminal tubule cells. BrdU treatment, however, caused no obvious pathologic alterations in the submandibular gland. Administration of isoproterenol for five days, beginning at five days of age, caused an apparent acceleration of the differentiation of acinar cells. In the glands of isoproterenol-treated rats the acinar cells were enlarged and were filled with homogeneous secretory granules. Pretreatment with BrdU partially inhibited the effects of isoproterenol on differentiation and hypertrophy of the submandibular gland. With the given dose of BrdU, approximately 5% of thymine was replaced by bromouracil in the DNA of the submandibular gland. Such a replacement would be consistent with a direct effect of BrdU on the cytodifferentiation in the submandibular gland. However, because of the severe retardation of growth of the BrdU-treated rats, indirect effects can not be excluded.

Postnatal development of acinar cells in rat submandibular gland as revealed by electron microscopic staining for carbohydrates

The postnatal differentiation of acinar cells in rat submandibular gland was studied by staining with periodic acid-thiosemicarbazide-silver proteinate to identify carbohydrate-containing macromolecules in the electron microscope. This method revealed glycogen particles and internal substructure in the secretory granules of developing acinar cells. On the basis of morphologic and histochemical criteria three phases of acinar cell development were defined. In the pro-acinar phase, during the first week after birth, pro-acinar cells and terminal tubular cells were the main components of the terminal tubules in the rudimentary gland. The secretory granules of the pro-acinar cells contained speckled or rod-like substructures which stained intensively for carbohydrates and were digested by proteolytic enzymes. During the second to third week after birth, which is the immature-acinar-cell phase, thread-like substructures were seen in the secretory granules. These structures, which were not digested by proteolytic enzymes, disappeared gradually. The acinar cells of 4-week-old or older rats displayed no particular substructure in the secretion granules and represented the final, mature phase of development.

Cell differentiation in the terminal tubule of fetal rat submandibular gland in organ culture

Submandibular glands from 17-day-old rat fetuses were maintained in organ culture for five days in a medium consisting of Eagle's MEM (87%), horse serum (10%), and chick embryo extract (3%). Each day of the culture period explants were incubated for the demonstration of peroxidase activity and processed for light and electron microscopic observations. In some experiments cultures were exposed to 3H-thymidine one hour prior to fixation and incubation for the demonstration of peroxidase activity. Labelling index was determined using radioautographs of 1 mu Epon-embedded sections. At the time of explantation the submandibular gland rudiment consisted of undifferentiated epithelial cells arranged in cords. On day 3 of culture two additional cell types could be distinguished: terminal tubule cells and proacinar cells. The proacinar cells were characterized by peroxidase activity in their granules and cytoplasm. By day 4 acinar cells begin to appear. On the fifth day of culture the four cell types of the terminal tubule were present in the following proportions: undifferentiated cells, 44%; terminal tubule cells, 19%; proacinar cells, 31%; acinar cells, 6%. These results indicate that the cytodifferentiation of the secretory unit of rat submandibular gland in vitro is comparable to the differentiation in vivo.

Cytodifferentiation of striated duct cells and secretory cells of the convoluted granular tubules of the rat submandibular gland

The structural and functional development of the striated ducts and convoluted granular tubules (CGT) of the rat submandibular gland (SMG) were studied by electron microscopy and alkaline protease chemistry. Development of the SMG was followed from 14 days of gestation through 30 weeks of age. The specialized morphology of the basal aspect of the striated duct cells arises from cellular extensions which are first seen at 20 days of gestation. These processes elongate and intertwine with similar processes from adjacent cells, and as the cells enlarge the processes are compressed together giving the appearance of "infolding" of the basal plasma membrane. Mitochondria migrate to the basal part of the cell and are seen in close relationship to the cellular extensions throughout the development of these cells. Development of the striated duct is complete by one week after birth. The CGT develop from the proximal portions of intralobular striated ducts. At one week after birth, cells of the proximal striated duct demonstrate apical vacuoles. By two weeks after birth these vacuoles are replaced by distinct zymogen-like granules. There is a progressive accumulation of large numbers of secretory granules in the CGT cells as the animals age. However, rough endoplasmic reticulum is a relatively inconspicuous cellular component throughout development. The accumulation of alkaline protease activity in the gland closely parallels the pattern of granule accumulation.