Ultrastructure of epithelial rests of Malassez in human periodontal ligament

Establishment and characterization of novel epithelial-like cell lines derived from human periodontal ligament tissue in vitro

In this study, novel human-derived epithelial-like cells (hEPLCs) lines were established from periodontal ligament (PDL) tissues, which were composed of a variety of cell types and exhibited complex cellular activities. To elucidate the putative features distinguishing these from epithelial rest of Malassez (ERM), we characterized hEPLCs based on cell lineage markers and tight junction protein expression. The aim of this study was, therefore, to establish and characterize hEPLCs lines from PDL tissues. The hEPLCs were isolated from PDL of third molar teeth. Cellular morphology and cell organelles were observed thoroughly. The characteristics of epithelial–endothelial-mesenchymal-like cells were compared in several markers by gene expression and immunofluorescence, to ERM and human umbilical-vein endothelial cells (HUVECs). The resistance between cellular junctions was assessed by transepithelial electron resistance, and inflammatory cytokines were detected by ELISA after infecting hEPLCs with periodontopathic bacteria. The hEPLCs developed into small epithelial-like cells in pavement appearance similar to ERM. However, gene expression patterns and immunofluorescence results were different from ERM and HUVECs, especially in tight junction markers (Claudin, ZO-1, and Occludins), and endothelial markers (vWF, CD34). The transepithelial electron resistance indicated higher resistance in hEPLCs, as compared to ERM. Periodontopathic bacteria were phagocytosed with upregulation of inflammatory cytokine secretion within 24 h. In conclusion, hEPLCs that were derived using the single cell isolation method formed tight multilayers colonies, as well as strongly expressed tight junction markers in gene expression and immunofluorescence. Novel hEPLCs lines exhibited differently from ERM, which might provide some specific functions such as metabolic exchange and defense mechanism against bacterial invasion in periodontal tissue.

Cytokeratin expression of engrafted three-dimensional culture tissues using epithelial cells derived from porcine periodontal ligaments

OBJECTIVE

This study investigated the differentiation and proliferation of epithelial cells derived from periodontal ligaments after three-dimensional culture using collagen gel with fibroblasts in vitro and in vivo.

METHODS

Epithelial cells and fibroblasts were derived from porcine periodontal ligaments. Epithelial cells were labeled using a fluorescent red membrane marker (PKH-26GL) and were seeded onto collagen gel with fibroblasts, followed by incubation in an air-liquid interface for 7 days. Three-dimensional cultures were grafted onto the backs of nude mice and removed at 1, 7, and 14 days after surgery (in vivo model). Unfixed sections (5 μm) were used to detect the presence of red fluorescent cells. Paraffin sections were analyzed histologically and immunohistochemically. Specimens were compared with three-dimensional culture tissues at 8, 14 and 21 days (in vitro model).

RESULTS

Grafted three-dimensional cultures formed a stratified epithelial structure similar to skin in vivo. Epithelial cells were sequenced in basal-layer-like structures at 14 days in vivo. Immunohistochemical findings showed that the expression of cytokeratin was detected in the epithelial layer in in vitro and in vivo models. Ck8 + 18 + 19 was expressed in the upper epithelial layer in the in vitro model at 14 and 21 days, but not in vivo. Involucrin was expressed in the certified layers in vitro at 14 days, but not in vivo. Laminin was detected at the dermo-epidermal junction in vivo at 7 and 14 days, but not in vitro.

CONCLUSION

These results suggest that differentiation of three-dimensional culture tissues differs in vivo and in vitro.

Epithelial and PGP9.5-immunoreactive cells of Malassez epithelium in the periodontal ligament of cats: a transmission electron microscopic study

OBJECTIVE

The aim of the study was to investigate the ultrastructural features of Malassez epithelium (ME) containing protein gene product 9.5 (PGP9.5)-immunoreactive (IR) cells in the cat periodontal ligament (PDL).

MATERIAL AND METHODS

Specimens from the teeth and tooth-supporting tissues of four mature cats of both sexes, 18 to 24 months of age, were used. The fixed jaws were decalcified in EDTA. Frozen sagittal sections 20 µm thick were immunostained for PGP9.5, and the ME, containing IR cells in the PDL, were evaluated under a transmission electron microscope.

RESULTS

Several epithelial cells and PGP9.5 IR cells formed clusters and were enveloped by a basal lamina and separated from the surrounding connective tissue. A large nucleus and scanty cytoplasm were observed in most of the ME cells, which contained abundant keratin filaments and mitochondria. Caveolae-like structures and vesicles were found in the periphery of the ME. The small cytoplasmic processes of some of the epithelial cells extended toward the surrounding connective tissues. The cytoplasmic matrix of one type of cell comprising the ME showed immunoreactivity for anti-PGP9.5 antibody. The IR cell in the cell clusters was connected to adjacent epithelial cells and extended cytoplasmic processes toward the adjacent epithelial cells. The IR cell contained keratin filaments and abundant densely cored vesicles approximately 100-250 nm in diameter.

CONCLUSIONS

The findings of the study suggest endocytotic capabilities of the epithelial cells and neuroendocrine functions of the IR cells. It is possible that the two different cell types react to extrinsic stimuli and interact with cells comprising the clusters and cords in the PDL. These ultrastructural evidences may imply functional heterogeneity of the ME in the PDL.

Epithelial cell rests of Malassez modulate cell proliferation, differentiation and apoptosis via gap junctional communication under mechanical stretching in vitro

Epithelial cell rests of Malassez (ERM) are involved in the maintenance and homeostasis of the periodontal ligament. The objective of this study was to investigate the effect of mechanical stretching on cell growth, cell death and differentiation in the ERM. Cultured porcine ERM were stretched for 24 hr in cycles of 18% elongation for 1 sec followed by 1 sec relaxation. The numbers of cells and TUNEL-positive cells were then counted. The expression of mRNAs encoding gap junction protein α1 (Gja1), ameloblastin, bone morphogenetic protein 2 (BMP2), bone morphogenetic protein 4 (BMP4) and noggin were evaluated using quantitative real-time PCR. The number of cells in the stretching group was approximately 1.3-fold higher than that in the non-stretching controls at 24 hr (p<0.01). Apoptotic cells ranged from 1.9-2.5% in the stretching group at 24 hr, but were only 0.6% in the control group (p<0.01). The expression of Gja1, ameloblastin and noggin mRNAs in the stretching group was decreased at 24 hr compared with in the non-stretching group (p<0.01), whereas the expression of BMP2 and BMP4 mRNAs in the stretching group was significantly higher than that in the control group (p<0.01). Incorporation of 18 α-glycyrrhetinic acid (18GA, a gap junction inhibitor) promoted proliferation and apoptosis and confirmed both the increase of BMP2 and BMP4 and the decline of Gja1, ameloblastin and noggin in ERM. Thus, the ERM modulate cell proliferation and apoptosis, and inhibit differentiation by reducing expression of Gja1 under mechanical stretching.

Effect of stretching force on the cells of epithelial rests of malassez in vitro

Background and Objective. The aim of this study was to investigate the behavior of cells from epithelial rest of Malassez (ERM) against stretching force. Material and Methods. ERM-cultured cells were stretched for 1 hour, at the cycle of 18% elongation for 1 second followed by 1-second relaxation. The cells without addition of stretching force were used as controls. The cells were observed by immunohistochmical staining using actin 0, 12, 24, 36, 48, and 72 hours. Furthermore, expressions of HSP70-, VEGF-, and OPN-mRNAs of cells were also evaluated using quantitative RT-PCR. Results. Actin filaments were randomly orientated in the cytoplasm in the control group, whereas in the stretching group, actin filaments were orientated comparatively parallel to the stretching direction. Expression of HSP70-mRNA in the stretching group was significantly higher than that of control group at 12, 24, 36 hours (P < .05). Expression of VEGF-mRNA in the stretching group was significantly higher than that of control group at 24, 36, 48, and 72 hours (P < .05). Expression of OPN-mRNA in the stretching group was significantly higher than that of control group at 12 and 24 hours (P < .05). Conclusion. ERM cells response against the stretching force by expressing HSP70, VEGF, and OPN.

Effects of epidermal growth factor and/or nerve growth factor on Malassez's epithelial rest cells in vitro: expression of mRNA for osteopontin, bone morphogenetic protein 2 and vascular endothelial growth factor

BACKGROUND AND OBJECTIVE

Malassez's epithelial rest (MER) cells are involved in the maintenance and homeostasis of the periodontal ligament (PDL). The purpose of this study was to determine the effects of epidermal growth factor (EGF) and/or nerve growth factor (NGF) in vitro on these functions of MER cells.

MATERIAL AND METHODS

MER cells from porcine PDL were incubated for 3 or 9 h after the addition of EGF and/or NGF to final concentrations of 10 ng/mL. Cells cultured without those growth factors were used as controls. The expression of mRNA for osteopontin, bone morphogenetic protein 2 (BMP-2) and vascular endothelial growth factor (VEGF) was analyzed using quantitative RT-PCR.

RESULTS

There was a decrease in the expression of osteopontin mRNA by MER cells treated for 9 h with NGF and the level of mRNA expressed was lower than that of the control and EGF-treated groups. The expression of BMP-2 mRNA by MER cells treated with NGF for 9 h also decreased, and was lower than that of the control and EGF-treated groups. The expression of VEGF mRNA by MER cells treated with EGF for 3 or 9 h was higher than in the control and NGF-treated groups. The expression of VEGF mRNA was lower in MER cells treated with NGF for 3 and 9 h than in the control and EGF-treated groups, and decreased from 3 to 9 h of treatment. EGF stimulated MER cells to secrete VEGF, which suggests that EGF plays an important role in maintaining the homeostasis of the PDL. NGF acts on MER cells to inhibit calcification in the PDL. Furthermore, in the EGF+NGF-treated MER cells, expression of mRNA for BMP-2 and VEGF was similar to that of the NGF-treated group, but cell proliferation and expression of osteopontin mRNA were similar to that of the EGF-treated group.

CONCLUSION

EGF and NGF play important roles in maintaining the PDL.

The epithelial cell rests of Malassez--a role in periodontal regeneration?

This article reviews general aspects about the epithelial cell rests of Malassez (ERM). The historical and general morphological features of the ERM are briefly described. The embryological derivation of the ERM is presented as an important consideration in understanding the events associated with their origin and possible functional roles within the periodontal ligament. The ultrastructural description of the ERM is also included to complement the morphological characteristics which distinguish these cells as the unique epithelial element of the periodontal ligament. The unique ability of these cells to synthesize and secrete a number of proteins usually associated with cells of mesenchymal origin, rather than ectodermal origin, is discussed in light of their role in cementum repair and regeneration. Such considerations lead to our hypothesis that one of the functional roles of the ERM may lie not only their role in maintaining and contributing to the normal periodontal cellular elements and function but also contributing, in a significant manner, to periodontal regeneration.

Morphology of Malassez's epithelial rest-like cells in the cementum: transmission electron microscopy, immunohistochemical, and TdT-mediated dUTP-biotin nick end labeling studies

BACKGROUND AND OBJECTIVE

It is known that epithelial islands are embedded in the cementum during tooth root formation, but details of this process remain unknown. The purpose of this study was to investigate the dynamic characteristics of Malassez's epithelial rest cells in the cementum during tooth root formation in pigs in vivo.

MATERIAL AND METHODS

The first molars of 6-mo-old pigs were used in this study. Specimens were decalcified before being embedded in paraffin. Paraffin sections were investigated using TdT-mediated dUTP-biotin nick end labeling (TUNEL), immunohistochemical, and ultrastructural techniques.

RESULTS

Malassez's epithelial rest cells were located close to the root surface at the apical one-third of the periodontal ligament, and epithelial clusters surrounded by distinct lamina cementia were sometimes observed in the cementum. TUNEL-positive cells were detected only in the cementum. Malassez's epithelial rest cells in the periodontal ligament were completely surrounded by basement membranes, but epithelial clusters in the cementum were only intermittently surrounded by such membranes. Cytokeratin-positive cells in the superstratum of the cementum were directly connected by cementocytes and by desmosome-like structures. However, organelles were scarce in the cytokeratin-positive cells in the substratum of the cementum, and the matrix of the cementum was deposited in the cells.

CONCLUSION

These results suggest that the majority of the fragmented Hertwig's root sheath remains in the periodontal ligament and that some cells, which are connected to cementoblasts, are embedded in the cementum and progress to apoptosis.

Apoptosis in the epithelial cells of the rests of Malassez of the periodontium of rat molars

BACKGROUND AND OBJECTIVES

Epithelial rests of Malassez are clusters of cells derived from Hertwig's root sheath that remain in the periodontal ligament throughout life. Although it is known that the cells of Malassez proliferate, there are no studies showing that they undergo programmed cell death, i.e. apoptosis. In most tissues, proliferation is balanced by apoptosis. Thus we examined regions of the periodontium of young and adult rat molars in the hope of detecting apoptosis.

METHODS

Wistar rats aged 29, 45 and 120 days were killed with chloral hydrate (600 mg/kg). Fragments containing maxillary molars were removed and fixed in formaldehyde, decalcified, and embedded in paraffin and glycol methacrylate. Sections were stained with hematoxylin/eosin and the Terminal deoxynucleotidyl transferase-mediated dUTP Nick End Labeling (TUNEL) method for detection of apoptosis. Specimens were also fixed in glutaraldehyde-formaldehyde, decalcified and processed for transmission electron microscopy.

RESULTS

Epithelial rests of Malassez containing round/ovoid basophilic dense bodies and TUNEL-positive structures were found in all specimens examined. Ultrastructural examination revealed that some cells of Malassez contained masses of condensed peripheral chromatin and a shrunken cytoplasm exhibiting intact organelles--images typical of apoptosis. Moreover, round/ovoid electron-opaque structures appeared to be in the process of being engulfed by neighboring epithelial cells of Malassez.

CONCLUSIONS

Our results demonstrate that epithelial cells of Malassez's rests undergo apoptosis in the developing and adult periodontium. Apoptosis may, together with proliferation, be part of the mechanism of turnover/remodelling of the cells of Malassez.

Production of osteopontin by cultured porcine epithelial cell rests of Malassez

BACKGROUND

The epithelial cell rests of Malassez (ERM) are derived from Hertwig's epithelial root sheath (HERS). During development the cells of HERS deposit a variety of molecules on the newly forming root surface. The possibility that ERM retain this potential after root development is completed and secrete bone or cementum-related proteins needs to be investigated. The purpose of this study was to determine the expression of the non-collagenous proteins osteopontin (OPN) and bone sialoprotein (BSP) by cells derived from the epithelial cell rests of porcine periodontium.

METHODS

ERM and fibroblasts were cultured from porcine periodontal ligament. The cells were identified and characterized using transmission electron microscopy, immunohistochemistry, western blot analysis of proteins, reverse transcription-polymerase chain reaction and ability to form mineralized nodules in culture. In particular the expression of the mineralized tissue-related proteins, BSP and OPN, was studied.

RESULTS

Cells from porcine periodontal ligaments were successfully cultured; separated and characterized as being of either an epithelial or fibroblastic phenotype. Although the ERM did not form mineralized nodules in culture, they did express a significant amount of mRNA for OPN.

CONCLUSION

The results from this study provide evidence that ERM express mRNA for at least one bone/cementum-related protein. Whether this function would be consistent with a role for ERM in tissue formation, inflammation and regeneration remains to be established.

Expression of type IV collagen and laminin at the interface between epithelial cells and fibroblasts from human periodontal ligament

The present study was undertaken to examine whether synthesis of type IV collagen and laminin around the epithelial rests of Malassez (ERM) requires direct contact between cells from ERM and periodontal ligament fibroblasts. Human periodontal ligament (HPDL) explants produced outgrowths containing both ERM cells and fibroblasts when cultured in a modified serum-free medium. The interface between ERM cells and fibroblasts was examined using phase-contrast microscopy (PCM) and scanning electron microscopy (SEM). Expression of type IV collagen and laminin was studied by immunohistochemistry and in situ hybridization. It was observed that ERM cells grew underneath fibroblasts or attached to them. At the interface, type IV collagen and laminin and their respective mRNAs were abundant in both ERM cells and fibroblasts, while these proteins and mRNAs showed little if any staining in cells further away from the interface. Hence, these findings indicate that synthesis of type IV collagen and laminin is induced by direct interaction between ERM cells and periodontal ligament fibroblasts.

NO-cGMP signaling molecules in the rat epithelial rests of Malassez

The epithelial rests of Malassez (ERM) are derived from the disintegrating epithelial root sheath of Hertwig that guides root formation during tooth development. Low concentrations of nitric oxide (NO) produced by NO-synthase I (NOS I) and NOS III activate intracellular soluble guanylate cyclase (sGC) to produce intracellular cyclic guanosine 3':5'-monophosphate (cGMP), which triggers rapid cellular responses such as cell proliferation, cell differentiation, and apoptosis under physiological conditions. The presence of NOS I-III, sGC (alpha2- and beta1-subunits) and cGMP in the ERM was investigated by immunohistochemistry. Rat molars with periodontium were perfusion and postfixed, decalcified, frozen-sectioned, and sections were immunostained. NOS I, NOS III, sGC (alpha2- and beta1-subunits) and cGMP were localized with different densities in the ERM. The presence of NOS II in the ERM varied. The localization of NOS I, NOS III, sGC and cGMP in the ERM indicates an involvement of NO and/or NO-cGMP signal pathway molecules in homeostasis of a variety of physiological processes in the ERM. These could include regulation of cell proliferation, cell differentiation and apoptosis.

Merkel-like cells in Malassez epithelium in the periodontal ligament of cats: an immunohistochemical, confocal-laser scanning and immuno electron-microscopic investigation

The cellular heterogeneity of Malassez epithelium (ME) residing in the periodontal ligament has recently been reported, and the presence and coexistence of the neuropeptides calcitonin gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP) in single cells in ME has been shown (1). However, the identity of these neuroendocrine cells has so far not been verified. This study was undertaken in order to elucidate the identity of the neuroendocrine cells in ME by means of transmission electron microscopy, confocal scanning microscopy and immunohistochemistry using antibodies to protein gene product (PGP) 9.5 and cytokeratin 20 (CK). Gingival tissue was included in the study as a positive control for identification of Merkel-like cells in oral epithelium. CK 20 immunopositive cells were present in both Malassez epithelium and in basal cell layers of gingival epithelium showing a distribution consistent with PGP 9.5 labelled cells in both epithelia. The results from PGP 9.5 immuno electron microscopy clearly evidenced the presence of single, intensely labelled cells and some nerve fibres invested between the Malassez epithelial cells. The conformity of the immunopositive cells in Malassez and gingival epithelium verified by double immunolabelling with PGP 9.5 and CK 20, indicates that the labelled neuroendocrine cells are identical in ME and in gingival epithelium. This demonstrates that Malassez epithelium not only exhibits neuroendocrine cells, but additionally that the neuroendocrine cells represent Merkel-like cells.

Neuroendocrine cells in Malassez epithelium and gingiva of the cat

Malassez epithelium has been designated as epithelial cell rests, the biological significance of which is still under debate. This study was designed to analyze Malassez epithelium for the presence of neuroendocrine cells. Gingival tissue was included as a positive control. Using immunohistochemistry, confocal and light microscopy, Malassez epithelium and gingival epithelium from mature cats (n = 5) were examined for cells containing the neuropeptides calcitonin gene-related peptide (CGRP), substance P (SP), and vasoactive intestinal peptide (VIP). Both Malassez epithelium and the basal epithelial cell layers in gingival rete pegs regularly displayed cells immunoreactive to CGRP, SP, and VIP. The immunopositive cells were most frequently present in the epithelial cell clusters and strands of Malassez located in the cervical half of thc periodontal ligament. Double immunolabeling revealed cellular co-expression of CGRP or SP with VIP, and the neuropeptides were co-localized in the cellular compartments. Labeled cells in both epithelia were occasionally supported by immunoreactive nerve fibers. This study shows that cells immunoreactive to CGRP, SP, and VIP arc located within the cat Malassez epithelium. The localization of neuroendocrine cells verifies the diversity of this epithelium and confirms that Malassez epithelium is composed of different cell types, in common with epithelia from other locations. The presence of neuroendocrine cells in Malassez epithelium strongly suggests biological functions of this tissue, and the neuropeptide content may thus indicate endocrine functions of the cells.

Immunofluorescence detection of cadherins in mouse tooth germs during root development

The distribution of two cell-adhesion molecules, E- and P-cadherin, was studied in relation to morphological changes in Hertwig's epithelial root sheath Before root dentinogenesis had started, the root sheath expressed both cadherins. As dentinogenesis proceeded, the sheath fragmented and lost P-cadherin rapidly and E-cadherin slowly, whereas the intact sheath at the apical end continued to express both. These results suggest that the two cadherins play a part in root as well as in crown development, and indicate that the decrease in the amount of these molecules and the fragmentation of the epithelial root sheath are interrelated.

Morphological changes of epithelial rests of Malassez in rat molars induced by local administration of N-methylnitrosourea

The objective was to examine the possibility that epithelial rests of Malassez can give origin to odontogenic tumours. A mixture of N-methylnitrosourea (MNU) and alginate impression material for dental use was injected onto the periosteum of the buccal side of the left mandible of 5-week-old, male Wistar rats (300 mg/kg body wt). The mixture was left at the site for several months. The rats were killed 1, 3, 5, and 8 months after the injection. After 5 and 8 months, the epithelial rests of Malassez in the cervical and bifurcational regions of the first, second, and third left mandibular molars were significantly enlarged and the alveolar bone around the lesion was resorbed by multinucleated cells in all rats. The epithelial masses were characterized by enamel organ-like structures, deposition of eosinophilic amorphous material, duct-like structures, and squamous metaplasia. In addition to these masses in the molar regions, odontogenic tumours were induced in the incisal region and carcinomas and sarcomas in the buccal region, knee, bladder, and skin. Local administration of a mixture of MNU and alginate impression material can induce odontogenic tumours from the epithelial rests of Malassez at high incidence.

The structure of bovine periodontal ligament with special reference to the epithelial cell rests

The aim of this study was to determine the structure of the bovine periodontal ligament, with special reference to epithelial cell rests (ECR) and their cytokeratin content. Periodontal ligament was obtained from bovine molar teeth and studied at both the light microscopic and electron microscopic levels. Cytokeratin content was determined using immunohistochemistry against a number of cytokeratin antibodies and specificity tested against bovine and human oral mucosa. Collagen fibril diameters and the area of a fiber bundle occupied by collagen were determined using a digital planimeter with a digitizing tablet. The majority of periodontal fibroblasts possessed considerable quantities of roughened endoplasmic reticulum, indicating rapid synthesis and secretion of collagen, but no intracellular collagen profiles were present. Endothelial cells showed Weibel-Palade bodies. Collagen fibril diameters showed a unimodal distribution with a mean collagen fibril diameter of 55.3 nm. The mean percentage area of the extracellular matrix occupied by collagen was 42%. Structurally, ECR were unusual in exhibiting large numbers of microvilli and conspicuous amounts of cytokeratin filaments. Bovine ECR showed a positive reaction to the pancytokeratin MNF116 (which reacts with the cytokeratins 5, 6, 8, 17, and probably 19), to PCK-26 (which reacts with the type II cytokeratins 1, 5, 6, and 8) and to cytokeratin 13. There was no reaction for cytokeratins 1, 4, 10, 11 and 18. Structurally, bovine periodontal ligament showed features common to other species. However, ECR in terms of both structure and cytokeratin content showed features indicative of important species differences which may have relevance when considering the etiology of radicular cysts.

Production of amelogenin by enamel epithelium of Hertwig's root sheath

OBJECTIVE

To study the multiple potentials of differentiating odontogenic epithelial cells.

STUDY DESIGN

Bilateral first and second maxillary molars of 30 immature rats were perforated into the pulp chambers with a round bur. The pulps were observed histologically and immunohistochemically for amelogenin 3, 7, and 14 days after the perforation.

RESULTS

On day 7, the enamel epithelium of Hertwig's root sheath migrated and formed oval-shaped epithelial islands that resembled epithelial rests of Malassez. On day 14, the islands consisted of epithelial cells with large nuclei and cytoplasm partially surrounded by a subsequently formed osteodentin and cementum. Immunoreactivity for amelogenin was observed in the large epithelial cells and in the area between the cells as well as the calcified tissues. Some of the enamel epithelium resembling columnar ameloblasts of the enamel organ were also positive for amelogenin.

CONCLUSIONS

The enamel epithelium of Hertwig's root sheath appeared to differentiate into ameloblasts and produce amelogenin.

The role of epithelium in the development, structure and function of the tissues of tooth support

The roles of epithelium in the development, structure and function of the tissues of tooth support are reviewed. Epithelium is involved in initiating odontogenesis which includes the tissues of tooth support and this role is discussed. Particular attention is paid to Hertwig's epithelial root sheath and its participation in the formation of the hyaline layer on the root surface as well as its possible role in initiating the differentiation of cementoblasts. The possible functions of the epithelial cell rests are reviewed and it is concluded that as yet no function can be ascribed to them. Evidence for an increasing role for dental epithelium in tooth eruption is presented and the role of dental epithelium in establishing the dentogingival junction is discussed, with the conclusion drawn that this role temporary.

Maintenance of cell-type-specific cytoskeletal character in epithelial cells out of epithelial context: cytokeratins and other cytoskeletal proteins in the rests of Malassez of the periodontal ligament

We have determined the patterns of synthesis of cytokeratins and other epithelial marker proteins in the "rests of Malassez" of the periodontium of rabbits and humans, by immunofluorescence microscopy of cryosections prepared from fixed and decalcified rabbit teeth with attached ligament or from manually isolated human periodontal ligaments. Proteins of the major cell structures characterizing epithelial differentiation are present in Malassez cells: a complex set of cytokeratins as well as desmosomal, hemidesmosomal and basal lamina proteins. In addition, we have shown these cytoskeletal and extracellular matrix structures by electron microscopy. The cytokeratin complement of Malassez cells was found to be highly complex, as 8 of the total of 20 known epithelial cytokeratins were detected (nos. 5, 7, 8, 14, 15, 17, 18, 19). This pattern, together with the presence of the desmosomal cadherins Dsg2 and Dsc2 and the cytoplasmic desmosome plaque-associated protein plakophilin 1, indicates that the cells of the rests of Malassez are derived from the basal cell layer of a stratified squamous epithelium rather than from simple epithelial or neuroendocrine epithelial cells. Our observations show that Malassez cells retain the major characteristics of epithelial cells throughout their differentiation from the root sheath epithelium into the rests of Malassez, even though the surface location and the polar tissue architecture that typify epithelial are lost during this process. From this study we further conclude that the specific cytoskeletal complement of the Malassez cells represents an intrinsic gene expression program that neither depends on nor causes the formation of a stratified epithelium. We also compare the specific cytoskeletal features of Malassez cells with those of other persisting epithelial residues and discuss the potential value of these findings in relation to the histogenesis and diagnostic classification of dental and periodontal cysts and tumors.