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

Strategies and Challenges in the Treatment of Dental Enamel

Enamel tissue, the hardest body tissue, which covers the outside of the tooth shields the living tissue, but it erodes and disintegrates in the acidic environment of the oral cavity. On the one hand, mature enamel is cell-free and, if damaged, does not regenerate. Tooth sensitivity and decay are caused by enamel loss. On the other hand, the tissue engineering approach is challenging because of the unique structure of tooth enamel. To develop an exemplary method for dental enamel rebuilding, accurate knowledge of the structure of tooth enamel, knowing how it is created and how proteins interact in its structure, is critical. Furthermore, novel techniques in tissue engineering for using stem cells to develop enamel must be established. This article aims to discuss current attempts to regenerate enamel using synthetic materials methods, recent advances in enamel tissue engineering, and the prospects of enamel biomimetics to find unique insights into future possibilities for repairing enamel tissue, perhaps the most fascinating of all tooth tissues.

Induction of maxillary and mandibular squamous epithelial cell proliferation in mink (Neovison vison) by β-naphthoflavone

A jaw lesion reported in mink exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and TCDD-like chemicals is considered a potential indicator of exposure to these chemicals. Many of the effects of TCDD-like chemicals are induced through interaction with the aryl hydrocarbon receptor. The present study indicates that mink dosed with β-naphthoflavone, which is an aryl hydrocarbon receptor ligand but not a TCDD-like chemical, also develop the lesion. Environ Toxicol Chem 2019;38:460-463. © 2018 SETAC.

Epithelial rests of Malassez: from latent cells to active participation in orthodontic movement

Introduction:

The epithelial rests of Malassez (ERM) represent a group of cells in the periodontal ligament classically consisting of latent or quiescent structures associated with pathological processes. However, recent evidence shows that these structures cannot be considered only as cellular debris. The ERM is a major tissue structure, with functions in maintaining the homeostasis of periodontal tissue, including the maintenance of orthodontic movement.

Objective:

The present literature review aims at presenting the potential functions of ERM, with emphasis on orthodontic movement and the functional structure of the periodontium.

Conclusion:

ERM cells have a functional activity in modulation of orthodontic movement, trough their potential for differentiation, maintenance functions and the capacity of repairing periodontium.

N-Methyl-N-nitrosourea as a mammary carcinogenic agent

The administration of chemical carcinogens is one of the most commonly used methods to induce tumors in several organs in laboratory animals in order to study oncologic diseases of humans. The carcinogen agent N-methyl-N-nitrosourea (MNU) is the oldest member of the nitroso compounds that has the ability to alkylate DNA. MNU is classified as a complete, potent, and direct alkylating compound. Depending on the animals' species and strain, dose, route, and age at the administration, MNU may induce tumors' development in several organs. The aim of this manuscript was to review MNU as a carcinogenic agent, taking into account that this carcinogen agent has been frequently used in experimental protocols to study the carcinogenesis in several tissues, namely breast, ovary, uterus, prostate, liver, spleen, kidney, stomach, small intestine, colon, hematopoietic system, lung, skin, retina, and urinary bladder. In this paper, we also reviewed the experimental conditions to the chemical induction of tumors in different organs with this carcinogen agent, with a special emphasis in the mammary carcinogenesis.

Cellular turnover in epithelial rests of Malassez in the periodontal ligament of the mouse molar

Fragments of Hertwig's epithelial root sheath persist in the periodontal ligament (PDL) in small clusters known as epithelial rests of Malassez (ERM). It is generally agreed that ERM are maintained as a quiescent and exclusively dental epithelial cluster in PDL. However, we speculate that homeostasis and cellular turnover underlies cluster maintenance. We also hypothesize that the fate of ERM clusters - diminishing or remaining - might be regulated via the presence or absence of epithelial stem cells therein. Histological analysis of aging mouse molar PDL showed that ERM clusters gradually increase in size with increasing age. Immunocytochemistry and cell culture revealed that ERM clusters contained Ki67-positive cells and were able to expand when brought in culture. The TdT-mediated biotin-dUTP nick-end labeling (TUNEL) procedure also detected signs of apoptosis. Finally, we identified putative epithelial stem cells in the clusters by 5-bromo-2'-deoxyuridine (BrdU) pulse-chase experiments and immunohistochemistry, using the stem-cell marker leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5). The results suggest that ERM clusters are maintained in the PDL, via cellular turnover, throughout life.

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.

N-methyl-N-nitrosourea-induced changes in epithelial rests of Malassez and the development of odontomas in rats

Morphological changes in the epithelial rests of Malassez (ERM) and the development of odontogenic tumors in the molars of female Lewis rats treated at 4 weeks of age with a single intraperitoneal injection of 50 mg/kg of N-methyl-N-nitrosourea (MNU) were examined at 12, 18 and 30 weeks of age. Following MNU exposure, the total number and average area of ERM in the cervical and furcational regions of the first, second and third molars of the mandible and maxilla were compared with age-matched control animals. The number of ERM at each time point was significantly greater in the MNU-treated group compared to the control group, but there was no time-dependent increase in the number of ERM in either group. The area of ERM was significantly larger in the MNU-treated group compared to the control group at each time point, and it increased in a time-dependent manner in the MNU-treated group. No increases in the number or area of ERM were observed in the control group. At 30 weeks of age, 23% of the MNU-treated rats had developed odontomas (complex type) in the molar region as well as in the incisor region. Immunohistochemically, the expression of tyrosine receptor kinase A (TrkA) and cytokeratin 14 (CK14) decreased, whereas p63 expression remained high during ERM enlargement. In tumors, ameloblast-like cells were positive for amelogenin, TrkA and CK14 but negative for p63, whereas odontoblast-like cells were negative for all antigens examined. In conclusion, a single intraperitoneal injection of MNU caused the development of odontomas in the molar region; these tumors were possibly derived from ERM.

Reactions of periodontal ligament epithelial cell clusters and OX6-immunopositive cells to experimental tooth movement and periodontitis

BACKGROUND AND OBJECTIVE

The aim of this study was to investigate reactions of periodontal ligament epithelial cell clusters and major histocompatibility complex class II (OX6)-immunopositive cells to simultaneously induced tooth movement and periodontitis employing Waldo's method.

MATERIAL AND METHODS

Elastic gums were inserted between the right upper first and second molars of rats. Animals were killed by intracardiac perfusion on days 1, 3, 7 and 14 after the experimental procedures, and maxillary molars were decalcified and processed for OCT compound. Cytokeratin and OX6 antibodies to detect epithelial and immunocompetent cells were used for double-fluorescence immunohistochemistry. Immunostained sections of rat upper molar regions were examined with a fluorescence microscope.

RESULTS

Large periodontal ligament epithelial cell clusters appeared and became contiguous with each other, and OX6-immunopositive cells surrounded the clusters over time in the periodontal ligament near the gum insertion site. In the periodontal ligament distant from the gum insertion site, epithelial cell clusters and OX6-immunopositive cells were scattered. After 14 d, thickened epithelium and elongated rete pegs were found close to large epithelial cell clusters in the periodontal ligament near the gum insertion site.

CONCLUSION

These findings suggest proliferation and/or aggregation of periodontal ligament epithelial cells, and interaction between OX6-immunopositive cells and the periodontal ligament epithelial cells, in response to tooth movement and periodontal inflammation. This method may be a useful experimental model to elucidate the relationship between rete pegs and periodontal ligament epithelial cell clusters in inflammatory conditions.

The Ca(2+)-binding protein calretinin is selectively enriched in a subpopulation of the epithelial rests of Malassez

During tooth development, the inner and outer enamel epithelia fuse by mitotic activity to produce a bilayered epithelial sheath termed Hertwig's epithelial root sheath (HERS). The epithelial rests of Malassez (ERM) are the developmental residues of HERS and remain in the adult periodontal ligament (PDL). Although the cellular regulation of the Ca(2+)-binding proteins parvalbumin, calbindin-D28k, and calretinin has been reported in the inner and outer enamel epithelia during tooth development, an involvement of Ca(2+)-binding proteins in the ERM has not so far been characterized. Among the three Ca(2+)-binding proteins tested (calbindin D28k, parvalbumin, calretinin), we have only been able to detect calretinin in a subpopulation of adult rat molar ERM, by using quantitative immunohistochemical and confocal immunofluorescence techniques. TrkA (a marker for ERM) is present in numerous epithelial cell clusters, whereas calretinin has been localized in the cytosol and perinuclear region of a subpopulation of TrkA-positive cells. We conclude that, in inner and outer enamel epithelial cells, Ca(2+) is regulated by calbindin, parvalbumin, and calretinin during tooth development, whereas in the ERM of adult PDL, Ca(2+) is regulated only by calretinin. The expression of Ca(2+)-binding proteins is restricted in a developmental manner in the ERM.

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.

WNT5A expression in ameloblastoma and its roles in regulating enamel epithelium tumorigenic behaviors

Odontogenic tumors originate from the remains of migrating enamel epithelium after the completion of normal tooth genesis. These enamel epithelium remnants exhibit the ability to recapitulate the events that occur during tooth formation. Several lines of evidence suggest that aberrance in the signaling pathways similar to the ones that are used during tooth development, including the WNT pathway, might be the cause of odontogenic tumorigenesis and maintenance. In this study we demonstrated that WNT5A expression was intense in both the epithelial component of ameloblastomas, the most common epithelial odontogenic tumor, and in this tumor's likely precursor cell, the enamel epithelium located at the cervical loop of normal developing human tooth buds. Additionally, when WNT5A was overexpressed in enamel epithelium cells (LS-8), the clones expressing high levels of WNT5A (S) exhibited characteristics of tumorigenic cells, including growth factor independence, loss of anchorage dependence, loss of contact inhibition, and tumor formation in immunocompromised mice. Moreover, overexpression of WNT5A drastically increased LS-8 cell migration and actin reorganization when compared with controls. Suppression of endogenous WNT5A in LS-8 cells (AS) greatly impaired their migration and AS cells failed to form significant actin reorganization and membrane protrusion was rarely seen. Taken together, our data indicate that WNT5A signaling is important in modulating tumorigenic behaviors of enamel epithelium cells in ameloblastomas.

Distribution of epithelial cells and their relationship to immunocompetent cells in rat molars: a confocal and transmission electron microscope study

This study sought to investigate the distribution of cytokeratin (CK)-immunopositive cells and their relationship to immunocompetent ED1- and OX6-immunopositive cells in rat periodontium by immunohistochemistry and electron microscopy. CK-immunopositive cells were generally distributed along the surface of the tooth root. They could also be found between root dentin and cementum, in the perivascular space, and close to or in the alveolar bone lacunae. ED1-immunopositive cells exhibited a compact shape with small processes and were widely distributed in the periodontium. Few sections demonstrated an intimate relationship between the CK- and ED1-immunopositive cells close to the cementum, in the perivascular space, and close to or in the alveolar bone. Numerous OX6-immunopositive cells with long branching processes were widely distributed in the periodontal ligament, surrounding and holding CK-immunopositive cells in the cell clusters, close to the cementum. Transmission electron microscopy revealed OX6-immunopositive cells that extended their cytoplasmic processes, which contained vesicles and occasionally lysosomes in between the epithelial cells. This study demonstrates the close relationship between the epithelial cells and the immunocompetent cells in a rat periodontium, indicating a functional interrelationship. It is possible that in a non-inflammatory periodontium, the epithelial cells act not independently, but through interaction with immunocompetent cells.

Quiescent epithelial cell rests of Malassez can differentiate into ameloblast-like cells

Epithelial cell rests of Malassez (ERM) are quiescent epithelial remnants of Hertwig's epithelial root sheath (HERS) that are involved in the formation of tooth roots. After completion of crown formation, HERS are converted from cervical loop cells, which have the potential to generate enamel for tooth crown formation. Cervical loop cells have the potential to differentiate into ameloblasts. Generally, no new ameloblasts can be generated from HERS, however this study demonstrated that subcultured ERM can differentiate into ameloblast-like cells and generate enamel-like tissues in combination with dental pulp cells at the crown formation stage. Porcine ERM were obtained from periodontal ligament tissue by explant culture and were subcultured with non-serum medium. Thereafter, subcultured ERM were expanded on 3T3-J2 feeder cell layers until the tenth passage. The in vitro mRNA expression pattern of the subcultured ERM after four passages was found to be different from that of enamel organ epithelial cells and oral gingival epithelial cells after the fourth passage using the same expansion technique. When subcultured ERM were combined with subcultured dental pulp cells, ERM expressed cytokeratin14 and amelogenin proteins in vitro. In addition, subcultured ERM combined with primary dental pulp cells seeded onto scaffolds showed enamel-like tissues at 8 weeks post-transplantation. Moreover, positive staining for amelogenin was observed in the enamel-like tissues, indicating the presence of well-developed ameloblasts in the implants. These results suggest that ERM can differentiate into ameloblast-like cells.

Epidermal growth factor receptor distribution in pericoronal follicles: relationship with the origin of odontogenic cysts and tumors

OBJECTIVE

Investigate the distribution of epidermal growth factor receptor (EGFR) in pericoronal follicles as a predictor of progression to odontogenic cysts and tumors.

STUDY DESIGN

Immunohistochemical EGFR staining patterns (membrane-only, cytoplasm-only, or combined membrane and cytoplasmic staining) in the reduced enamel epithelium and nests of odontogenic epithelium associated with follicles of impacted molar teeth were evaluated. The staining pattern of 20 specimens of pericoronal follicle was compared with that of 16 normal oral mucosa samples and to squamous cell carcinoma samples.

RESULTS

Combined membrane and cytoplasmic staining was observed for normal oral mucosa mostly in proliferating layers (basal and suprabasal), decreasing in intensity toward the surface. Seven epithelial nests presented membrane-only staining, and the majority presented either a cytoplasm-only or a combined staining pattern. The staining patterns observed in reduced enamel epithelium were cytoplasm-only and combined.

CONCLUSION

EGFR membrane-only staining may be an indicator of increased potential for epithelial nests to become odontogenic cysts or tumors.

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.

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.

Periodontal regeneration of transplanted rat molars after cryopreservation

The effects of cryopreservation on periodontal regeneration of transplanted rat molars were investigated histologically and histochemically in rats. Bilateral first and second maxillary molars of 4-week-old Wistar rats were gently extracted and transplanted into the abdominal subcutaneous connective tissue immediately or after cryopreservation in liquid nitrogen overnight. Donor teeth were slowly frozen by a rate-controlling freezer (program freezer) using 5% dimethylsulfoxide (DMSO) and 6% hydroxyethyl starch (HES) as cryoprotectants. One-four weeks after transplantation, they were carefully excised with the surrounding tissues. Regeneration of acellular cementum, periodontal ligament, and alveolar bone were observed 2 weeks after immediate transplantation. The pulp was repaired by the ingrowth of granulation tissue from the root apex followed by the formation of calcified tissue. The regenerated periodontal ligament was positive for alkaline phosphatase (ALP). Small or mononuclear tartrate resistant acid phosphatase (TRAP) positive cells were scattered on the newly formed alveolar bone and on the hard tissue in the pulp, but there was no external or internal progressive root resorption at 4 weeks. Cryopreserved teeth had acellular cementum with a rough surface at 1 week, but with the increase of cementoblasts and the appearance of periodontal ligament and alveolar bone, the surface became smooth at 3 weeks. Epithelial rests of Malassez (ERM) also revived. After regeneration of the periodontal tissues at 4 weeks, there was no evidence of root resorption. Although the process proceeded slowly, the cryopreserved teeth showed the periodontal regeneration substantially similar to that of the immediately transplanted teeth without progressive root resorption, indicating that they could be applicable for clinical use.

Proliferation of maxillary and mandibular periodontal squamous cells in mink fed 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

This report characterizes squamous cell proliferation in young farm mink (Mustela vison) fed a diet supplemented with 0.024 ppm 3,3',4,4',5-pentachlorobiphenyl (polychlorinated biphenyl [PCB] congener 126). One to 2 months of dietary exposure to PCB 126 resulted in gross lesions of the upper and lower jaws consisting of mandibular and maxillary nodular proliferation of the gingiva and loose teeth. The maxilla and mandible of the PCB-treated mink were markedly porous because of loss of alveolar bone. Histologically, this osteoporosis was caused by proliferation of squamous cells that formed infiltrating cords. This report clearly documents the fact that the environmental contaminant PCB 126 can cause osteoinvasive squamous proliferation in young mink, although the dose used in the present study was 7 and 36 times higher than what is typically encountered in contaminated bird eggs and fish, respectively.