A review of the proliferative capacity of major salivary glands and the relationship to current concepts of neoplasia in salivary glands

Organoid Models for Salivary Gland Biology and Regenerative Medicine

The salivary gland is composed of an elegant epithelial network that secrets saliva and maintains oral homeostasis. While cell lines and animal models furthered our understanding of salivary gland biology, they cannot replicate key aspects of the human salivary gland tissue, particularly the complex architecture and microenvironmental features that dictate salivary gland function. Organoid cultures provide an alternative system to recapitulate salivary gland tissue in vitro, and salivary gland organoids have been generated from pluripotent stem cells and adult stem/progenitor cells. In this review, we describe salivary gland organoids, the advances and limitations, and the promising potential for regenerative medicine.

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.

Salivary Gland Hypofunction and Xerostomia in Head and Neck Radiation Patients

BACKGROUND

The most manifest long-term consequences of radiation therapy in the head and neck cancer patient are salivary gland hypofunction and a sensation of oral dryness (xerostomia).

METHODS

This critical review addresses the consequences of radiation injury to salivary gland tissue, the clinical management of salivary gland hypofunction and xerostomia, and current and potential strategies to prevent or reduce radiation injury to salivary gland tissue or restore the function of radiation-injured salivary gland tissue.

RESULTS

Salivary gland hypofunction and xerostomia have severe implications for oral functioning, maintenance of oral and general health, and quality of life. Significant progress has been made to spare salivary gland function chiefly due to advances in radiation techniques. Other strategies have also been developed, e.g., radioprotectors, identification and preservation/expansion of salivary stem cells by stimulation with cholinergic muscarinic agonists, and application of new lubricating or stimulatory agents, surgical transfer of submandibular glands, and acupuncture.

CONCLUSION

Many advances to manage salivary gland hypofunction and xerostomia induced by radiation therapy still only offer partial protection since they are often of short duration, lack the protective effects of saliva, or potentially have significant adverse effects. Intensity-modulated radiation therapy (IMRT), and its next step, proton therapy, have the greatest potential as a management strategy for permanently preserving salivary gland function in head and neck cancer patients.Presently, gene transfer to supplement fluid formation and stem cell transfer to increase the regenerative potential in radiation-damaged salivary glands are promising approaches for regaining function and/or regeneration of radiation-damaged salivary gland tissue.

Plasmacytoid cells in salivary pleomorphic adenoma: an alternative interpretation of their immunohistochemical characteristics highlights function and capability for epithelial-mesenchymal transition

OBJECTIVES

Plasmacytoid cells (PLCs) in salivary pleomorphic adenoma (SPA) are regarded as modified neoplastic myoepithelia and define plasmacytoid myoepithelioma (pMYO). However, histochemically, immunohistochemically and ultrastructurally, PLCs fail to demonstrate frank myogenous properties. Epithelial-mesenchymal transition (EMT) may explain the phenotypes in SPA. Our aim was to evaluate (1) PLCs with accepted or purported myoepithelial and EMT-related markers; and (2) pMYOs for PLAG1 aberrations by using fluorescence in situ hybridization.

STUDY DESIGN

Eight SPAs with or without PLC-predominance and 3 pMYOs were immunohistochemically studied.

RESULTS

PLCs in SPA and pMYO exhibited strong, scattered to diffuse positivity for K7, rare K14 positivity and were mostly negative for α-smooth muscle actin, h-caldesmon, and p63/p40. S100 staining was strong and diffuse, whereas calponin was variable. DOG1 was negative. PLCs in pMYO and PLC-rich SPA exhibited selective or diffuse WT1 and D2-40 immunoreactivity. EMT markers SNAIL/SLUG exhibited strong and variable immunoreactivity in PLCs in contrast to weak or absent E-cadherin expression. SOX10 was diffusely and strongly positive. PLAG1 rearrangement was present in 1 pMYO.

CONCLUSIONS

PLCs mostly fail to express myoepithelial markers; PLCs are neoplastic cells adapting to microenvironmental changes and capable of EMT; and tumors composed solely of PLCs are apparently SPAs depleted of a ductal component.

Proliferative Activity of Myoepithelial Cells in Normal Salivary Glands and Adenoid Cystic Carcinomas Based on Double Immunohistochemical Labeling

Objective

To investigate the proliferative activity of myoepithelial cells (MEC) in normal salivary glands (NSG) and adenoid cystic carcinomas (ACC)) Study design. Twenty -three salivary gland specimens (13 ACC, 10 NSG) were studied using double immunohistochemical labeling for α smooth muscle actin (a-SMA) and proliferative cell nuclear antigen (PCNA)).

Results

There was a significant difference in PCNA reactivity in normal samples between myoepithelial cells of the parotid glands and of the submandibular glands, rates being higher in the latter. Neoplastic myoepithelial cells exhibited higher expression than neoplastic epithelial cells. In addition, myoepithelial cells of the cribriform type of ACC showed PCNA reactivity lower than those of the tubular type, whereas there was no statistically significant difference in epithelial cell rates. We could not identify myoepithelial cells in solid pattern due to α-SMA negativity; although high PCNA reactivity was evident.

Conclusion

These data suggest that the myoepithelial cell has a key role in ACC oncogenesis, more so than its epithelial cell counterparts. Moreover, the data provide a histopathological interpretation for aggressive clinical features of submandibular ACC, as the myoepithelial cells were less differentiated as compared to those of parotid glands.

Salivary Glands: Stem Cells, Self-duplication, or Both?

Understanding the intrinsic potential for renewal and regeneration within a tissue is critical for the rational design of reparative strategies. Maintenance of the salivary glands is widely thought to depend on the differentiation of stem cells. However, there is also new evidence that homeostasis of the salivary glands, like that of the liver and pancreas, relies on self-renewal of differentiated cells rather than a stem cell pool. Here, we review the evidence for both modes of turnover and consider the implications for the process of regeneration. We propose that the view of salivary glands as postmitotic and dependent on stem cells for renewal be revised to reflect the proliferative activity of acinar cells and their role in salivary gland homeostasis.

Squamous/epidermoid differentiation in normal breast and salivary gland tissues and their corresponding tumors originate from p63/K5/14-positive progenitor cells

A small group of tumors of breast and salivary glands contains squamous/epidermoid elements as a constitutive feature (e.g., squamous carcinoma, syringomatous tumors, and mucoepidermoid carcinoma). Other tumors (e.g., pleomorphic adenoma, adenomyoepithelial tumors, and adenoid cystic carcinoma) may show occasionally squamous differentiation. Furthermore, squamous metaplasia may be observed in non-neoplastic breast and salivary tissues. However, the histogenesis of these squamous differentiations is far from being understood. Based on our earlier in situ triple immunofluorescence and quantitative reverse transcription (RT)-PCR experiments for basal keratins K5/14 and p63 as well as for glandular keratins (K7/K8/18), squamous keratins (K10 and K13), and myoepithelial lineage markers (smooth muscle actin, SMA), we here traced the squamous/epidermoid differentiation lineage of 60 tumors of the breast and/or salivary glands, cultured tumor cells of 2 tumors, and of 7 squamous metaplasias of non-neoplastic breast and salivary tissues. Our results indicate that both the neoplastic lesions as well as the non-neoplastic squamous metaplasia contain p63/K5/14+ cells that differentiate toward K10/13+ squamous cells. Thus, cells with squamous/epidermoid differentiation undergo a transition from its original p63/K5/14+ precursor state to K10/13+ squamous lineage state, which can be pictured by triple-immunofluorescence experiments. Given the immunophenotypic similarity of p63/K5/14+ tumor cells to their physiological p63/K5/14+ counterparts in normal breast and salivary duct epithelium, we suggest that these cells provide an important histogenetic key to understanding the pathogenesis of squamous differentiation both in normal breast/salivary gland tissues and their corresponding tumors.

Bone marrow-derived mesenchymal stem cells migrate to healthy and damaged salivary glands following stem cell infusion

Xerostomia is a severe side effect of radiation therapy in head and neck cancer patients. To date, no satisfactory treatment option has been established. Because mesenchymal stem cells (MSCs) have been identified as a potential treatment modality, we aimed to evaluate stem cell distribution following intravenous and intraglandular injections using a surgical model of salivary gland damage and to analyse the effects of MSC injections on the recruitment of immune cells. The submandibular gland ducts of rats were surgically ligated. Syngeneic adult MSCs were isolated, immortalised by simian virus 40 (SV40) large T antigen and characterized by flow cytometry. MSCs were injected intravenously and intraglandularly. After 1, 3 and 7 days, the organs of interest were analysed for stem cell recruitment. Inflammation was analysed by immunohistochemical staining. We were able to demonstrate that, after intravenous injection, MSCs were recruited to normal and damaged submandibular glands on days 1, 3 and 7. Unexpectedly, stem cells were recruited to ligated and non-ligated glands in a comparable manner. After intraglandular injection of MSCs into ligated glands, the presence of MSCs, leucocytes and macrophages was enhanced, compared to intravenous injection of stem cells. Our data suggest that injected MSCs were retained within the inflamed glands, could become activated and subsequently recruited leucocytes to the sites of tissue damage.

Canalicular adenoma--search for the cell of origin: ultrastructural and immunohistochemical analysis of 7 cases and review of the literature

Canalicular adenoma (CA) is a rare, benign epithelial neoplasm of the salivary glands. Historically considered to be a variant of basal cell adenoma, this "monomorphic" adenoma has a distinct clinical, morphologic, and immunohistochemical profile. The putative cell of origin remains a topic of debate. A combination of morphology, immunohistochemistry, and ultrastructural analyses have been employed to determine histogenesis, but the interpretations of these studies have implicated multiple different cell types along the salivary gland duct as the cell of origin. The authors sought to further characterize CA using electron microscopy, immunohistochemistry, and special and immuno-stains on 7 cases. Their morphologic, immunohistochemical, and ultrastructural findings support a cell of origin demonstrating features of both the intercalated duct cells and the striated duct luminal epithelial cells.

A perspective of comparative salivary and breast pathology. Part I: microstructural aspects, adaptations and cellular events

This is the first part of a review comparing the pathology of salivary and mammary glands. Here, less obvious similarities and differences in functional histology and their influences on pathology are examined with emphasis on myoepithelial cells, stromal components, analogues of mucosa-associated lymphoid tissue, steroid receptors, and intraparenchymal cells of monocytic lineage. Particular cell phenotypes (oncocytic, apocrine, neuroendocrine and clear) are critically evaluated and responses to atrophy, infarction and fine-needle aspiration biopsy procedures are highlighted together with aspects of metaplasia, regeneration, ageing and microcalcification. Areas of controversy or uncertainty which may benefit from further investigations are also discussed.

Isolation and propagation of mesenchymal stem cells from the lacrimal gland

PURPOSE

Previously, it was reported that the murine lacrimal gland is capable of repair after experimentally induced injury and that the number of stem/progenitor cells was increased during the repair phase (2-3 days after injury). The aim of the present study was to determine whether these cells can be isolated from the lacrimal gland and propagated in vitro.

METHODS

Lacrimal gland injury was induced by injection of interleukin (IL)-1, and injection of saline vehicle served as control. Two and half days after injection, the lacrimal glands were removed and used to prepare explants or acinar cells for tissue culture. Cells derived from the explants and the acinar cells were grown in DMEM supplemented with 10% fetal bovine serum. Cells were stained for the stem cells markers, nestin, vimentin, ABCG2, and Sca-1. Cell proliferation was measured using an antibody against Ki67 or a cell-counting kit. The adipogenic capability of these cells was also tested in vitro.

RESULTS

Results show that nestin-positive cells can be isolated from IL-1-injected, but not saline-injected, lacrimal glands. A population of nestin-positive cells was also positive for vimentin, an intermediate filament protein expressed by mesenchymal cells. In addition, cultured cells expressed two other markers of stem cells, ABCG2 and Sca-1. These cells proliferated in vitro and can be induced to form adipocytes, attesting to their mesenchymal stem cell property.

CONCLUSIONS

Murine lacrimal glands contain mesenchymal stem cells that seem to play a pivotal role in tissue repair.

Mechanisms involved in injury and repair of the murine lacrimal gland: role of programmed cell death and mesenchymal stem cells

The non-keratinized epithelia of the ocular surface are constantly challenged by environmental insults, such as smoke, dust, and airborne pathogens. Tears are the sole physical protective barrier for the ocular surface. Production of tears in inadequate quantity or of inadequate quality results in constant irritation of the ocular surface, leading to dry eye disease, also referred to as keratoconjunctivitis sicca (KCS). Inflammation of the lacrimal gland, such as occurs in Sjogren syndrome, sarcoidosis, chronic graft-versus-host disease, and other pathological conditions, results in inadequate secretion of the aqueous layer of the tear film and is a leading cause of dry eye disease. The hallmarks of lacrimal gland inflammation are the presence of immune cell infiltrates, loss of acinar epithelial cells (the secreting cells), and increased production of proinflammatory cytokines. To date, the mechanisms leading to acinar cell loss and the associated decline in lacrimal gland secretion are still poorly understood. It is also not understood why the remaining lacrimal gland cells are unable to proliferate in order to regenerate a functioning lacrimal gland. This article reviews recent advances in exocrine tissue injury and repair, with emphasis on the roles of programmed cell death and stem/progenitor cells.

Ascl3 expression marks a progenitor population of both acinar and ductal cells in mouse salivary glands

Ascl3, also know as Sgn1, is a member of the mammalian achaete scute (Mash) gene family of transcription factors, which have been implicated in cell fate specification and differentiation. In the mouse salivary gland, expression of Ascl3 is restricted to a subset of duct cells. Salivary gland function depends on the secretory acinar cells, which are responsible for saliva formation, and duct cells, which modify the saliva and conduct it to the oral cavity. The salivary gland ducts are also the putative site of progenitor cells in the adult gland. Using a Cre recombinase-mediated reporter system, we followed the fate of Ascl3-expressing cells after the introduction of an EGFP-Cre expression cassette into the Ascl3 locus by homologous recombination. Lineage tracing shows that these cells are progenitors of both acinar and ductal cell types in all three major salivary glands. In the differentiated progeny, expression of Ascl3 is down-regulated. These data directly demonstrate a progenitor-progeny relationship between duct cells and the acinar cell compartment, and identify a population of multipotent progenitor cells, marked by expression of Ascl3, which is capable of generating both gland cell types. We conclude that Ascl3-expressing cells contribute to the maintenance of the adult salivary glands.

A single injection of interleukin-1 induces reversible aqueous-tear deficiency, lacrimal gland inflammation, and acinar and ductal cell proliferation

Emerging studies from our laboratory demonstrate that interleukin-1 (IL-1) family members play a major role in impairing lacrimal gland functions. Here we have extended our investigations to observe the effects of IL-1 on aqueous tear production, lacrimal gland secretion, lacrimal gland histology, and acinar and ductal cell proliferation. We demonstrate that a single injection of IL-1 into the lacrimal glands inhibited neurally- as well as agonist-induced protein secretion resulting in decreased tear output. Meanwhile, IL-1 injection induced a severe, but reversible (7-13 days), inflammatory response that led to destruction of lacrimal gland acinar epithelial cells. Finally, we demonstrate that as the inflammatory response subsided and lacrimal gland secretion and tear production returned to normal levels, there was increased proliferation of acinar and ductal epithelial cells. Our work uncovers novel effects of IL-1 on lacrimal gland functions and the potential regenerative capacity of the mouse lacrimal gland.

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

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

Regeneration in chronic sialadenitis: an analysis of proliferation and apoptosis based on double immunohistochemical labelling

The understanding of regeneration in salivary glands as a finely tuned balance of cellular proliferation, differentiation and apoptosis has been limited by the difficulty of identifying proliferating cells. This has been overcome in the present investigation by double immunohistochemical labelling for the proliferation-associated antigen Ki67 and for different cell-type-specific antigens applied to 8 specimens of normal parotids and 16 specimens of chronic parotid sialadenitis with particular reference to acini and intercalated ducts. In comparison with low baseline rates of proliferation in normal parotids, proliferative indices were significantly increased in chronic sialadenitis in mature acinar cells, intercalated ductal cells and myoepithelial cells without evidence of proliferation by an additional population of cells. In accordance with findings in glands of experimental animals, the present data do not support the previously postulated concept of regeneration of acini and intercalated ducts by a hypothetical population of uncommitted ductal stem cells. The demonstration of a profound capacity for intrinsic glandular regeneration from differentiated cells represents a biological basis for the good results obtained from conservative therapy of chronic sialadenitis and offers hope for novel therapies designed to reconstitute impaired salivary flow.

Differential diagnosis of salivary acinic cell carcinoma and adenocarcinoma (NOS). A comparison of (immuno-)histochemical markers

A correct histologic differential diagnosis between salivary acinic cell carcinoma (ACC) and adenocarcinoma not otherwise specified (AC-NOS) is highly relevant because of the strikingly different biologic behavior and related therapeutical strategies. The distinction between both tumor types can be difficult because of an enormous variation in histologic appearance, with either type showing partially overlapping morphologic features. Owing to a lack of approved markers, the expression of PAS-staining, alpha-Amylase, alpha-1 Anti-trypsin, cytokeratin (CK)-subtypes 7/18 and Ki-67 was evaluated in 16 cases of ACC and 16 cases of AC-NOS. CK 7 is identified as the most reliable marker with strong positivity in AC-NOS, and complete or preponderant negativity in ACC. The characteristic membranous staining pattern of CK 18 in ACC, in contrast to a diffuse cytoplasmic pattern in AC-NOS, proved to be an additional valuable criterion. PAS and alpha-Amylase are only of little value when ACC is diagnosed, as many cases are only faintly positive or completely negative. The proliferation index (Ki-67) proved to be significantly higher in AC-NOS; however, the diagnostic usefulness is limited by a relevant overlap. In conclusion, we recommend CK 7 and 18 as the most valuable markers in cases with difficult differential diagnosis between ACC and AC-NOS.

Multifocal ductal cell hyperplasia in the submandibular salivary glands of Wistar rats chronically treated with a novel steroidal compound

A high incidence of multifocal ductal hyperplasia was observed in the submandibular salivary gland of rats treated for 26 weeks with a high dose of a novel synthetic steroid with combined estrogenic and progestagenic properties. Hyperplastic foci consisted of microcystic duct-like structures lined by a single or multilayered epithelium, sometimes showing a tendency towards a cribiform growth pattern. The hyperplastic ducts wereembedded in a collagen-rich stroma and surrounded by numerous myoepithelial cells. Immunohistochemical methods used for the detection of estrogen- and progesterone receptors revealed that progesterone receptors were abundantly present in the nucleus of epithelial cells within the lesions, exclusively. Estrogen receptors could not be detected in both the normal tissue and hyperplasic foci. The morphological, ultrastructural, and immunohistochemical characteristics strongly suggest that these hyperplastic lesions originated from the intercalated ducts. The rodent-specific granular duct cell was not involved in the pathogenesis as was clearly demonstrated by the lack of immunoreactive epidermal growth factor within the lesions. Lesions were not observed in studies with progestagens and estrogens alone or with other combined estrogen/progestagen compounds, suggesting that the specific ratio of estrogenic and progestagenic activity of the present steroid had played an important role in the development of ductal hyperplasia in this study. Lesions of the intercalated ducts, as described in this study, have not been reported before in the literature.

Expression of cytoskeletal proteins in developing human minor salivary glands

The presence of an epithelium at different stages of proliferation and differentiation raises interesting questions concerning the histogenesis, cell turnover and differentiation of normal salivary glands. In order to expand knowledge of these aspects, we investigated the expression of cytokeratins (CKs) 7,8,10,13,14,16,18 and 19, vimentin (VIM), and smooth muscle actin (SMA) in developing human minor salivary glands using monoclonal antibodies. Labial, buccal, palatine, and lingual salivary glands and those from the floor of the mouth were obtained from human fetuses (forensic postmortem) ranging in age from gestational weeks 10 to 29. Serial sections, 3 microm thick, were immunostained using a strepto-avidin-biotin technique. Reactivity for all antibodies was negative in the salivary gland epithelium during the developmental stages of bud formation, cord growth, and branching of cord. During canalization and cytodifferentiation, the glandular epithelial cells showed a positive reaction to some CKs and SMA. Cytokeratins 7, 8, 18, and 19 showed strong labeling in luminal duct cells that exhibited some degree of morphological differentiation. Myoepithelial cellc were recognized by antibodies to SMA. Cytoskeletal protein expression changes according to the cell type, degree of differentiation, and stage of morphological development of the glandular structure. These changes occur independently of the localization of the gland.

Salivary gland hyperplasia

Thirteen cases of adenomatous ductal hyperplasia (ADH) of the major salivary glands coexisting with salivary gland tumors or chronic parotitis are reported. The proliferating ducts have the morphology and immunohistochemistry similar to intercalated duct epithelium. Adenomatous ductal hyperplasia is compared to adenomatoid acinar hyperplasia (AAH), a lesion found predominantly in intraoral salivary glands and histologically composed of hyperplastic acinar cells. The exact nature of both lesions is not clear. However, ADH may be a precursor lesion of salivary gland tumors (especially epithelial-myoepithelial carcinomas), whereas AAH may represent a reactive process of idiopathic nature.

Adenomatous ductal proliferation of the salivary gland

OBJECTIVE

The objective of this study is to report a series of patients with adenomatous ductal proliferation of the salivary gland and to investigate the nature and histogenesis of this process.

STUDY DESIGN

The histologic and immunohistochemical studies of 13 patients with adenomatous ductal proliferation of the salivary gland were performed.

RESULTS

Ten cases coexisted with other diseases, such as salivary gland tumor or chronic parotitis. Most of the lesions were limited to 1 or 2 lobules and showed prominent ductal proliferation with some acinar cell complexes. The proliferative ducts had a compact growth pattern with little stroma. Dedifferentiation of acinar cells, identified by the loss of their secretory granules and appearing similar to intercalated duct epithelium, was seen in the early stages of adenomatous ductal proliferation.

CONCLUSION

Acinar cells play a role in the histogenesis of salivary gland tumors because acinar cells clearly participate in the formation of the proliferative ducts.

Flow cytometric analysis of DNA content in parotid tumor and its contiguous acini

To investigate the relationship between proliferative capacity of salivary gland cells in contiguous acini of parotid tumors and recurrent neoplasma, DNA contents of 30 fresh specimens of parotid were studied by using cytometry in tumors, normal and shallow or deep lobe acini of the masses. The results showed that the DI was 1.369, S% 16.95, PI 26.18 in malignant tumors; DI was 1.171, S% 12.41, PI 15.54 in recurrent pleomorphic adenoma; DI was 1.141, S% 12. 74, PI 13.07 in pleomorphic adenoma, DI was 0.999, S% 5.10, PI 8.00 in normal acini. Analysis of variance showed there was a significant difference (P < 0.01). The average DNA contents of shallow on deep lobe of contiguous tumors was 1.08 in DI, 10.65 in S%, 13.49 in PI in malignant tumor, 1.06 in DI, 8.96 in S% and 9.85 in PI in pleomorphic adenoma, which were all higher than in normal acini (P > 0.05). It was concluded that the levels of DI and S% of parotid tumor and its contiguous acini are related to degree of malignancy or recurrent condition of the tumors, suggesting contiguous acini of parotid tumors had the strong capacity of proliferation, which might play an important role in recurrent or malignant change of the parotid tumors.

Lymphoma development in Sjögren's syndrome: novel p53 mutations

OBJECTIVE

Sjögren's syndrome (SS) is a chronic autoimmune disease characterized by lymphocytic infiltrations of the exocrine glands. Disease progression may lead to uncontrolled clonal proliferation of B lymphocytes and development of lymphoma. This study was undertaken to examine the possible involvement of the cell cycle checkpoint genes p53 and p21 in the pathophysiology of the syndrome.

METHODS

Protein expression of p53 and p21 was studied, by immunohistochemistry and Western blot analysis, in minor salivary gland (MSG) biopsy specimens from 7 patients with SS and 5 control subjects. In addition, sequence analysis of the p53 gene was performed on DNA samples obtained from MSG biopsy samples of the same 7 patients with SS and from 4 patients with SS and in situ non-Hodgkin's lymphoma (NHL).

RESULTS

The study revealed increased protein expression of p53 and p21 in MSG biopsy specimens from patients as compared with controls, while sequence analysis showed that the p53 gene was of the wild type. Furthermore, sequence analysis of the p53 gene from patients with SS and in situ NHL revealed 2 novel mutations in exon 5 of the p53 gene. These mutations are single-base substitutions and appear to be functional since exon 5 is included in the coding region of the p53 gene.

CONCLUSION

This is the first report on wild-type p53 gene activation in SS. Our findings indicate a probable role for the DNA damage response genes in the pathogenesis of this syndrome. The novel mutations of the p53 gene implicate dysregulation of this tumor suppressor gene as a possible mechanism for lymphoma development in SS.

Modes of epithelial cell death and repair in Sjögren's syndrome (SS)

We evaluated possible modes of epithelial cell destruction and restoration in minor salivary gland biopsies from patients with SS. Minor salivary gland biopsies from 10 primary Sjögren's syndrome (pSS) patients and eight control individuals were evaluated by immunohistochemical staining for the expression of apoptosis-related molecules, substances released by activated cytotoxic T cells, as well as proteins involved in epithelial cell repair. The results were analysed by computer screen analysis and they were expressed as average percentages. Apoptosis-promoting molecules, Fas antigen and Fas ligand were observed in ductal and acinar epithelial cells as well as in infiltrating mononuclear cells of minor salivary glands from SS patients in comparison with control biopsies. Bax protein, which acts as a death-promoter message, was expressed in the ductal and acinar epithelial cells and in mononuclear infiltrating cells of SS patients compared with control individuals, while Bcl-2, an inhibitor of apoptosis, was primarily found in the lymphocytic infiltrates. In situ DNA fragmentation assay (TUNEL) revealed that epithelial cells were apoptotic in patients with SS compared with control subjects. Immunohistochemical staining for perforin and granzyme B, released from granules of activated cytotoxic lymphocytes, revealed their presence in lymphocytic infiltrates of patients with SS compared with control biopsies. pS2, a member of the trefoil protein family which functions as promoter of epithelial cell repair and cell proliferation, was expressed in epithelial cells in biopsies from SS patients. These studies suggest that the functional epithelium of minor salivary glands in patients with SS appears to be influenced by both intrinsic and extrinsic mechanisms of destruction, while a defensive mechanism of epithelial restoration seems to be active.

Histogenesis and development of membranous basal cell adenoma

The histogenesis of salivary gland tumors remains controversial. A histologic and immunohistochemical study was carried out on 12 cases of membranous basal cell adenoma in the salivary glands to investigate its histogenesis and development. In 4 cases, multiple focal proliferations of striated ducts in the surrounding salivary gland, which were conspicuous, allowed tracing of the development of the tumors from their origin. The earliest changes were characterized by hyperplasia of the basal cells of striated ducts. Continuous ductal proliferation led to the formation of microadenomas. The observations suggest that proliferating basal cells of striated ducts apparently participate in and play a main role in the histogenesis of some membranous basal cell adenomas of the salivary glands.

Study of minor salivary gland mucoepidermoid carcinoma differentiation based on immunohistochemical expression of cytokeratins, vimentin and muscle-specific actin

The expression of cytokeratins (CKs) 7,8,10,13,14,18,19, vimentin and muscle-specific actin (MSA) was investigated in 17 mucoepidermoid carcinomas (MEC) by the streptavidin-biotin technique. The results revealed that CKs 7, 8 and 18 were positive for intermediate, luminal columnar and mucous cells. For epidermoid cells, the expression was heterogeneous and discrete. The reaction with CK19 was similar to that seen for the above CKs, except for the fact that mucous cells were negative. CK 14 was preferentially expressed in the intermediate cells localised in basal, parabasal and epidermoid cells. CK13 was localised in intermediate, epidermoid and luminal columnar cells. In stratified epithelium, CK13 was expressed in intermediate cells and negative in basal cells. These findings were more expressive in cystic areas of the tumours. CK10 was negative for all the cases studied. MSA was positive only in stromal elements, and only two cases of CME were heterogeneously positive for vimentin. The result obtained showed that the immunoprofile of MEC, for the studied antigens, is similar to that exhibited by the excretory duct of normal salivary glands.

Cell proliferation in salivary gland adenocarcinomas with myoepithelial participation. A study of 78 cases

We used three markers of cell proliferation mitotic counts, mitotic index and expression of proliferating cell nuclear antigen--to assess the proliferative activity of a series of 78 low-grade salivary adenocarcinomas with myoepithelial participation classified according to: their histological type, the predominant architectural type, and the predominant cytological type. The series included adenoid cystic carcinomas (40), epithelial-myoepithelial carcinomas (19), polymorphous low-grade adenocarcinomas (12) and basal cell adenocarcinomas (7). The proliferation indicators were found to be similar in the first three groups, being significantly lower than in the last. Tumours formed by basal cells had statistically significant higher mitotic indexes than those predominantly composed of clear cells of myoepithelial type and ductal cells. Tubular tumours, irrespective of the histological classification of the neoplasm, had proliferation indexes similar to those found in cribriform neoplasms. Solid tumours, whether formed by ductal or clear myoepithelial-type cells, had higher indexes than the neoplasms with differentiated (cribriform and tubular) patterns. The highest mean values for every proliferation indicator used were found in tumours with solid organization that were predominantly formed by basal cells. These results agree with the hypothesis that cell proliferation is inversely related to neoplastic differentiation. The identification of the prevalent cell phenotype and architecture may extend our knowledge from adenoid cystic carcinoma, whose solid variant carries a worse prognosis, and supports that the usual classification of this group of salivary adenocarcinomas would benefit to be complemented with information on tumour architecture and cellular composition.

Poorly differentiated carcinoma of the parotid gland in a six-week-old Sprague-Dawley rat

A poorly differentiated spontaneous tumor was found in the parotid gland region of a 6-wk-old female Sprague-Dawley rat. The tumor consisted of undifferentiated cells forming dense clusters, well-defined nests, or compact sheets. In some areas, spindle-shaped cells were arranged in a fascicular fashion. Neither ductular nor acinar structures were seen. No keratinization was observed, either. Mitotic figures were frequent. All neoplastic cells were immunoreactive for pankeratin but negative for vimentin, alpha-smooth muscle actin and S-100 protein. Ultrastructurally, the tumor cells were characterized by desmosome-like junctional complexes, poorly developed cytoplasmic organelles, and microvilli-like cytoplasmic processes. Based on these findings and its anatomical location, the present tumor was diagnosed as a poorly differentiated carcinoma of the parotid gland.

Myoepithelial cells actively proliferate during atrophy of rat parotid gland

Despite limited supporting evidence, salivary gland myoepithelial cells are said to be differentiated cells with little or no capacity to replicate; they presumably develop from stem cells. This study investigated the proliferative potential of myoepithelial cells with an antibody to proliferating cell nuclear antigen and a rat model. This model involved clamping of the parotid duct causing atrophy of the gland and then releasing the duct followed by gland regeneration. Rats were sacrificed at time points during atrophy and regeneration phases and the number and location of cycling myoepithelial cells assessed. Cycling myoepithelial cells were identified with double immunohistochemical staining, cycling cells with proliferating cell nuclear antigen-positive nuclei within muscle-specific actin-positive cytoplasm (the latter identified with antibody HHF35). The results show that baseline proliferative rates of myoepithelial cells in both the resting and fully regenerated gland ranged from 0.3% to 2%, similar to rates for other major cell types in the normal rat gland. A peak myoepithelial cell proliferative rate of 23% occurred at day 5 during the atrophy phase. Rates during the regenerative phase were not significantly different than the baseline levels. Similarities of rat and human parotid gland and the definite proliferative capacity of myoepithelial cells indicates that these specialized cells must be considered one of the potential progenitor cells for human salivary gland tumors.

Differentiating myoepithelial and acinar cells in rat neonatal parotid gland and histogenetic concepts for salivary gland tumors

Histogenetic concepts for salivary gland tumors are predicated on the presence of reserve or undifferentiated cells in normal glands, presumably the source for cell renewal and induction of tumors. Developing rat parotid gland, which remains fetal-like at birth, provides the opportunity to study differentiation and observe whether cytologically undifferentiated cells do or do not have functional indicators of specific differentiation pathways. Immunohistochemistry and immuno-electron microscopy, when applied to parotid gland at birth, at 12 days of age and in the adult gland, indicate that commitment to myoepithelial cell differentiation occurs prior to development of structural changes characteristic of these cells. Conversely, secretory granules are evident in differentiating acinar cells prior to synthesis of amylase. The results suggest that an appearance of undifferentiation does not confer reserve cell status either in the normal salivary gland or their tumors.

Cytomegalovirus in the principal submandibular gland of the little brown bat, Myotis lucifugus

In the course of a large scale study of salivary gland ultrastructure in chiropterans, enlarged cells infected with numerous virus particles were encountered in some acinar cells in the principal submandibular glands of two of 34 little brown bats (Myotis lucifugus). The characteristic morphology of the viruses, together with the cytomegaly that they induced, led to their identification as cytomegalovirus (CMV). In a reversal of the situation in other animal species, bat virus particles within cytoplasmic vacuoles lacked capsomeres, whereas the latter were prominent in particles free in the cytosol. The generally accepted schemes for CMV production cannot explain this seemingly aberrant morphology. This report extends to four the number of mammalian orders in which CMV has been documented by means of electron microscopy as occurring in salivary glands.

Proliferative activity by cell type in the developing rat parotid gland

BACKGROUND

In contrast to the considerable amount of research that has been done on the proliferative activity of the several types of parenchymal cells in the developing submandibular glands of rodents, systematic studies of cellular proliferation in the developing parotid gland have been confined to the acinar cells. The purpose of the present study was to attempt to fill this knowledge gap.

METHODS

Tritiated thymidine was parenterally administered to Sprague-Dawley rats at ages representative of the pre- and postnatal development of the parotid gland, and glands were harvested for autoradiography 90 min after injection. Mitotic activity among all cell types was verified by electron microscopy.

RESULTS

At all ages, the % labeled cells was much greater among the acini than any other cell type, including well-differentiated cells at 25 and 40 days. However, there were only small alterations in the proportions of cells comprised by the major cell types.

CONCLUSIONS

Current theories on the histogenesis of salivary glands and their neoplasms are based on the renewing population model, in which both normal differentiated cells and neoplastic cells arise from undifferentiated stem cells in the ducts. However, these results suggest that most of the migration and redifferentiation in the developing rat parotid gland must be in the opposite direction, i.e., the acinar cells redifferentiate into ductal cells. They also indicate that until there are precise data on the rates of cell death among the several cell types, it remains more appropriate for salivary glands to be categorized as an expanding, rather than renewing, population.

Expression of bcl-2 in salivary glands and salivary gland adenomas. A contribution to the reserve cell theory

The bcl-2 proto-oncogene product inhibits apoptosis. It has been suggested that bcl-2 assists the survival of stem cells. Bcl-2 also plays a role in the development of adenomas. In salivary glands it is expressed in basal cells of striated and excretory ducts which may indicate that these cells are reserve cells. Acinar cells, myoepithelial cells and most luminal cells are negative for bcl-2. In basal cell adenomas and Warthin's tumors it is found predominantly in cells with basal cell differentiation. In pleomorphic adenomas bcl-2 is expressed mainly in basal cells of tubulo-ductal structures, at various degrees and patterns in solid and trabecular areas and at low degree in myxoid areas. In chondroid areas of pleomorphic adenomas, in myoepitheliomas and oncocytomas it is only focally expressed or missing. If the inhibition of apoptosis plays a significant role in the genesis of these neoplasms, then factors other than bcl-2 must be effective.

Regeneration of parotid acinar cells after high radiation doses. A morphological study in rat

The acute and late effects of fractionated irradiation on rat parotid gland acinar cells were studied by light and electron microscopy. At 10 days after the last irradiation session (6 Gy or 9 Gy daily during five consecutive days) no effects were seen. At 180 days, minor loss of acini was detectable after a total dose of 30 Gy. After 45 Gy a massive acinar loss was seen at that time; the number of acini had diminished and minor duct-like structures and scattered amounts of fibrous stroma dominated the slides. The remaining acini were disorganized and usually larger compared with the control side and to non-irradiated animals. The acinar cells appeared larger than in the controls. The ducts were better preserved but the intercalated ducts often seemed to be larger than normal. We suggest that this phenomenon indicates a remaining capacity of the parotid gland to regenerate acinar cells even after high radiation doses.

Inverted ductal papilloma of minor salivary glands

Inverted ductal papillomas are rare tumours of minor salivary glands. A case is reported on the lower lip of a 50-year-old man. The tumour showed a characteristic endophytic growth pattern and was composed of bulbous papillary projections of basaloid cells dilating and filling the superficial portion of the excretory duct. Histologically, inverted ductal papillomas resemble the inverted papilloma of the nose and paranasal sinuses, but are completely benign and are not associated with malignant change.

Myoepithelium of salivary glands

In salivary glands and other exocrine organs, there are starfish-shaped cells that lie between the basal lamina and the acinar and ductal cells. These have structural features of both epithelium and smooth muscle cells, and so are called myoepithelial cells. Their functions include contraction when the gland is stimulated to secrete, compressing or reinforcing the underlying parenchymal cells, thus aiding in the expulsion of saliva and preventing damage to the other cells. They also may aid in the propagation of secretory and other stimuli. Their common developmental origin with the basal cells of the larger ducts is displayed in the mature glands by shared structural and immunohistochemical features, but most such basal cells do not have the distinguishing features of myoepithelial cells, such as myofibrils. Although myoepithelial cells can be identified by light microscopy through enzyme histochemistry and special stains and immunohistochemistry for their myofibrils, these techniques can be misleading in salivary gland neoplasms. Thus, the most reliable means of identifying neoplastic myoepithelial cells is with a combination of histochemistry and electron microscopy. The extent to which these cells are derived from undifferentiated stem cells in both normal and neoplastic growth is controversial. The presentation here of transmission electron microscopy (TEM) of well-differentiated myoepithelial cells in mitotic division indicates that stem cells are not necessarily the only source of myoepithelial cells in the later stages of salivary gland development or in neoplasia.

The pathobiology of salivary gland. III. PCNA-localization of cycling cells induced in rat submandibular gland by low-dose x-radiation

Application of ionizing radiation to adult rat major salivary glands tested tenets of the bicellular reserve cell hypothesis for the induction of salivary gland tumors, namely, that stem cells preferentially located to luminal cells of the intercalated duct and basal cells of the excretory duct in normal salivary glands. The effect of a single, low dose (3000 cGy) of x-radiation administered to the parotid and submandibular glands was followed with the use of immunocytochemistry and an antibody to the cell cycle-related protein proliferating cell nuclear antigen to detect the kinetics and localization of cycling cells up to 15 days postirradiation. Maximal responses occurred in acinar cells (12.6-fold increase) of submandibular glands on day 7 postirradiation. Similar but less dramatic concurrent increases in proliferating cells were evident in intercalated (3.4-fold) and striated (2.2-fold) duct cells, but little response was seen in basal or luminal cells of submandibular gland excretory ducts. A limited but maximal proliferative response again occurred on day 7 in the parotid gland. Neither in the steady state nor irradiated submandibular gland was there evidence of specific stem ("reserve") cells associated with the intercalated or excretory ducts. It appears unnecessary to invoke stem cells in a model of cellular proliferation in salivary glands. Therefore current concepts of salivary gland tumorigenesis require modification because all cell types, including acinar cells, are at risk in the carcinogenic process.

Current status of histogenetic and morphogenetic concepts of salivary gland tumorigenesis

Because of their complexity and relative infrequency, salivary gland tumors commonly result in diagnostic problems. Histogenetic and morphogenetic concepts of tumorigenesis in these glands are reviewed and their relevance to routine diagnosis and classification of salivary gland tumors evaluated. Evidence is presented from animal and human studies that under steady-state and pathophysiological conditions, all cell types present in the normal gland, including acinar cells, are capable of rapidly entering the cell cycle and are, therefore, possible targets for neoplastic transformation.

Pathobiology of salivary glands. IV. Histogenetic concepts and cycling cells in human parotid and submandibular glands cultured in floating collagen gels

Localization of cells with proliferative capacity in human major salivary glands lacks extensive study. Minced fragments of human parotid (n = 3) and submandibular (n = 3) glands embedded in a floating collagen gel matrix and cultured for up to 28 days allowed maintenance of the three-dimensional relationship of the various cell types in these glands. Immunocytochemistry and electron microscopy of a time-dependent series of cultured gland fragments showed gradual cytologic modification of acinar cells so that acini became duct-like but also established that even after 28 days of culture certain cellular features allowed continued identification of acinar cells. Serial section immunostaining for amylase, cytokeratins, and proliferating cell nuclear antigen (a specific marker for cycling cells) revealed that acinar, intercalated duct, and excretory duct (both basal and luminal) cells are all capable of entering the cell cycle. At day 5 of culture, the number of cycling cells increased 16-fold in the parotid gland and 9-fold in the submandibular gland over that in the respective in situ gland. In this in vitro system, which perhaps simulates regenerative processes in human salivary glands, none of the samples showed cycling cells localized only to segments of intercalated duct or the basal cells of excretory duct as suggested by current histogenetic concepts.

Comparative cytofluorometric DNA analysis of pleomorphic adenoma and adenoid cystic carcinoma of the salivary glands

The nuclear DNA content in the tumor cells of 17 pleomorphic adenomas (PA) and 26 adenoid cystic carcinomas (ACC) was assayed by cytofluorometry to clarify the etiology of the differences in their biological nature. Aneuploidy was found in only two solid-pattern ACC samples; all the other samples had diploid stemlines. With respect to the polyploid cell rate and the S + G2M fraction, the differences between the four histological patterns of PA were not significant. In the ACC, the S + G2M fraction was significantly higher in solid pattern than in cribriform or trabecular pattern tumors, which may indicate that proliferative activity is higher in the solid pattern than in the other two. The polyploid cell rate in ACC was significantly higher in the solid pattern than in the cribriform pattern carcinomas. There was no significant difference in the mean polyploid cell rate between PAs and ACCs. The mean S + G2M fraction was significantly higher in all histological patterns of ACCs than in PAs. These results suggest that proliferative activity is greater in any one of the histological patterns of ACC than in PA. These findings may largely explain the biological differences between the two tumors.

Comparative histogenesis and morphogenesis of mucoepidermoid carcinoma and pleomorphic adenoma. An ultrastructural study

Current classifications of salivary gland tumors separate mucoepidermoid carcinoma from other neoplasms on the basis of a number of histological features, in particular the lack of participation of neoplastic myoepithelial cells. However, ultrastructural examination of low- and intermediate-grade mucoepidermoid carcinomas and pleomorphic adenomas reveals many common organizational and cellular features. Of prime importance is the relationship of intermediate cells to the luminal cells in mucoepidermoid carcinomas, which is remarkably similar to that seen between modified myoepithelial cells and luminal cells in pleomorphic adenomas. The results suggest that intermediate cells of mucoepidermoid carcinoma are the counterpart of the modified myoepithelial cells of pleomorphic adenoma. The generally accepted hypothesis that the former tumor develops from an excretory duct reserve cell, while the latter originates from an intercalated duct stem cell does not seem to be valid; pleomorphic adenoma and mucoepidermoid carcinoma appear to be closely related morphologically.