Migration of fibroblasts in the periodontal ligament of the mouse incisor as revealed by autoradiography

RANKL deletion in periodontal ligament and bone lining cells blocks orthodontic tooth movement

The bone remodeling process in response to orthodontic forces requires the activity of osteoclasts to allow teeth to move in the direction of the force applied. Receptor activator of nuclear factor-κB ligand (RANKL) is essential for this process although its cellular source in response to orthodontic forces has not been determined. Orthodontic tooth movement is considered to be an aseptic inflammatory process that is stimulated by leukocytes including T and B lymphocytes which are presumed to stimulate bone resorption. We determined whether periodontal ligament and bone lining cells were an essential source of RANKL by tamoxifen induced deletion of RANKL in which Cre recombinase was driven by a 3.2 kb reporter element of the Col1α1 gene in experimental mice (Col1α1.CreERTM+.RANKLf/f) and compared results with littermate controls (Col1α1.CreERTM-.RANKLf/f). By examination of Col1α1.CreERTM+.ROSA26 reporter mice we showed tissue specificity of tamoxifen induced Cre recombinase predominantly in the periodontal ligament and bone lining cells. Surprisingly we found that most of the orthodontic tooth movement and formation of osteoclasts was blocked in the experimental mice, which also had a reduced periodontal ligament space. Thus, we demonstrate for the first time that RANKL produced by periodontal ligament and bone lining cells provide the major driving force for tooth movement and osteoclastogenesis in response to orthodontic forces.

Histological evidence for a dynamic dental battery in hadrosaurid dinosaurs

The first histological study of an entire hadrosaurid dental battery provides a comprehensive look at tooth movement within this complex structure. Previous studies have focused on isolated teeth, or in-situ batteries, but this is the first study to examine an entire dental battery of any dinosaur. The absence of direct tooth-to-tooth contact across the entire battery and a unique arrangement of the dental tissues in hadrosaurids led us to compare their teeth with the ever-growing incisors of mammals. The similarity in the distributions of tissues along the incisor, coupled with continuous eruption, make for helpful comparisons to hadrosaurid teeth. The mammalian ever-growing incisor can be used as a model to extrapolate the soft tissue connections and eruptive mechanisms within the hadrosaurid dental battery. Serial sections across the adult dental battery reveal signs of gradual ontogenetic tooth migration. Extensive remodeling of the alveolar septa and the anteroposterior displacement of successive generations of teeth highlight the gradual migration of tooth generations within the battery. These eruptive and ontogenetic tooth movements would not be possible without a ligamentous connection between successive teeth and the jaws, underscoring the dynamic nature of one of the most unique and complex dental systems in vertebrate history.

Mechanoresponsive Properties of the Periodontal Ligament

The periodontal ligament (PDL) functions as an enthesis, a connective tissue attachment that dissipates strains created by mechanical loading. Entheses are mechanoresponsive structures that rapidly adapt to changes in their mechanical loading; here we asked which features of the PDL are sensitive to such in vivo loading. We evaluated the PDL in 4 physiologically relevant mechanical environments, focusing on mitotic activity, cell density, collagen content, osteogenic protein expression, and organization of the tissue. In addition to examining PDLs that supported teeth under masticatory loading and eruptive forces, 2 additional mechanical conditions were created and analyzed: hypoloading and experimental tooth movement. Collectively, these data revealed that the adult PDL is a remarkably quiescent tissue and that only when it is subjected to increased loads—such as those associated with mastication, eruption, and orthodontic tooth movement—does the tissue increase its rate of cell proliferation and collagen production. These data have relevance in clinical scenarios where PDL acclimatization can be exploited to optimize tooth movement.

Osteostimulatory effect of bone grafts on fibroblast cultures

Objective:

We analyzed the morphological changes and alkaline phosphatase (ALP) level in fibroblast, which is indicative of their functional ability when cultured in three different commercially available graft materials with osseoconductive property.

Materials and Methods:

Fibroblasts obtained from fifth passage were seeded within three different bone substitutes (bovine hydroxyapatite [HA] [Osseo-graft^®], β-tricalciumphosphate [RTR^®], bovine HA [Bio-oss^®]) and incubated under standard cell culture conditions. 10 samples in each group were evaluated for cell morphology and alkaline phosphates activity using scanning electron microscopy and spectrophotometric analysis on the 7^th day of culture.

Results:

Fibroblast cultured with RTR^® showed changes in morphology and increase in ALP activity when compared to fibroblast cultured with Osseo-graft^® and Bio-oss^®.

Conclusion:

Alkaline phosphatase activity was observed in fibroblasts when cultured with three types of commercially available bone grafts. ALP activity was highest when cultured with β-tricalcium phosphate graft material indicating its better bone regenerating capacity of this graft material.

Periostin of human periodontal ligament fibroblasts promotes migration of human mesenchymal stem cell through the αvβ3 integrin/FAK/PI3K/Akt pathway

BACKGROUND AND OBJECTIVE

The periodontal ligament (PDL) is characterized by rapid turnover, high remodeling capacity and high inherent regenerative potential compared with other connective tissues. Periostin, which is highly expressed in the fibroblasts in the PDL, has been widely discussed in relation to collagen fibrillogenesis in the PDL. Recently, several reports have indicated periostin in cell migration. The aim of this study was to examine whether human PDL fibroblasts (hPDLFs) with high levels of periostin expression promote the migration of human bone marrow mesenchymal stem cells (hMSCs).

MATERIAL AND METHODS

The migration of hMSCs was examined by transwell chamber migration assay under different conditions: medium alone, hPDLFs, human dermal fibroblasts, recombinant periostin, integrin αvβ3 blocking antibody (anti-CD51/61 antibody) and inhibitors of FAK (PF431396) and PI3K (LY294002). Phosphorylation of FAK and Akt in hMSCs under stimulation of periostin was examined by western blotting.

RESULTS

The migration assay revealed that the number of migrated hMSCs by hPDLFs was significantly larger than those by dermal fibroblasts, periostin small interfering RNA hPDLFs and medium alone. Furthermore, recombinant periostin also strongly induced hMSC migration. The addition of anti-CD51/61 antibody, PF431396 and LY294002 caused a significant reduction in the number of migrated hMSCs respectively. The anti-CD51/61 antibody inhibited both FAK and Akt phosphorylations under periostin stimulation. PF431396 inhibited both FAK and Akt phosphorylations. LY294002 inhibited only Akt phosphorylation, and FAK phosphorylation was not influenced under periostin stimulation.

CONCLUSION

Periostin expression in hPDLFs promotes the migration of hMSCs through the αvβ3 integrin/FAK/PI3K/Akt pathway in vitro.

Function of chemokine (CXC motif) ligand 12 in periodontal ligament fibroblasts

The periodontal ligament (PDL) is one of the connective tissues located between the tooth and bone. It is characterized by rapid turnover. Periodontal ligament fibroblasts (PDLFs) play major roles in the rapid turnover of the PDL. Microarray analysis of human PDLFs (HPDLFs) and human dermal fibroblasts (HDFs) demonstrated markedly high expression of chemokine (CXC motif) ligand 12 (CXCL12) in the HPDLFs. CXCL12 plays an important role in the migration of mesenchymal stem cells (MSCs). The function of CXCL12 in the periodontal ligament was investigated in HPDLFs. Expression of CXCL12 in HPDLFs and HDFs was examined by RT-PCR, qRT-PCR and ELISA. Chemotactic ability of CXCL12 was evaluated in both PDLFs and HDFs by migration assay of MSCs. CXCL12 was also immunohistochemically examined in the PDL in vivo. Expression of CXCL12 in the HPDLFs was much higher than that in HDFs in vitro. Migration assay demonstrated that the number of migrated MSCs by HPDLFs was significantly higher than that by HDFs. In addition, the migrated MSCs also expressed CXCL12 and several genes that are familiar to fibroblasts. CXCL12 was immunohistochemically localized in the fibroblasts in the PDL of rat molars. The results suggest that PDLFs synthesize and secrete CXCL12 protein and that CXCL12 induces migration of MSCs in the PDL in order to maintain rapid turnover of the PDL.

MT1-MMP expression in the odontogenic region of rat incisors undergoing interrupted eruption

MT1-MMP (membrane type matrix metalloproteinase-1) has been considered an important membrane-type matrix metalloproteinase involved in the remodeling process in tissue and organ development, including the processes of the tooth and root growth and dental eruption. Therefore, the aims of this study were to evaluate MT1-MMP expression in the odontogenic region, as well as the eruption rate and morphology of the lower-left rat incisor, where the eruption process was interrupted for 14 days by a steel wire attached from the center of the incisor labial face and braced to the first molar. In the interrupted eruption group, the eruption rate was significantly reduced, producing drastic morphological alterations in the tooth germ and socket area. The MT1-MMP expression was widespread in the dental follicle, in both groups studied (normal and interrupted eruption groups); however a significant decrease in immunostaining was observed in the interrupted eruption group. Results indicate that MT1-MMP may have an important role in the process of dental eruption.

Isolation and characterization of multipotent mesenchymal stromal cells from the gingiva and the periodontal ligament of the horse

BACKGROUND

The equine periodontium provides tooth support and lifelong tooth eruption on a remarkable scale. These functions require continuous tissue remodeling. It is assumed that multipotent mesenchymal stromal cells (MSC) reside in the periodontal ligament (PDL) and play a crucial role in regulating physiological periodontal tissue regeneration. The aim of this study was to isolate and characterize equine periodontal MSC. Tissue samples were obtained from four healthy horses. Primary cell populations were harvested and cultured from the gingiva, from three horizontal levels of the PDL (apical, midtooth and subgingival) and for comparison purposes from the subcutis (masseteric region). Colony-forming cells were grown on uncoated culture dishes and typical in vitro characteristics of non-human MSC, i.e. self-renewal capacity, population doubling time, expression of stemness markers and trilineage differentiation were analyzed.

RESULTS

Colony-forming cell populations from all locations showed expression of the stemness markers CD90 and CD105. In vitro self-renewal capacity was demonstrated by colony-forming unit fibroblast (CFU-F) assays. CFU-efficiency was highest in cell populations from the apical and from the mid-tooth PDL. Population doubling time was highest in subcutaneous cells. All investigated cell populations possessed trilineage differentiation potential into osteogenic, adipogenic and chondrogenic lineages.

CONCLUSIONS

Due to the demonstrated in vitro characteristics cells were referred to as equine subcutaneous MSC (eSc-MSC), equine gingival MSC (eG-MSC) and equine periodontal MSC (eP-MSC). According to different PDL levels, eP-MSC were further specified as eP-MSC from the apical PDL (eP-MSCap), eP-MSC from the mid-tooth PDL (eP-MSCm) and eP-MSC from the subgingival PDL (eP-MSCsg). Considering current concepts of cell-based regenerative therapies in horses, eP-MSC might be promising candidates for future clinical applications in equine orthopedic and periodontal diseases.

Endoglin is involved in BMP-2-induced osteogenic differentiation of periodontal ligament cells through a pathway independent of Smad-1/5/8 phosphorylation

The periodontal ligament (PDL), a connective tissue located between the cementum of teeth and the alveolar bone of mandibula, plays a crucial role in the maintenance and regeneration of periodontal tissues. The PDL contains fibroblastic cells of a heterogeneous cell population, from which we have established several cell lines previously. To analyze characteristics unique for PDL at a molecular level, we performed cDNA microarray analysis of the PDL cells versus MC3T3-E1 osteoblastic cells. The analysis followed by validation by reverse transcription-polymerase chain reaction and immunochemical staining revealed that endoglin, which had been shown to associate with transforming growth factor (TGF)-beta and bone morphogenetic proteins (BMPs) as signaling modulators, was abundantly expressed in PDL cells but absent in osteoblastic cells. The knockdown of endoglin greatly suppressed the BMP-2-induced osteoblastic differentiation of PDL cells and subsequent mineralization. Interestingly, the endoglin knockdown did not alter the level of Smad-1/5/8 phosphorylation induced by BMP-2, while it suppressed the BMP-2-induced expression of Id1, a representative BMP-responsive gene. Therefore, it is conceivable that endoglin regulates the expression of BMP-2-responsive genes in PDL cells at some site downstream of Smad-1/5/8 phosphorylation. Alternatively, we found that Smad-2 as well as Smad-1/5/8 was phosphorylated by BMP-2 in the PDL cells, and that the BMP-2-induced Smad-2 phosphorylation was suppressed by the endoglin knockdown. These results, taken together, raise a possibility that PDL cells respond to BMP-2 via a unique signaling pathway dependent on endoglin, which is involved in the osteoblastic differentiation and mineralization of the cells.

The myofibroblast: phenotypic characterization as a prerequisite to understanding its functions in translational medicine

The phrase ‘translational research’conveys the idea of the pursuit of applications for the treatment of human disease.The myofibroblast, long known for having a role in wound-healing, and for its presence in fibrotic conditions and tumour stroma, is becoming a focus for translational research, not least through its increasingly documented role as a tumour-promoting cell.In fibroproliferative conditions, cancer and tissue engineering, the myofibroblast, derived partly and possibly from circulating bone-marrow-derived cells and epithelial-to-mesenchymal transformation, is attracting great attention.In cancer, this cell was initially regarded as a barrier to tumour dissemination, but there is now a growing body of evidence to indicate that it is an active participant in tumour progression.While the involvement of the myofibroblast in these pathological processes is pushing the myofibroblast into the limelight of translational medicine as a target for potential anti-fibrotic and anti-cancer therapy, there are still numerous indications from the literature that the myofibroblast is a poorly understood cell in terms of its differentiation.Partly, this is due to a failure to appreciate the contribution of electron microscopy to understanding the nature of this cell.This paper, therefore, is devoted to detailing the principal phenotypic characteristics of the myofibroblast and promotes the argument that understanding how the myofibroblast carries out its roles in normal biological and in pathological processes will be enhanced by a sound understanding of its cellular differentiation, which in turn arguably demands a significant ultrastructural input.

The equine periodontium as a continuously remodeling system: morphometrical analysis of cell proliferation

OBJECTIVE

The hypsodont equine cheek tooth erupts continuously throughout life. Tooth eruption is inevitably associated with a remodeling of the periodontium. One major process of remodeling in the PDL is cell proliferation. The aim of this study was to detect cell proliferation at different sites of the equine PDL in order to examine the dynamics of the periodontal cell population.

DESIGN

Specimens from nine warm-blood horses were taken-- containing the PDL interposed between the cementum and the alveolar bone--at three designated levels: subgingival, middle, and apical. Cell proliferation was detected immunohistochemically by use of anti-Ki-67 in combination with an elaborate morphometrical procedure using an image-analysis program. Three zones of the PDL were distinguished in each specimen: a zone next to the dental cementum, a central zone, and a zone next to the alveolar bone. The PDL was divided into three levels and three zones, resulting in nine defined regions of interest.

RESULTS

The proliferation index was high at the apical level and in the zone next to the alveolar bone. The opposite was true of cell density, which was high at the subgingival level and in a zone next to the dental cementum.

CONCLUSIONS

Evaluation and statistical analysis of the parameters proliferation index and cell density in distinct, biologically appropriate sites of the equine PDL suggest the presence of dynamic processes like cell migration by which the cells of the PDL move from an area next to the alveolar bone towards the dental cementum, and in an apico-occlusal direction. Furthermore, our study confirms the close relationship between cell proliferation, cell migration, and eruption.

Fractal analysis of the PDL-bone interface and implications for orthodontic tooth movement

BACKGROUND

The periodontal ligament (PDL) is a soft tissue interposed between the tooth and the alveolar bone. It is responsible for transmission of forces in vivo; this promotes bone remodeling. The purpose our study was to use fractal analysis to quantify the complex morphology of the PDL-bone interface.

METHODS

We used Scion Image (Scion Corp, Frederick, Md) and Benoit fractal analysis (Tru Soft International, St. Petersburg, Fla) programs to calculate the fractal dimension of the PDL-bone interface in rats via the box-counting method. Rats in the experimental groups received an initial force of 0.1N or 0.5N with customized springs for 6 hours.

RESULTS

Our studies showed an increase in normal fractal dimension at the root apices of the rats' maxillary molars. We also found evidence that the fractal dimension varies along the entire root length from the apex to the cementoenamel junction.

CONCLUSIONS

Mechanical loading might lead to an increase in fractal dimension at the PDL-bone interface apart from mechanisms of bone cell directed remodeling. These changes in fractal dimension are proportional to loading and could provide a new parameter for force determination in orthodontic tooth movement.

Alveolar bone Sharpey fibers of the rat incisor in normal and altered functional conditions examined by scanning electron microscopy

The morphology and the area density of Sharpey fibers in the socket of the rat incisor under normo-, hyper-, and hypofunctional conditions were evaluated using scanning electron microscopy. Sharpey fibers appeared either as dome-shaped projections, when highly mineralized, or as depressions when less mineralized. Near the alveolar crest, most of the fibers were fully mineralized and arranged in compact longitudinal rows. Toward the basal end of the socket, the rows became interrupted, forming islets of gradually smaller size and number. The density of the Sharpey fibers was higher (P < 0.01) in the mesial and distal faces than in the lingual face in most of the socket length. In normofunctional conditions, in all faces the density decreased 70 to 90 times from the crestal toward the basal region of the socket (P < 0.01). The degree of mineralization of the Sharpey fibers also decreased steadily in the same direction, indicating that, for support, the periodontal ligament matures from basal to incisal and is fully developed only in the crestal region. In hyper- and hypofunctional conditions, the same distribution was observed. The area density of the Sharpey fibers in the hyperfunctional condition showed a slight increase at the basal levels of the socket mesial and distal faces (P < 0.01 or P < 0.05). In hypofunctional incisors, the density decreased significantly (P < 0.01) at the mesial and distal faces in all regions of the socket, and at the lingual face, the decrease (P < 0.05) was restricted to the incisal regions. This may be one of the factors for the weakening of the periodontal ligament in hypofunctional incisors.

A cell line with characteristics of the periodontal ligament fibroblasts is negatively regulated for mineralization and Runx2/Cbfa1/Osf2 activity, part of which can be overcome by bone morphogenetic protein-2

The periodontal ligament (PDL) is a connective tissue located between the cementum of teeth and the alveolar bone of the mandibula. It plays an integral role in the maintenance and regeneration of periodontal tissue. The cells responsible for maintaining this tissue are thought to be fibroblasts, which can be either multipotent or composed of heterogenous cell populations. However, as no established cell lines from the PDL are available, it is difficult to assess what type of cell promotes all of these functions. As a first step to circumvent this problem, we have cloned and characterized cell lines from the PDL from mice harboring a temperature-sensitive SV 40 large T-antigen gene. RT-PCR and in situ hybridization studies demonstrated that a cell line, designated PDL-L2, mimics the gene expression of the PDL in vivo: it expresses genes such as alkaline phosphatase, type I collagen, periostin, runt-related transcription factor-2 (Runx2) and EGF receptor, but does not express genes such as bone sialoprotein and osteocalcin. Unlike osteoblastic cells and a mixed cell population from the PDL, PDL-L2 cells do not produce mineralized nodules in the mineralization medium. When PDL-L2 cells were incubated in the presence of recombinant human bone morphogenetic protein-2 alkaline phosphatase activity increased and mineralized nodules were eventually produced, although the extent of mineralization is much less than that in osteoblastic MC3T3-E1 cells. Furthermore, PDL-L2 cells appeared to have a regulatory mechanism by which the function of Runx2 is normally suppressed.

Phagocytosis and intracellular digestion of collagen, its role in turnover and remodelling

Collagens of most connective tissues are subject to continuous remodelling and turnover, a phenomenon which occurs under both physiological and pathological conditions. Degradation of these proteins involves participation of a variety of proteolytic enzymes including members of the following proteinase classes: matrix metalloproteinases (e.g. collagenase, gelatinase and stromelysin), cysteine proteinases (e.g. cathepsin B and L) and serine proteinases (e.g. plasmin and plasminogen activator). Convincing evidence is available indicating a pivotal role for matrix metalloproteinases, in particular collagenase, in the degradation of collagen under conditions of rapid remodelling, e.g. inflammation and involution of the uterus. Under steady state conditions, such as during turnover of soft connective tissues, involvement of collagenase has yet to be demonstrated. Under these circumstances collagen degradation is likely to take place particularly within the lysosomal apparatus after phagocytosis of the fibrils. We propose that this process involves the following steps: (i) recognition of the fibril by membrane-bound receptors (integrins?), (ii) segregation of the fibril, (iii) partial digestion of the fibril and/or its surrounding non-collagenous proteins by matrix metalloproteinases (possibly gelatinase), and finally (iv) lysosomal digestion by cysteine proteinases, such as cathepsin B and/or L. Modulation of this pathway is carried out under the influence of growth factors and cytokines, including transforming growth factor beta and interleukin 1 alpha.

Immunohistochemical characterization and localization of MHC class II antigen-presenting cells in the periodontal ligament of rat incisors

Dendritic cells have been identified both in lymphoid and nonlymphoid tissues as non-phagocytic antigen-presenting cells, equipped with extensive flamelike cytoplasmic projections. Our immunohistochemical study revealed presence of a large population of dendritic cells and other immunocompetent cells, showing a region-specific distribution, in the lingual periodontal ligament of continuously erupting rat incisors. This study aims to reveal the kinetics and cytological characterization of immunocompetent cells in the periodontal ligament of rat incisors with special reference to their differentiation pathway in the unique local environment.

Cellular origins and differentiation control mechanisms during periodontal development and wound healing

In the context of cellular origins, odontogenic epithelium and oral epithelium are the sources for junctional epithelium during development and during wound healing respectively. In contrast, both odontogenic and non-odontogenic mesenchyme contain the progenitors for gingival fibroblasts in developing tissues while in wounded tissues, gingival fibroblasts are derived from gingival connective tissues and comprise a heterogeneous population of cells with diverse properties and functions. Periodontal ligament, bone and cementum cell populations apparently originate from dental follicle progenitor cells during development, but during wound healing derive from ancestral cells in periodontal ligament and bone. Cellular differentiation in developing periodontium is governed in part by epithelial-mesenchymal interactions that generate specific signals which regulate selective cell populations in time and space. On the other hand, differentiation during wound healing and regeneration is regulated by a vast array of extracellular matrix informational molecules and by cytokines that induce both selective and non-selective responses in the different cell lineages and their precursors. Further, several important signalling systems are irretrievably lost after development is complete. Thus, in the context of cellular origins and differentiation, developing and wounded periodontal tissues exhibit fundamental differences. Future prospects for improved healing and regeneration of periodontal tissues may derive from identification and isolation of informational molecules that are stored in connective tissue matrices. These molecules and elucidation of their functions may open new perspectives in our understanding of the biology of periodontal wound healing and may provide novel approaches to periodontal regeneration.

Effect of loading on the migration of periodontal fibroblasts in the rat incisor

The influence of occlusal loading on periodontal fibroblasts was investigated in hypoloaded (shortened out of occlusion), functionally loaded and hyperloaded (constant linguointrusive mechanical loads of 9.4 +/- 0.06 g) lower left rat incisors. One hour following injection of 3H-thymidine, half of the animals in each group were killed, while the remaining rats were killed 2 weeks later. The decalcified incisors were embedded in glycolmethacrylate and sectioned (2 microns) serially, perpendicularly to the long tooth axis. Labeled and unlabeled fibroblasts in the tooth-related periodontal ligament were counted in 8 x 80 microns consecutive layers. Cell density (CD) and labeling index (LI) were plotted according to their location on the apico-incisal and cementum-bone axes. Loading caused a decrease in CD and a shift of cells from the cementum towards the middle of the ligament, proportionally to load intensity and duration. The average tooth-to-bone movement of the cells was 2 microns/day in the hypoloaded and 4 microns/day in the two loaded groups. The mean daily tooth eruption rate was 975 +/- 60 microns, 499 +/- 18 microns and 103 +/- 27 microns in the hypo-, functionally- and hyperloaded teeth, respectively. The respective concomitant average daily cell migration rates in the incisal direction were 786 microns, 500 microns, and 500 microns, i.e. 80%, 100% and 485% of the tooth eruption rates. The gross disparity between cell velocity and tooth movement under conditions of restrained eruption indicates active motility of the fibroblasts, rather than their passive tooth-eruption dependent translation.

Cell generation within the interdental gingival septum of the rat

The aims of this investigation were to determine whether connective tissue progenitor cells in the interdental gingival septum (IGS) have a paravascular origin, and how the distribution of 3HTdR-labelled cells within the IGS changes with time after a single injection. 30 male hooded Lister rats aged 6 weeks, were killed in groups of ten, 3, 75 and 171 h after a single injection of tritiated thymidine. Autoradiographs were examined of 3 transverse Historesin sections of the papilla between second and third mandibular molars in 29 specimens, taken at equidistant intervals between the col and alveolar bone crest. At all times and levels, 73.9-93.0% of labelled cells and 72.0-79.0% of unlabelled cells lay within 50 microns of blood vessels (BVs). The highest percentages of labelled cells (PLCs) occurred within 5 microns of BVs (P < 0.001) although mean nuclear density here was lowest (P < 0.017), and there was a significant diminution of PLC (P < 0.05) with increasing distance from BVs, occurring most precipitously 5-10 microns from vessel walls. Sites of significantly increased PLC at 3 h also approximated to sites in which mean BV densities were greatest. At 3 h, a number of discrete sites with significantly increased PLCs (P < 0.05) were found within two zones, each equivalent to 40% of IGS volume and juxtaposed with each tooth. At later times, additional sites of raised PLC appeared throughout the IGS. Overall PLCs in the upper two levels and within 5 microns of BVs also increased over the time-course (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative enzyme histochemical analysis of the distribution of alkaline phosphatase activity in the periodontal ligament of the rat incisor

The spatial distribution of alkaline phosphatase (ALP) activity was examined in the periodontal ligament of the continuously growing rat incisor. With the indoxyl-tetrazolium salt method, enzyme activity was demonstrated in undecalcified cryosections, and the amount of reaction product was quantified. ALP activity appeared to be distributed heterogeneously. Its highest activity was found in the bone-related compartment of the ligament. In the tooth-related compartment and the supracrestal extension of the ligament, enzyme activity was significantly lower, but still higher than in the lamina propria of the gingiva. In the part of the ligament bordering the cementum, highest activity was found in the apical region just occlusal to Hertwig's epithelial root sheath, where formation of acellular cementum begins. From there toward the incisal edge, the activity of the enzyme gradually decreased. It is suggested that differences among the various parts of the periodontal ligament are related to local variations in phosphate metabolism and cementum deposition.

The effects of cytoskeletal inhibitors on the collagen gel contraction by dog periodontal ligament fibroblasts in vitro

Periodontal ligament fibroblasts (PLF) were incorporated into small collagen gel discs (100 microliters, dia 10.0 mm), cultured in media containing various concentrations of cytochalasin D or colchicine and the diameters of the discs then measured. Cytochalasin D (3.1-800 nM) inhibited contraction of the gel discs in a dose-dependent manner and 800 nM of cytochalasin D completely inhibited contraction. This complete inhibition was still observed when cell density in the gel was increased. Although colchicine at a concentration of 0.98-15.7 nM also inhibited the contraction in a dose-dependent manner, contraction at 50% of the control level was still observed in the presence of a high concentration (0.25 mM, 100 micrograms/ml). These results strongly indicate that actin filaments are more intimately involved in PLF-mediated collagen gel contraction than microtubules. Although it is suggested that PLF are involved in tooth eruption, earlier studies have shown that microtubule inhibitors inhibit eruption of rat incisors whereas the microfilament inhibitor, cytochalasin B, has no effect. Thus, the PLF theory alone seems insufficient to explain the mechanism of tooth eruption.

Bundle formation of principal fibers in rat molars

The bundling of principal fibers was investigated in tangential sections through the tooth-related portion in developing rat molars by light and electron microscopy. When root dentin calcification began, cross sections of principal fibers emerged as fibril aggregates in the narrow intercellular spaces in a densely packed population of periodontal ligament cells. Subsequently, these cells changed shape and location to widen the intercellular spaces. The fibril aggregates became thicker in these spaces. With root development, the collagen fibrils formed loosely aggregated bundles and the periodontal ligament cells extended cell processes between the bundles. The cell processes usually contained microfilaments suggestive of actin filaments, and as the cell processes extended and came in close apposition, they formed delimited compartments. These compartments appeared to be a sheath-like structure, and the loose fibril bundles developed into tight fibril bundles in the compartments. Finally the principal fibers consisted of many tight fibril bundles, which were partially or entirely surrounded by cell processes and cell bodies. The findings suggest that the sheath-like, cellular compartments cause the tight bundling of the principal fibers.

Role of fibroblast subpopulations in periodontal physiology and pathology

Fibroblasts are the principal cell type in the soft connective tissues of the periodontium; they perform important functions in development, physiology, and disease. A growing number of reports have indicated site-specific phenotypic variation of fibroblasts. Heterogeneity of metabolic traits has been demonstrated in cells from healthy and diseased tissues. The tissue distribution and relative proportions of fibroblast subpopulations have a significant impact on the regulation of connective tissue function in health and disease.

Measurements of the isometric contractile forces generated by dog periodontal ligament fibroblasts in vitro

One hypothesis for the mechanism of tooth eruption is that the periodontal ligament fibroblasts generate the eruptive force. To assess the force generated, these fibroblasts were obtained by explant culture of ligament from mandibular premolars of a dog and were cultured in collagen gel matrices. The forces generated by them under isometric conditions were continuously measured for 120 h with a strain gauge. At the same time the number of cells in the gel was counted and the force measured was calculated as the force generated by 10(4) cells. Shortly after the start of culture, the force per 10(4) cells increased rapidly; it reached 5.2 X 10(-4) N at 8 h, and then remained at the same level for about 48 h. Our findings suggest that fibroblasts of the periodontal ligament may generate sufficient force for tooth eruption.

Comparison of trigeminal receptor location and structure in the periodontal ligament of different types of teeth from the rat, cat, and monkey

The periodontal ligament is richly innervated by mechanoreceptors whose cell bodies are located either in the trigeminal ganglion (TG) or the mesencephalic (MS) trigeminal nucleus. Both are sensitive to stretch of the ligament induced by tooth movement, but their thresholds, central connections, and functional significance differ. This study compared the location of TG and MS receptors in the periodontal ligament of cat teeth after labeling each by anterograde axonal transport. We also compared the location and ultrastructure of the feline TG receptors with labeled TG receptors in the periodontal ligament of monkey teeth and rat incisors in order to determine their location and ultrastructural properties. We found that the MS and TG receptors had a different distribution in the periodontal ligament of cat teeth; the MS terminals were concentrated below and next to the base of the roots, whereas the TG receptors were most numerous around the middle of the roots. The TG receptors of monkey teeth had a similar location to the feline TG receptors, but those of rat incisors were very different. Rat incisors are curved, continuously erupting teeth, and their TG receptors were located primarily on the lingual side in the alveolar (nonerupting) portion of the ligament. Ultrastructural comparisons found that most mechanoreceptors in the periodontal ligament of all the teeth had an unencapsulated branched Ruffini-like structure. The TG receptors in the rat incisor ligament were the largest; those of monkey had the most varied form. Some coiled or encapsulated receptors were found in the monkey and cat ligament, but not in the rat incisor ligament. The TG receptors appear to be located at sites that would be most easily stretched during tooth contact. The different sites and intensity of the stretch forces occurring during the use of different types of teeth may determine the variations in the size and location of the TG mechanoreceptors and of their associated support cells. The different distribution of MS receptors may contribute to their response thresholds and static properties, which differ from those of TG receptors.

Three-dimensional presentation of cell migration in the periodontal ligament of the rat incisor

The progenitor compartments and cell migration were examined in the tooth-related periodontal ligament (t-PDL) of rat incisors. A pulse injection of 3H-Tdr was administered to 15 rats (200 gm each) and the animals were killed in groups of five, at 1 hr and at 1 and 2 weeks after injection. Three-dimensional analysis of cell counts and labeling index demonstrated the existence of two progenitor compartments (PC). The apical PC (responsible for 70% of synthesizing cells) was concentrated in the apical 5 mm of the t-PDL. The paracemental PC (30% of synthesizing cells) was located along and around the cementum, occupying 24 micron of the t-PDL. The cells from the apical PC migrated incisally at a rate of 6 mm/week, which is a rate similar to that of tooth eruption. The cells from the paracemental PC moved in a transverse direction toward either bone or cementum at the much slower rate of 16 micron/week.

Innervation of periodontal ligament and dental pulp in the rat incisor: an immunohistochemical investigation of neurofilament protein and glia-specific S-100 protein

Nervous elements in the periodontal ligament and dental pulp of rat incisors were investigated by means of immunohistochemistry for neurofilament protein (NFP) and glia-specific S-100 protein. The periodontal ligament in the incisors was densely innervated by NFP-immunoreactive nerve fibers; the distribution of the nerve fibers and their terminations differed markedly from those in molars. NFP-positive, thick nerve bundles entered the lingual periodontal ligament through slits located in the mid-region of the alveolar socket, and immediately formed numerous Ruffini-like corpuscles. In the labial periodontal ligament, all of the NFP-immunoreactive nerve fibers terminated in free endings. The restricted location of the stretch receptor, Ruffini-like corpuscle, in the lingual periodontal ligament appears to be an essential element, because this region is regularly extended during mastication. The nervous elements were restricted to the alveolar half of the periodontal ligament in every region; they avoided the dental half of the periodontal ligament, which presumably moves continuously with the tooth. Pulpal nerve fibers in incisors also showed a characteristic distribution different from those in molars; individual nerve fibers with beaded structures ran in the center of the pulp toward the incisal edge, and did not form the subodontoblastic nerve plexus of Raschkow. Immunostaining for S-100 protein revealed a distribution pattern of nervous elements similar to that for NFP, suggesting that the nerves supplying the periodontal ligament and dental pulp were mostly covered by a Schwann sheath.

Three-dimensional presentation of the fibroblast progenitor compartment in the periodontal ligament of the rat incisor

The purpose of this study was to determine the shape, size, and spatial configuration of the fibroblast progenitor compartment (PC) of the tooth-related periodontal ligament (PDL). Five albino rats (weight 200 gm) were killed 1 hr following intraperitoneal injection of 3H-thymidine. The incisors were decalcified, embedded in glycol-methacrylate, and sectioned serially, perpendicularly to the long axis of the tooth. Ten 2-microns-thick sections along each tooth were selected, processed autoradiographically, and the distance from the apex measured. Labeled and unlabeled fibroblasts were counted in 8 x 80-microns consecutive layers. Cell density (CD) and labeling index (LI) were plotted on a three-dimensional system of coordinates depicting the directions: apex to incisal edge, cementum to bone, and cemento-enamel junction (CEJ) to lingual tooth surface. The CD along the tooth was constant, except at 2-6 mm distant from the apex, where it was significantly higher. The highest concentration of cells around the tooth was in the 20-microns region adjacent to the cementum. The PC on the mesial and lateral tooth sides extended 1-4 mm from the apex, with the LI peak (6.5%) occurring at 2 mm. The PC on the lingual tooth side was located at 2-6 mm from apex, peaking (14%) at 4 mm. On the cementum-bone coordinate, the PC extended to about 50 microns from the cementum on all sides, the LI peak appearing at a distance of 20-40 microns from the tooth. The LI was consistently higher on the lingual side than on the mesial and lateral aspects.

Movement of fibroblasts in the periodontal ligament of the mouse incisor is related to eruption

Movement of fibroblasts in the periodontal ligament of the lower incisor of the mouse was studied by pulse-labeling with tritiated thymidine and proline. 3H-Thymidine was administered to mark the nuclei of the cells in the proliferative compartment near the basal end of the tooth; 3H-proline gave rise to a narrow band of radioactivity in the dentin, which served as a reference line for measurement of eruption. One or three weeks after injection in each animal, the lower right incisor was prevented from further eruption by being pinned to its alveolar process. The animals were killed 0, 1, or 2 weeks later, and their mandibles processed for LM-radioautography. It was found that in the left incisors, which were not inhibited in their eruption, labeled cells in the tooth-half of the periodontal ligament moved incisally at a rate similar to the eruption rate. In the pinned incisors, no further incisal migration could be established. It is concluded that fibroblast migration in the tooth-half of the ligament is strictly coupled to the eruptive process.

Long-term effect of loading on the fibroblast population of the periodontal ligament in the rat lower incisor

The number and density of the cells in the tooth-related periodontal ligament (t-PDL) were related to the recovery of eruptive potential 2.5 months after load removal. Four left incisors in which eruption returned to normal (group A) and six in which eruption remained grossly impaired after a two-week application of latero-intrusive loads of 18.5 +/- 0.69 (group B) were compared to four control rats with unimpeded eruption. The incisors were demineralized, embedded in paraplast and cut into 6 micron transverse serial sections from which a three-dimensional reconstruction of the PDL was made by computerized histomorphometry, and the fibroblast population counted. The t-PDL volumes in controls and in groups A and B reached 1.15, 1.05 and 1.53 mm3, respectively; the estimated number of cells in the same PDL volumes were 2244 X 10(3), 1.659 X 10(3) and 2094 X 10(3) cells, respectively. The mean cell count/segment of PDL and the calculated number of cells/unit of periodontal tissue were significantly lower (p less than 0.001) in the formerly stressed, lateral periodontium of both experimental groups. In group B, these values were also decreased in the mesial periodontium.

Microfilaments in human cementoblasts and periodontal fibroblasts

An electron microscopic survey was carried out on the human periodontal ligament (PDL), including a part of the gingival connective tissue attached to extracted tooth roots (11 functioning premolars and 6 nonfunctioning third molars) in order to examine the characteristics of microfilaments (6 nm) in cementoblasts and PDL fibroblasts. Microfilaments which were grouped in bundles with semiperiodic dense nodes or in meshworks just beneath the cell membrane were seen predominantly in the cells characterized by their ultrastructurally immature appearance. These microfilaments were more commonly observed in third molar PDL than in premolar PDL and, in general, more conspicuous in cementoblasts than in fibroblasts. The significance of microfilaments in human PDL is discussed, particularly in relation to cell differentiation and morphogenesis.

Odontoblast turnover in the impeded and unimpeded rat incisor derived from computerized histomorphometry

A computerized histomorphometric method was devised to estimate the kinetics of odontoblast turnover and dentinogenesis in rat incisors. The method was applied to two groups of rats: one group with lower incisors in impeded eruption and another group with the left lower incisor in the unimpeded state. The teeth were divided into six equal segments, from which consecutive ground sections were obtained. The distance of each ground section from the posterior border of the alveolar bone was calculated. Each section was magnified, traced, and the tracings fed into a computer by a sonic digitizer. The perimeters and areas of dentine and pulp in each ground section were calculated by the computer. The mean odontoblast density along the predentine was evaluated from histological sections taken both from the same tooth segments and from teeth sectioned midsagittally. These served for the estimation of the predentine area occupied by the average odontoblast. In the impeded group, this area was 11% larger than in the unimpeded one. Outer dimensions of teeth, namely the circumference of the dentine, the labiolingual width, and the mesiolateral width remained constant and equal for both groups. Daily rates of dentine apposition were computed and were found to vary according to the age of the odontoblasts. Odontoblasts of impeded teeth started to secrete matrix at a rate of 17 microns/day, which increased slightly to 19 microns/day and later declined to 7 microns/day on the 38th day. Dentine production of unimpeded odontoblasts, on the other hand, started at a rate of 16 microns/day and gradually increased to 34 microns/day on the 17th day.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related and 224Ra-induced abnormalities in the teeth of male mice

A high incidence of incisor abnormalities was found in aged control and aged 224Ra-treated male CBA mice. Visual examination of the abnormalities in both controls and treated mice revealed extreme shortening of the upper incisors and hypoplastic, grooved or undulating enamel. The administration of 865 or 1730 nCi of 224Ra hastened the onset of incisor abnormalities although no specific feature was attributable solely to radium toxicity. Radiography and histology revealed corrugated incisors, obliteration of the pulp cavity, extension and disorganized growth of incisors basally, secondary incisors, open pulp and fractures within the alveoli. There was a statistically-significant reduction in the number of molars present in animals given 432, 865 or 1730 nCi 224Ra.

Effects of colchicine and hydrocortisone on unimpeded eruption rates of root-resected mandibular incisors of rats

A single injection of colchicine and daily injections of hydrocortisone have opposite effects on tooth eruption, respectively causing marked reduction and marked increase in unimpeded eruption rates in the root-resected incisors of rats. Unimpeded eruption rates of both root-resected and normal, non-resected, incisors responded identically to these two drugs, providing support for the view that the eruptive process following root resection is physiological. It was not possible to obtain direct evidence relating to either the hydrostatic pressure or the periodontal fibroblast hypotheses of tooth eruption; the drugs used to affect the eruption rates of resected rat incisors could also affect either the tissue hydrostatic pressure or the motility or contractibility of periodontal fibroblasts or both.

A periodontal attachment mechanism without alveolar bone. Case report

A 22-year-old black male was referred for periodontal therapy because of radiographic evidence of advanced bone loss associated with the posterior teeth. Clinical examination revealed gingivitis, normal sulcus depths, and minimal loss of clinical attachment. Complete blood counts, serum chemistry, and neutrophil function were within normal limits. Histological, histochemical and ultrastructural analysis of an extracted tooth revealed no loss of attachment; large areas of the cementum were collagen-poor and, ultrastructurally, resembled afibrillar cementum. It is proposed that the periodontal attachment mechanism present in this case was associated with a localized failure in normal periodontal development.

An in-vitro model for tooth eruption utilizing periodontal ligament fibroblasts and collagen lattices

An in-vitro model that simulates tooth eruption was devised to determine the possible role of periodontal ligament fibroblasts (PLF) in providing a major force for eruption. The model consisted of agarose-coated wells in multiwell tissue-culture plates to which wire mesh was attached at the rim and a slice of demineralized root was attached to the floor. When the wells were filled with a collagen lattice containing PLF, the lattice contracted and elevated the root slice. The root slice in models that were unattached at the rim was not elevated. Cultures without cells did not contract and root slices were not elevated. Elevation was inhibited by cytochalasin D and Colcemid. Histologically, cultures that had elevated showed orientated cells and collagen fibres. The results suggest that PLF may be capable of generating forces that are sufficient to produce tooth eruption.

The effect of a microfilament-disrupting drug, cytochalasin B, on 6-hourly and daily eruption rates of the rat mandibular incisor

The drug did not affect 6-hourly and daily eruption rates of the unimpeded mandibular incisor of the rat. It is suggested that the tooth eruption is not inhibited by the disruption of microfilaments because the disruption was not associated with the motility of the cells and/or that the motility of the cells is not associated with the motive force of tooth eruption. Further studies may be necessary to confirm that the effects of cytochalasin B are on both microfilaments and motility of the periodontal fibroblasts.