Salivary glands: a paradigm for diversity of gland development

Genome-wide statistical evidence elucidates candidate factors of life expectancy in dogs

It is well-established that large and heavy dogs tend to live shorter lives. In this study, we aimed to determine whether traits other than body size are associated with the life expectancy of dogs. We compiled a dataset of 20 phenotypes, including body size, lifespan, snout ratio, and shedding, into a single matrix for 149 dog breeds using data from the American Kennel Club and other peer-reviewed sources. The analysis revealed that drooling might be associated with both the lifespan and body mass index of dogs. Furthermore, a genome-wide association study with adjusted phenotypes and statistical verification methods, such as Mendelian randomization. Additionally, conducting differential gene expression analysis with the salivary gland for the 2 cases, hypersalivation/less drooling vs various body sizes, we could observe the hypersalivation-related proteins. This genetic analysis suggests that body size and drooling might be candidate factors influencing lifespan. Consequently, we identified several candidate genes, including IGSF1, PACSIN2, PIK3R1, and MCCC2, as potential genetic factors influencing longevity-related phenotypes.

Nkx2.3 transcription factor is a key regulator of mucous cell identity in salivary glands

Saliva is vital to oral health, fulfilling multiple functions in the oral cavity. Three pairs of major salivary glands and hundreds of minor salivary glands contribute to saliva production. The secretory acinar cells within these glands include two distinct populations. Serous acinar cells secrete a watery saliva containing enzymes, while mucous acinar cells secrete a more viscous fluid containing highly glycosylated mucins. Despite their shared developmental origins, the parotid gland (PG) is comprised of only serous acinar cells, while the sublingual gland (SLG) contains predominantly mucous acinar cells. The instructive signals that govern the identity of serous versus mucous acinar cell phenotypes are not yet known. The homeobox transcription factor Nkx2.3 is uniquely expressed in the SLG. Disruption of the Nkx2.3 gene was reported to delay the maturation of SLG mucous acinar cells. To examine whether Nkx2.3 plays a role in directing the mucous cell phenotype, we analyzed SLG from Nkx2.3-/- mice using RNAseq, immunostaining and proteomic analysis of saliva. Our results indicate that Nkx2.3, most likely in concert with other transcription factors uniquely expressed in the SLG, is a key regulator of the molecular program that specifies the identity of mucous acinar cells.

Tlx1 regulates acinar and duct development in mouse salivary glands

Tlx1 encodes a transcription factor expressed in several craniofacial structures of developing mice. The role of Tlx1 in salivary gland development was examined using morphological and immunohistochemical analyses of Tlx1 null mice. Tlx1 is expressed in submandibular and sublingual glands but not parotid glands of neonatal and adult male and female C57Bl/6J (Tlx1+/+ ) mice. TLX1 protein was localized to the nuclei of terminal tubule cells, developing duct cells and mesenchymal cells in neonatal submandibular and sublingual glands, and to nuclei of duct cells and connective tissue cells in adult glands. Occasionally, TLX1 was observed in nuclei of epithelial cells in or adjacent to the acini. Submandibular glands were smaller and sublingual glands were larger in size in mutant mice (Tlx1-/- ) compared to wild-type mice. Differentiation of terminal tubule and proacinar cells of neonatal Tlx1-/- submandibular glands was abnormal; expression of their characteristic products, submandibular gland protein C and parotid secretory protein, respectively, was reduced. At 3 weeks postnatally, terminal tubule cells at the acinar-intercalated duct junction were poorly developed or absent in Tlx1-/- mice. Granular convoluted ducts in adult mutant mice were decreased, and epidermal growth factor and nerve growth factor expression were reduced. Along with normal acinar cell proteins, adult acinar cells of Tlx1-/- mice continued to express neonatal proteins and expressed parotid proteins not normally present in submandibular glands. Sublingual gland mucous acinar and serous demilune cell differentiation were altered. Tlx1 is necessary for proper differentiation of submandibular and sublingual gland acinar cells, and granular convoluted ducts. The mechanism(s) underlying Tlx1 regulation of salivary gland development and differentiation remains unknown.

Evaluating the transcriptional landscape and cell-cell communication networks in chronically irradiated parotid glands

Understanding the transcriptional landscape that results in chronic salivary hypofunction after irradiation will help identify injury mechanisms and develop regenerative therapies. We present scRNA-seq analysis from control and irradiated murine parotid glands collected 10 months after irradiation. We identify a population of secretory cells defined by specific expression of Etv1, which may be an acinar cell precursor. Acinar and Etv1+ secretory express Ntrk2 and Erbb3, respectively while the ligands for these receptors are expressed in myoepithelial and stromal cells. Furthermore, our data suggests that secretory cells and CD4+CD8+T-cells are the most transcriptionally affected during chronic injury with radiation, suggesting active immune involvement. Lastly, evaluation of cell-cell communication networks predicts that neurotrophin, neuregulin, ECM, and immune signaling are dysregulated after irradiation, and thus may play a role in the lack of repair. This resource will be helpful to understand cell-specific pathways that may be targeted to repair chronic damage in irradiated glands.

Identifying stem cells in the main excretory ducts of rat major salivary glands: adventures with commercial antibodies

We investigated the entire length of the main excretory ducts (MED) of the major sublingual, parotid and submandibular salivary glands of mature laboratory rats for mucous (goblet) and luminal ciliated cells, biomarkers of cell proliferation, apoptosis, and five biomarkers of stem cells. Spleen and testis were used as positive controls. We used formalin fixed, paraffin embedded tissues. No mucous cells or cells with luminal cilia were observed in hematoxylin and eosin, alcian blue or periodic acid-Schiff stained sections. Immunohistochemistry using rabbit anti-rat antibodies produced anomalous reactions with cleaved caspase-3 for apoptosis, Ki-67 for proliferative activity and Sox 2. Following antigen retrieval, no primary antibody and all three negative controls, labeled macrophages appeared in the spleen. TUNEL staining revealed a few cells per section undergoing apoptosis. Reactions deemed valid occurred in MED with cytokeratin-5 and c-Kit and stem cell antigen 1 (Sca-1) mostly in the gland and middle segments. Other ducts, but not acini or myoepithelial cells, also were variably stained with c-Kit and Sca-1.

ΔNp63 maintains the fidelity of the myoepithelial cell lineage and directs cell differentiation programs in the murine salivary gland

Salivary glands consist of several epithelial cell types of distinct lineages and functional characteristics that are established by directed differentiation programs of resident stem and progenitor cells. We have shown that ΔNp63, a crucial transcriptional regulator of stem/progenitor cells, is enriched in both the basal and myoepithelial cell (MEC) populations and that ΔNp63 positive cells maintain all the descendent epithelial cell lineages of the adult mouse salivary glands (mSGs). Although this pivotal role of ΔNp63 in driving the broader epithelial cell fate and identity in the mSG has been demonstrated, how ΔNp63 functions specifically in the commitment and differentiation of the MEC population is less understood. Using multiple genetic mouse models that allow for cell tracing, we show that ΔNp63 is critical in maintaining and renewing MECs, in part through the transcriptional regulation of Acta2 gene expression, a defining marker of this cell population. We demonstrate that during adult mSG homeostasis, ΔNp63 enriched MECs function as bipotent progenitor cells that maintain not only the MEC population, but also the distinctly different ductal cell lineages. The fidelity of this process is dependent on ΔNp63 expression, since MEC-specific ablation of ΔNp63 results in altered MEC differentiation and affects cellular plasticity resulting in aberrant differentiation of the intercalated ducts and acinar cells. In contrast, we find that the contribution of MECs to ductal and acinar cell regeneration following severe injury is independent of ΔNp63. Our observations offer new insights into cellular mechanisms driving MEC fate choices and differentiation programs in the context of salivary gland homeostasis and in response to injury and regeneration. Long term, these findings have implications for better treatment of salivary gland dysfunction through stem cell-based approaches.

Organ-specific extracellular matrix directs trans-differentiation of mesenchymal stem cells and formation of salivary gland-like organoids in vivo

BACKGROUND

Current treatments for salivary gland (SG) hypofunction are palliative and do not address the underlying cause or progression of the disease. SG-derived stem cells have the potential to treat SG hypofunction, but their isolation is challenging, especially when the tissue has been damaged by disease or irradiation for head and neck cancer. In the current study, we test the hypothesis that multipotent bone marrow-derived mesenchymal stem cells (BM-MSCs) in a rat model are capable of trans-differentiating to the SG epithelial cell lineage when induced by a native SG-specific extracellular matrix (SG-ECM) and thus may be a viable substitute for repairing damaged SGs.

METHODS

Rat BM-MSCs were treated with homogenates of decellularized rat SG-ECM for one hour in cell suspension and then cultured in tissue culture plates for 7 days in growth media. By day 7, the cultures contained cell aggregates and a cell monolayer. The cell aggregates were hand-selected under a dissecting microscope, transferred to a new tissue culture dish, and cultured for an additional 7 days in epithelial cell differentiation media. Cell aggregates and cells isolated from the monolayer were evaluated for expression of SG progenitor and epithelial cell specific markers, cell morphology and ultrastructure, and ability to form SG-like organoids in vivo.

RESULTS

The results showed that this approach was very effective and guided the trans-differentiation of a subpopulation of CD133-positive BM-MSCs to the SG epithelial cell lineage. These cells expressed amylase, tight junction proteins (Cldn 3 and 10), and markers for SG acinar (Aqp5 and Mist 1) and ductal (Krt 14) cells at both the transcript and protein levels, produced intracellular secretory granules which were morphologically identical to those found in submandibular gland, and formed SG-like organoids when implanted in the renal capsule in vivo.

CONCLUSIONS

The results of this study suggest the feasibility of using autologous BM-MSCs as an abundant source of stem cells for treating SG hypofunction and restoring the production of saliva in these patients.

Salivary gland function, development, and regeneration

Salivary glands produce and secrete saliva, which is essential for maintaining oral health and overall health. Understanding both the unique structure and physiological function of salivary glands, as well as how they are affected by disease and injury, will direct the development of therapy to repair and regenerate them. Significant recent advances, particularly in the OMICS field, increase our understanding of how salivary glands develop at the cellular, molecular, and genetic levels: the signaling pathways involved, the dynamics of progenitor cell lineages in development, homeostasis, and regeneration, and the role of the extracellular matrix microenvironment. These provide a template for cell and gene therapies as well as bioengineering approaches to repair or regenerate salivary function.

Reversely immortalized mouse salivary gland cells presented a promising metabolic and fibrotic response upon BMP9/Gdf2 stimulation

The submandibular gland (SMG) and the sublingual gland (SLG) are two of the three major salivary glands in mammals. In mice, they are adjacent to each other and open into the oral cavity, producing saliva to lubricate the mouth and aid in food digestion. Though salivary gland dysfunction accompanied with fibrosis and metabolic disturbance is common in clinic, in-depth mechanistic research is lacking. Currently, research on how to rescue salivary function is challenging, as it must resort to using terminally differentiated acinar cells or precursor acinar cells with unknown differentiation. In this study, we established reversely immortalized mouse primary SMG cells (iSMGCs) and SLG cells (iSLGCs) on the first postnatal day (P0). The iSMGCs and iSLGCs grew well, exhibited many salivary gland characteristics, and retained the metabolism-related genes derived from the original tissue as demonstrated using transcriptome sequencing (RNA-seq) analysis. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of these two cell lines, which overlapped with those of the SMG and SLG, were enriched in cysteine and methionine metabolism. Furthermore, we investigated the role of bone morphogenetic protein 9 (BMP9), also known as growth differentiation factor 2(Gdf2), on metabolic and fibrotic functions in the SMG and SLG. We demonstrated that iSMGCs and iSLGCs presented promising adipogenic and fibrotic responses upon BMP9/Gdf2 stimulation. Thus, our findings indicate that iSMGCs and iSLGCs faithfully reproduce characteristics of SMG and SLG cells and present a promising prospect for use in future study of salivary gland metabolism and fibrosis upon BMP9/Gdf2 stimulation.

An Overview on the Histogenesis and Morphogenesis of Salivary Gland Neoplasms and Evolving Diagnostic Approaches

Simple Summary

Diagnosing salivary gland neoplasms (SGN) remain a challenge, given their underlying biological nature and overlapping features. Evolving techniques in molecular pathology have uncovered genetic mutations resulting in these tumors. This review delves into the molecular etiopatho-genesis of SGN, highlighting advanced diagnostic protocols that may facilitate the identification and therapy of a variety of SGN.

Abstract

Salivary gland neoplasms (SGN) remain a diagnostic dilemma due to their heterogenic complex behavior. Their diverse histomorphological appearance is attributed to the underlying cellular mechanisms and differentiation into various histopathological subtypes with overlapping fea-tures. Diagnostic tools such as fine needle aspiration biopsy, computerized tomography, magnetic resonance imaging, and positron emission tomography help evaluate the structure and assess the staging of SGN. Advances in molecular pathology have uncovered genetic patterns and oncogenes by immunohistochemistry, fluorescent in situ hybridization, and next–generation sequencing, that may potentially contribute to innovating diagnostic approaches in identifying various SGN. Surgical resection is the principal treatment for most SGN. Other modalities such as radiotherapy, chemotherapy, targeted therapy (agents like tyrosine kinase inhibitors, monoclonal antibodies, and proteasome inhibitors), and potential hormone therapy may be applied, depending on the clinical behaviors, histopathologic grading, tumor stage and location, and the extent of tissue invasion. This review delves into the molecular pathways of salivary gland tumorigenesis, highlighting recent diagnostic protocols that may facilitate the identification and management of SGN.

Observation Rather than Surgery for Benign Parotid Tumors: Why, When, and How

Surgery is the preferred treatment of benign parotid lesions, but it carries a risk of complications. Therefore, the approach toward the surgery of these lesions should seek to avoid complications. There are no guidelines or recommendations for when not to operate. Integration of comorbidities and other factors shift the scales from surgery toward observation in a small subset of patients presenting with parotid tumors. When observation is chosen, the patient should be followed frequently and cautiously, and the surgeon should be prepared to change strategy to surgical excision if in doubt.

Sox9 function in salivary gland development

BACKGROUND

Organogenesis is regulated by morphogen signaling and transcription networks. These networks differ between organs, and identifying the organ-specific network is important to clarify the molecular mechanisms of development and regeneration of organs. Several studies have been conducted to identify salivary gland-specific networks using a mouse submandibular gland model. The submandibular glands (SMGs) of mice manifest as a thickening of the oral epithelium at embryonic day 11.5 and invaginate into the underlying mesenchyme. The network between Fgf10 and Sox9 is involved in SMG development in mice.

HIGHLIGHT

Sox9, a member of the Sox family, is expressed in the SMG in mice from the embryonic stage to the adult stage, although the distribution changes during development. A null mutation of mouse Sox9 is lethal during the neonatal period due to respiratory failure, whereas deletion of Sox9 in the oral epithelium using the Cre/lox P system, can lead to smaller initial buds of SMGs in conditional knockout (cKO) mice than in normal mice. In addition, we showed that adenoviral transduction of Sox9 and Foxc1 genes into mouse embryonic stem cell-derived oral ectoderm could induce salivary gland rudiment in an organoid culture system. ChIP-sequencing revealed that Sox9 possibly regulates several tube- and branching-formation-related genes.

CONCLUSION

Sox9 may serve as an essential transcription factor for salivary gland development. The Sox9-mediated pathway can be a promising candidate for regenerating damaged salivary glands.

Diverse epithelial cell populations contribute to the regeneration of secretory units in injured salivary glands

Salivary glands exert exocrine secretory function to provide saliva for lubrication and protection of the oral cavity. Its epithelium consists of several differentiated cell types, including acinar, ductal and myoepithelial cells, that are maintained in a lineage-restricted manner during homeostasis or after mild injuries. Glandular regeneration following a near complete loss of secretory cells, however, may involve cellular plasticity, although the mechanism and extent of such plasticity remain unclear. Here, by combining lineage-tracing experiments with a model of severe glandular injury in the mouse submandibular gland, we show that de novo formation of acini involves induction of cellular plasticity in multiple non-acinar cell populations. Fate-mapping analysis revealed that, although ductal stem cells marked by cytokeratin K14 and Axin2 undergo a multipotency switch, they do not make a significant contribution to acinar regeneration. Intriguingly, more than 80% of regenerated acini derive from differentiated cells, including myoepithelial and ductal cells, that appear to dedifferentiate to a progenitor-like state before re-differentiation into acinar cells. The potential of diverse cell populations serving as a reserve source for acini widens the therapeutic options for hyposalivation.

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.

In vitro three-dimensional culture systems of salivary glands

Dry mouth can be caused by salivary gland hypofunction due to Sjögren's syndrome (SS) or radiation therapy for head and neck cancer, and it can also be a side effect of medications. The use of sialagogues effectively increases saliva secretion in patients with dry mouth. However, the application of sialagogues is not always satisfactory because of their side effects, such as sweating, nausea, runny nose and diarrhea. Two-dimensional (2D) cell cultures have been used not only for drug screening and discovery but also to clarify disease mechanisms. However, three-dimensional (3D) cell cultures are expected to be even more advantageous than 2D cell cultures. Therefore, we have tried to develop an in vitro cell culture system that can reconstitute 3D salivary glands. Sox9 and Foxc1 were identified as important genes that differentiate mouse embryonic stem cell-derived oral ectoderm into salivary gland placode. Using these genes and organoid culture systems, we succeeded in generating salivary gland organoids that exhibited a morphology and gene expression profile that were similar to those of the embryonic rudiment from which salivary glands arise in normal mice. These organoids are expected to be a promising tool for disease modeling, drug discovery and regenerative medicine in salivary glands.

Effects of spaceflight on the mouse submandibular gland

OBJECTIVE

This study was conducted to determine if the morphology and biochemistry of the mouse submandibular gland is affected by microgravity and the spaceflight environment.

DESIGN

Tissues from female mice flown on the US space shuttle missions Space Transportation System (STS)-131 and STS-135 for 15 and 13 d, respectively, and from male mice flown on the 30 d Russian Bion-M1 biosatellite, were examined using transmission electron microscopy and light and electron microscopic immunohistochemistry.

RESULTS

In contrast to the parotid gland, morphologic changes were not apparent in the submandibular gland. No significant changes in protein expression, as assessed by quantitative immunogold labeling, occurred in female mice flown for 13-15 d. In male mice, however, increased labeling for salivary androgen binding protein alpha (in acinar cell secretory granules), and epidermal growth factor and nerve growth factor (in granular convoluted duct cell granules) was seen after 30 d in space.

CONCLUSION

These results indicate that spaceflight alters secretory protein expression in the submandibular gland and suggest that the sex of the animals and the length of the flight may affect the response. These findings also show that individual salivary glands respond differently to spaceflight. Saliva contains proteins secreted from salivary glands and is easily collected, therefore is a useful biofluid for general medical analyses and in particular for monitoring the physiology and health of astronauts.

Transcriptomic and Single-Cell Analysis of the Murine Parotid Gland

The salivary complex of mammals consists of 3 major pairs of glands: the parotid, submandibular, and sublingual glands. While the 3 glands share similar functional properties, such as saliva secretion, their differences are largely based on the types of secretions they produce. While recent studies have begun to shed light on the underlying molecular differences among the glands, few have examined the global transcriptional repertoire over various stages of gland maturation. To better elucidate the molecular nature of the parotid gland, we have performed RNA sequencing to generate comprehensive and global gene expression profiles of this gland at different stages of maturation. Our transcriptomic characterization and hierarchical clustering analysis with adult organ RNA sequencing data sets has identified a number of molecular players and pathways that are relevant for parotid gland biology. Moreover, our detailed analysis has revealed a unique parotid gland-specific gene signature that may represent important players that could impart parotid gland-specific biological properties. To complement our transcriptomic studies, we have performed single-cell RNA sequencing to map the transcriptomes of parotid epithelial cells. Interrogation of the single-cell transcriptomes revealed the degree of molecular and cellular heterogeneity of the various epithelial cell types within the parotid gland. Moreover, we uncovered a mixed-lineage population of cells that may reflect molecular priming of differentiation potentials. Overall our comprehensive studies provide a powerful tool for the discovery of novel molecular players important in parotid gland biology.

Morphogenesis and Mucus Production of Epithelial Tissues of Three Major Salivary Glands of Embryonic Mouse in 3D Culture

Embryonic mouse submandibular epithelia initiate branching morphogenesis within two days when embedded in Matrigel and stimulated by members of the epidermal growth factor family. However, it is unknown whether the end buds further branch over longer culture periods, and whether saliva-producing cells differentiate there. In the present study, we cultivated three major (submandibular, sublingual and parotid) salivary epithelia from 13-day embryos for 14 days in mesenchyme-free cultures. All epithelia continued to grow and branch to form numerous acinus-like structures in medium supplemented with neuregulin 1, fibroblast growth factor 1, and insulintransferrin-sodium selenite. Alcian blue staining to detect mucous cells showed that each epithelium differentiated via three distinct modes, as seen in normal development, although the staining intensities were weaker than in normal development. RT-PCR analysis of the amylase gene showed that no epithelia expressed amylase after 14 days of culture, which is inconsistent with the fact that only parotid epithelium does so at postnatal day 7 during normal development. These results suggest that cytodifferentiation progresses to a lesser extent in mesenchyme-free cultures than in vivo.

Initial stages of development of the submandibular gland (human embryos at 5.5-8 weeks of development)

The aim of this study was to determine the main stages of submandibular salivary gland development during the embryonic period in humans. In addition, we studied submandibular salivary gland development in rats on embryonic days 14-16 and expression in the submandibular salivary gland region with the monoclonal antibody HNK-1. Serial sections from 25 human embryos with a greatest length ranging from 10 to 31 mm (Carnegie stages 16-23; weeks 5.5-8 of development) and Wistar rats of embryonic days (E) 14-16 were analysed with light microscopy. Five stages of submandibular salivary gland development were identified. The prospective stage (1), between weeks 5.5 and early week 6, is characterized by a thickening of the epithelium of the medial paralingual groove in the floor of the mouth corresponding to the primordium of the submandibular salivary gland parenchyma. At this stage, the primordium of the parasympathetic ganglion lies below the lingual nerve. The primordium of the submandibular salivary gland parenchyma is observed in rats on E14 in the medial paralingual groove with mesenchymal cells, underlying the lingual nerve. These cells are HNK-1-positive, corresponding to the primordium of the parasympathetic ganglion. The bud stage (2), at the end of week 6 in humans and on E15 in rats, is characterized by the proliferation and invagination of the epithelial condensation, surrounded by an important condensation of the mesenchyme. The pseudoglandular stage (3) at week 6.5 is characterized by the beginning of the formation of lobes in the condensed mesenchyme. The canalicular stage (4), between week 7 and 7.5, is characterized by the appearance of a lumen in the proximal part of the submandibular duct. The innervation stage (5) occurs during week 8, with the innervation of the submandibular and interlobular ducts. Nervous branches arriving from the parasympathetic ganglion innervate the glandular parenchyma. Numerous blood vessels are observed nearby. Our results suggest that submandibular salivary gland development requires interactions among epithelium, mesenchyme, parasympathetic ganglion and blood vessels.

c-Kit+ Cells in Adult Salivary Glands do not Function as Tissue Stem Cells

A rare population of salivary gland cells isolated based on c-Kit immunoreactivity are thought to represent tissue stem cells since they exhibit the most robust proliferative and differentiation capacity ex vivo. Despite their high promise for cell-based therapies aimed at restoring salivary function, the precise location and in vivo function of c-Kit⁺ stem cells remain unclear. Here, by combining immunostaining with c-KitCre^ERT2-based genetic labeling and lineage tracing in the adult mouse salivary glands, we show that c-Kit is expressed in a relatively large and heterogeneous cell population that consists mostly of differentiated cells. Moreover, c-Kit does not mark ductal stem cells that are known to express cytokeratin K14. Tracking the fate of in vivo-labeled c-Kit⁺ or that of K14⁺ cells in spheroid cultures reveals a limited proliferative potential for c-Kit⁺ cells and identifies K14⁺ cells as the major source of salispheres in these cultures. Long-term in vivo lineage tracing studies indicate that although c-Kit marks at least two discrete ductal cell lineages, c-Kit⁺ cells do not contribute to the normal maintenance of any other cell lineages. Our results indicate that c-Kit is not a reliable marker for salivary gland stem cells, which has important implications for salivary gland regenerative therapies.

Genetic and scRNA-seq Analysis Reveals Distinct Cell Populations that Contribute to Salivary Gland Development and Maintenance

Stem and progenitor cells of the submandibular salivary gland (SMG) give rise to, maintain, and regenerate the multiple lineages of mature epithelial cells including those belonging to the ductal, acinar, basal and myoepithelial subtypes. Here we have exploited single cell RNA-sequencing and in vivo genetic lineage tracing technologies to generate a detailed map of the cell fate trajectories and branch points of the basal and myoepithelial cell populations of the mouse SMG during embryonic development and in adults. Our studies show that the transcription factor p63 and alpha-smooth muscle actin (SMA) serve as faithful markers of the basal and myoepithelial cell lineages, respectively and that both cell types are endowed with progenitor cell properties. However, p63⁺ basal and SMA⁺ myoepithelial cells exhibit distinct cell fates by virtue of maintaining different cellular lineages during morphogenesis and in adults. Collectively, our results reveal the dynamic and complex nature of the diverse SMG cell populations and highlight the distinct differentiation potential of the p63 and SMA expressing subtypes in the stem and progenitor cell hierarchy. Long term these findings have profound implications towards a better understanding of the molecular mechanisms that dictate lineage commitment and differentiation programs during development and adult gland maintenance.

FGF2-dependent mesenchyme and laminin-111 are niche factors in salivary gland organoids

Epithelial progenitor cells are dependent upon a complex 3D niche to promote their proliferation and differentiation during development, which can be recapitulated in organoids. The specific requirements of the niche remain unclear for many cell types, including the proacinar cells that give rise to secretory acinar epithelial cells that produce saliva. Here, using ex vivo cultures of E16 primary mouse submandibular salivary gland epithelial cell clusters, we investigated the requirement for mesenchymal cells and other factors in producing salivary organoids in culture. Native E16 salivary mesenchyme, but not NIH3T3 cells or mesenchymal cell conditioned medium, supported robust protein expression of the progenitor marker Kit and the acinar/proacinar marker AQP5, with a requirement for FGF2 expression by the mesenchyme. Enriched salivary epithelial clusters that were grown in laminin-enriched basement membrane extract or laminin-111 together with exogenous FGF2, but not with EGF, underwent morphogenesis to form organoids that displayed robust expression of AQP5 in terminal buds. Knockdown of FGF2 in the mesenchyme or depletion of mesenchyme cells from the organoids significantly reduced AQP5 levels even in the presence of FGF2, suggesting a requirement for autocrine FGF2 signaling in the mesenchyme cells for AQP5 expression. We conclude that basement membrane proteins and mesenchyme cells function as niche factors in salivary organoids.

Effect of Biochanin A versus 17β estradiol in rat submandibular salivary gland

The epigenetic nature of development mandates the observation of the effect of any exogenous substance, especially those with estrogenic activities, during critical phases of development. The submandibular gland (SMG) presents as a great model due to extensive postnatal development, and is known to be regulated and affected by hormones as well as growth factors. Herein, we observed postnatal development following low doses of Biochanin A (BCA) and 17β estradiol (E2) in rats. The pups were randomly divided into four groups: control, BCA, E2, and dimethyl sulfoxide (DMSO), and euthanized at the 6th, 15th, 30th, and 60th postnatal days (PND). SMG morphogenesis was assessed. The nuclear expression of estrogen receptor beta (ERβ) was evaluated immunohistochemically; ERβ expression was up-regulated by BCA and down-regulated by E2. Similarly, caspase three gene expression, assessed by real time polymerase chain reaction was increased in the BCA group but decreased in the E2 group. A significant decrease in epidermal growth factor gene expression was noted at PND 30. The results presented by this study provide evidence that the effect of a postnatal exposure of the SMG to Biochanin A during development could be linked to sex hormone-dependent disorders.

BPI-fold (BPIF) containing/plunc protein expression in human fetal major and minor salivary glands

The aim of this study was to determine expression, not previously described, of PLUNC (palate, lung, and nasal epithelium clone) (BPI-fold containing) proteins in major and minor salivary glands from very early fetal tissue to the end of the second trimester and thus gain further insight into the function of these proteins. Early fetal heads, and major and minor salivary glands were collected retrospectively and glands were classified according to morphodifferentiation stage. Expression of BPI-fold containing proteins was localized through immunohistochemistry. BPIFA2, the major BPI-fold containing protein in adult salivary glands, was detected only in the laryngeal pharynx; the lack of staining in salivary glands suggested salivary expression is either very late in development or is only in adult tissues. Early expression of BPIFA1 was seen in the trachea and nasal cavity with salivary gland expression only seen in late morphodifferentiation stages. BPIFB1 was seen in early neural tissue and at later stages in submandibular and sublingual glands. BPIFA1 is significantly expressed in early fetal oral tissue but BPIFB1 has extremely limited expression and the major salivary BPIF protein (BPIFA2) is not produced in fetal development. Further studies, with more sensitive techniques, will confirm the expression pattern and enable a better understanding of embryonic BPIF protein function.

Three-dimensional organotypic culture of human salivary glands: the slice culture model

OBJECTIVE

A challenge in studying human salivary glands is to maintain the cells ex vivo in their three-dimensional (3D) morphology with an intact native extracellular matrix (ECM) environment. This paper established a human salivary 3D organotypic slice culture model that could maintain its physiological functions as well as allowing a direct visualization of the cells.

METHODS

Human salivary biopsies from six patients were embedded in agarose and submerged in cold buffer for thin (50 μm) sectioning using a vibratome. 'Salivary slices' were mechanically supported by a porous membrane insert that allowed an air-liquid interface and cultured in serum-free culture media. Cell viability, proliferation, apoptosis, physiological functions, and gene expression were assessed during 14 days of culture.

RESULTS

Human salivary slices maintained cell survival (70-40%) and proliferation (6-17%) for 14 days ex vivo. The protein secretory (amylase) function decreased, but fluid (intracellular calcium mobilization) function was maintained. Acinar, ductal, and myoepithelial cell populations survived and maintained their 3D organization within the slice culture model.

CONCLUSION

The human salivary slice culture model kept cells alive ex vivo for 14 days as well as maintaining their 3D morphology and physiological functions.

Sialolithiasis of an Accessory Parotid Gland

We report a case of sialolithiasis of an accessory parotid gland in the cheek demonstrated by computed tomography and sialography. The accessory parotid gland was located anterolateral to the masseter muscle and was isolated from the main parotid gland. The calculus developed from this accessory parotid gland, and the main parotid gland was free of sialolithiasis and inflammation. To our knowledge, this is the first report concerning sialolithiasis in an accessory parotid gland. The calculus was removed without facial nerve injury or salivary fistula via a peroral approach.

Immunocytochemical Localization of Histatins in Human Salivary Glands

Histatins are a family of salivary proteins with bactericidal and fungicidal activities that contribute to the innate defense of the oral cavity. Histatins are present in the serous granules of the parotid and submandibular glands. The important role of histatins in saliva, and the limited information on their cellular and subcellular distribution, prompted us to further define the localization of histatins in the major salivary glands. Immunogold-silver staining of 1-μm sections of plastic-embedded tissue with anti-histatin antibody revealed histatin immunoreactivity in the serous acinar cells of the parotid and submandibular glands, the serous demilune cells of the submandibular and sublingual glands, and in occasional intercalated duct cells. No reactivity was seen in mucous cells or in striated or excretory duct cells. Electron microscopic observations of thin sections labeled with anti-histatin and gold-labeled secondary antibodies revealed immunoreactivity associated with the rough endoplasmic reticulum and Golgi complex and in secretory granules of serous acinar and demilune cells. The granules of parotid acinar cells exhibited relatively uniform labeling of their content, whereas the granules of serous cells in the submandibular and sublingual glands showed variable labeling of the dense and light regions of their content. A few intercalated duct cells adjacent to the acinar cells also exhibited labeled granules. These results suggest that the serous cells of the major glands are the main source of histatins in human saliva. They are also consistent with several previous studies demonstrating the variable distribution of different proteins within the granule content.

β-Actin protein expression differs in the submandibular glands of male and female mice

β-actin, a cytoskeletal protein, is the most widely used housekeeping gene. Although housekeeping genes are expressed in all tissues, the β-actin gene is expressed in certain cell types because of differential binding of transcriptional factors to the regulatory elements of the gene. The expression and localization of β-actin protein in the submandibular glands (SMG) of mice were investigated in this study, using Western blot analysis and immunohistochemistry. In ICR and C57BL/6J mice, the levels of β-actin protein in the SMG of females are significantly higher than those in the SMG of males. β-actin protein is majorly distributed in acinar cells of SMG. There is no significant difference in the expression level of β-actin protein between females and castrated males. After castrated male ICR mice are treated with 10 mg/kg/day testosterone propionate (TP) for 3 weeks, the levels of β-actin protein in SMG decrease. The numbers of duct per unit area increase, whereas the numbers of acinus per unit area decrease after TP administration. These data suggest that β-actin protein is mainly distributed in acinar cells of SMG and results in a marked sexual dimorphism in mice.

Identification of Stem Cells in the Secretory Complex of Salivary Glands

Salivary glands have an essential secretory function for maintaining oral and overall health. The epithelial compartment of the gland is composed of several highly specialized cell types that cooperate to secrete and deliver saliva to the oral cavity. The mouse submandibular gland has been used as a model for major salivary glands in human. The secretory complex in this model is composed of 2 secretory compartments, including acini and granular ducts connected by intercalated ducts. Contractile myoepithelial cells surround the secretory complex to facilitate salivary flow. Whether differentiated cells in the secretory complex are maintained by self-duplication or contribution from stem cells has remained an open question. Here, in analyzing the expression of basal cytokeratin (K) 14 in the secretory complex, we discovered a subset of K14(+) ductal cells in the intercalated ducts of the adult gland. These cells are distinct from the K14-expressing basal/myoepithelial cells, proliferate at a significantly higher rate than any other epithelial cell type in the gland, and reside in a spatially defined domain within the intercalated duct. Using inducible genetic lineage tracing, we show that K14(+) ductal cells represent a long-lived yet cycling population of stem cells that are established during development and contribute to the formation and maintenance of the granular ducts throughout life. Our data provide direct evidence for the existence of stem cells contributing to homeostasis of salivary glands, as well as new insights into glandular pathobiology.

Prediction of Growth Factor-Dependent Cleft Formation During Branching Morphogenesis Using A Dynamic Graph-Based Growth Model

This study considers the problem of describing and predicting cleft formation during the early stages of branching morphogenesis in mouse submandibular salivary glands (SMG) under the influence of varied concentrations of epidermal growth factors (EGF). Given a time-lapse video of a growing SMG, first we build a descriptive model that captures the underlying biological process and quantifies the ground truth. Tissue-scale (global) and morphological features related to regions of interest (local features) are used to characterize the biological ground truth. Second, we devise a predictive growth model that simulates EGF-modulated branching morphogenesis using a dynamic graph algorithm, which is driven by biological parameters such as EGF concentration, mitosis rate, and cleft progression rate. Given the initial configuration of the SMG, the evolution of the dynamic graph predicts the cleft formation, while maintaining the local structural characteristics of the SMG. We determined that higher EGF concentrations cause the formation of higher number of buds and comparatively shallow cleft depths. Third, we compared the prediction accuracy of our model to the Glazier-Graner-Hogeweg (GGH) model, an on-lattice Monte-Carlo simulation model, under a specific energy function parameter set that allows new rounds of de novo cleft formation. The results demonstrate that the dynamic graph model yields comparable simulations of gland growth to that of the GGH model with a significantly lower computational complexity. Fourth, we enhanced this model to predict the SMG morphology for an EGF concentration without the assistance of a ground truth time-lapse biological video data; this is a substantial benefit of our model over other similar models that are guided and terminated by information regarding the final SMG morphology. Hence, our model is suitable for testing the impact of different biological parameters involved with the process of branching morphogenesis in silico, while reducing the requirement of in vivo experiments.

Aquaporins in Salivary Glands: From Basic Research to Clinical Applications

Salivary glands are involved in saliva secretion that ensures proper oral health. Aquaporins are expressed in salivary glands and play a major role in saliva secretion. This review will provide an overview of the salivary gland morphology and physiology of saliva secretion, and focus on the expression, subcellular localization and role of aquaporins under physiological and pathophysiological conditions, as well as clinical applications involving aquaporins. This review is highlighting expression and localization of aquaporins in human, rat and mouse, the most studied species and is pointing out possible difference between major salivary glands, i.e., parotid, submandibular and sublingual glands.

Long-Term In Vitro Expansion of Salivary Gland Stem Cells Driven by Wnt Signals

Adult stem cells are the ultimate source for replenishment of salivary gland (SG) tissue. Self-renewal ability of stem cells is dependent on extrinsic niche signals that have not been unraveled for the SG. The ductal compartment in SG has been identified as the location harboring stem cells. Here, we report that rare SG ductal EpCAM(+) cells express nuclear β-catenin, indicating active Wnt signaling. In cell culture experiments, EpCAM(high) cells respond potently to Wnt signals stimulating self-renewal and long-term expansion of SG organoids, containing all differentiated SG cell types. Conversely, Wnt inhibition ablated long-term organoid cultures. Finally, transplantation of cells pre-treated with Wnt agonists into submandibular glands of irradiated mice successfully and robustly restored saliva secretion and increased the number of functional acini in vivo. Collectively, these results identify Wnt signaling as a key driver of adult SG stem cells, allowing extensive in vitro expansion and enabling restoration of SG function upon transplantation.

Melatonin localization in human salivary glands

BACKGROUND

Circulating melatonin is believed to reach body fluids by crossing passively the cell membranes, but alternative ways for melatonin transport also are hypothesized. This investigation was carried out to furnish ultrastructural evidences for melatonin transport by salivary gland cells in order to indicate plausible routes by which circulating melatonin can reach saliva.

METHODS

Bioptic samples of parotid, submandibular and labial glands were processed for the electron microscopy and treated to demonstrate melatonin reactivity by the immunogold staining method.

RESULTS AND CONCLUSIONS

The preferential sites of melatonin reactivity were the granules and vesicles of serous cells. Our results suggested that the acinar cells are able to store melatonin and that the hormone can be released into saliva through granule and vesicle exocytosis. The quantitative evaluation of labelling showed that the parotid gland is the most involved in the release of melatonin in saliva.

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.

Salivary amylase - The enzyme of unspecialized euryphagous animals

INTRODUCTION

Alpha-amylase (EC 3.2.1.1) is the most abundant enzyme in the saliva of man and of several vertebrates. In humans, salivary amylase is mainly formed in the parotid gland; its activity is of high inter-individual and intra-individual variability. The physiological functions of α-amylase have not yet been explored completely. It is well known that the enzyme cleaves the α-(1,4)-glycosidic bonds of polysaccharides. Furthermore it plays an important role in initial bioadhesion in man, facilitating carbohydrate metabolism and bacterial adherence at the tooth surface and therewith caries initiation. Nevertheless, it is still a matter of interest why humans have such high amounts of salivary amylase.

OBJECTIVE

The review presents an evolutionary approach by considering salivary amylase in the animal kingdom with special focus on mammalians divided into the three main nutritional types carnivores, herbivores, and omnivores; it was postulated that for most mammalian animals salivary α-amylase is essential.

RESULTS

The enzyme has been detected in saliva of some herbivores and many omnivorous animals, but not in pure carnivores. Focusing on ruminants, negligible levels or an absence of α-amylase was determined. Presence and activity probably differ depending on the species-specific diet. Animals feeding on unripe fruits, seeds, roots and bulbs exhibit higher activity of salivary α-amylase than species consuming ripe fruits, insects, and vertebrates.

CONCLUSION

In contrast to carnivores and most herbivores, omnivores have considerable amounts of amylase in their saliva. Though, the starch-digesting enzyme has been investigated well, the physiological function of amylase in saliva has not yet been explored completely. It can be hypothesized that nutritional habits affect expression of enzymes in the saliva of animals. It has to be verified, whether α-amylase is genetically or epigenetically determined. As a consequence of the development of agriculture, and following dietary changes, amylase can be recognized as a nutritional and evolutionary marker. Interdisciplinary evolutionary research might offer new perspectives for preventive dentistry.

Analysis of histone H2BGFP retention in mouse submandibular gland reveals actively dividing stem cell populations

The purpose of this study was to use histone 2B-green fluorescent protein (H2BGFP) pulse-chase experiments to provide a broad view of population dynamics in salivary gland and to identify the quiescent stem cells that had previously been suggested to reside in the gland. Two transgenic mouse models in which inducible H2BGFP expression was regulated either by keratin (K)14 promoter or by a ubiquitous promoter were generated. The level of fluorescent label in the submandibular gland induced by a pulse of H2BGFP expression was monitored over a period of 18 weeks of chase. Efficient targeting of H2BGFP label to the relatively undifferentiated ductal cells by K14 promoter did not identify a quiescent population of stem cells. Ubiquitous targeting of all ductal cells identified label-retaining cells but these cells were mapped to the more differentiating ductal compartments. Furthermore, they did not display the major characteristics of quiescent stem cells including the undifferentiated phenotype, mobilization in response to injury, and the clonogenicity in culture. Quantitative assessment of H2BGFP loss in various ductal compartments and short-term lineage tracing of K14(+) ductal cells were consistent with the presence of actively dividing pools of stem/progenitor cells in the intercalated ducts and the basal layer of excretory ducts functioning independently during homeostasis.

Expression and Localization of α-amylase in the Submandibular and Sublingual Glands of Mice

In the major salivary glands of mice, acinar cells in the parotid gland (PG) are known to be the main site for the production of the digestive enzyme α-amylase, whereas α-amylase production in the submandibular gland (SMG) and sublingual gland (SLG), as well as the cell types responsible for α-amylase production, has been less firmly established. To clarify this issue, we examined the expression and localization of both the mRNA and protein of α-amylase in the major salivary glands of male and female mice by quantitative and histochemical methods. α-amylase mRNA levels were higher in the order of PG, SMG, and SLG. No sexual difference was observed in α-amylase mRNA levels in the PG and SLG, whereas α-amylase mRNA levels in the female SMG were approximately 30% those in the male SMG. Using in situ hybridization and immunohistochemistry, signals for α-amylase mRNA and protein were found to be strongly positive in acinar cells of the PG, serous demilune cells of the SLG, and granular convoluted tubule (GCT) cells of the male SMG, weakly positive in seromucous acinar cells of the male and female SMG, and negative in mucous acinar cells of the SLG. These results clarified that α-amylase is produced mainly by GCT cells and partly by acinar cells in the SMG, whereas it is produced exclusively by serous demilune cells in the SLG of mice.

Spatial and temporal expression of c-Kit in the development of the murine submandibular gland

The c-Kit pathway is important in the development of many mammalian cells and organs and is indispensable for the development of hematopoiesis, melanocytes, and primordial germ cells. Loss-of-function mutations in c-Kit lead to perinatal death in mouse embryos. Previously, c-Kit has been used as one of salivary epithelial stem or progenitor cell markers in mouse, its specific temporo-spatial expression pattern and function in developing murine submandibular gland (SMG) is still unclear. Here we used quantitative real-time PCR, in situ hybridization, and immunohistochemistry analysis to detect c-Kit expression during the development of the murine SMG. We found that c-Kit was expressed in the epithelia of developing SMGs from embryonic day 11.5 (E11.5; initial bud stage) to postnatal day 90 (P90; when the SMG is completely mature). c-Kit expression in the end bud epithelium increased during prenatal development and then gradually decreased after birth until its expression was undetectable in mature acini at P30. Moreover, c-Kit was expressed in the SMG primordial cord at the initial bud, pseudoglandular, canacular, and terminal end bud stages. c-Kit was also expressed in the presumptive ductal cells adjacent to the developing acini. By the late terminal end bud stage on P14, c-Kit expression could not be detected in ductal cells. However, c-Kit expression was detected in ductal cells at P30, and its expression had increased dramatically at P90. Taken together, these findings describe the spatial and temporal expression pattern of c-Kit in the developing murine SMG and suggest that c-Kit may play roles in epithelial histo-morphogenesis and in ductal progenitor cell homeostasis in the SMG.

Effects of double ligation of Stensen's duct on the rabbit parotid gland

Salivary gland duct ligation is an alternative to gland excision for treating sialorrhea or reducing salivary gland size prior to tumor excision. Duct ligation also is used as an approach to study salivary gland aging, regeneration, radiotherapy, sialolithiasis and sialadenitis. Reports conflict about the contribution of each salivary cell population to gland size reduction after ductal ligation. Certain cell populations, especially acini, reportedly undergo atrophy, apoptosis and proliferation during reduction of gland size. Acini also have been reported to de-differentiate into ducts. These contradictory results have been attributed to different animal or salivary gland models, or to methods of ligation. We report here a bilateral double ligature technique for rabbit parotid glands with histologic observations at 1, 7, 14, 30, 60 days after ligation. A large battery of special stains and immunohistochemical procedures was employed to define the cell populations. Four stages with overlapping features were observed that led to progressive shutdown of gland activities: 1) marked atrophy of the acinar cells occurred by 14 days, 2) response to and removal of the secretory material trapped in the acinar and ductal lumens mainly between 30 and 60 days, 3) reduction in the number of parenchymal (mostly acinar) cells by apoptosis that occurred mainly between 14-30 days, and 4) maintenance of steady-state at 60 days with a low rate of fluid, protein, and glycoprotein secretion, which greatly decreased the number of leukocytes engaged in the removal of the luminal contents. The main post- ligation characteristics were dilation of ductal and acinar lumens, massive transient infiltration of mostly heterophils (rabbit polymorphonuclear leukocytes), acinar atrophy, and apoptosis of both acinar and ductal cells. Proliferation was uncommon except in the larger ducts. By 30 days, the distribution of myoepithelial cells had spread from exclusively investing the intercalated ducts pre-ligation to surrounding a majority of the residual duct-like structures, many of which clearly were atrophic acini. Thus, both atrophy and apoptosis made major contributions to the post-ligation reduction in gland size. Structures also occurred with both ductal and acinar markers that suggested acini differentiating into ducts. Overall, the reaction to duct ligation proceeded at a considerably slower pace in the rabbit parotid glands than has been reported for the salivary glands of the rat.