Scanning electron microscopy of the cellular elements of rat periodontal ligament

Anti-inflammatory effects of low-level laser therapy on human periodontal ligament cells: in vitro study

Periodontal disease is a chronic inflammatory disease that is commonly treated with surgical and nonsurgical techniques. However, both approaches have limitations. Low-level laser therapy (LLLT) has been widely applied in reducing inflammatory reactions, and research indicates that LLLT induces an anti-inflammatory effect that may enhance periodontal disease therapy. The purpose of this study was to investigate the anti-inflammatory effect of LLLT on human periodontal ligament cells (hPDLCs) in an inflammatory environment and aimed to determine the possible mechanism of action. Cells were cultured and treated with or without lipopolysaccharide (LPS) from Porphryromonas gingivalis or Escherichia coli, followed by irradiation with a gallium-aluminum-arsenide (GaAlAs) laser (660 nm) at an energy density of 8 J/cm². Quantitative real-time polymerase chain reactions were used to assess the expression of pro-inflammatory genes, including tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6, and IL-8. The dual-luciferase reporter assay was used to examine nuclear factor-κB (NF-κB) transcriptional activity. An enzyme-linked immunosorbent assay was used to monitor the concentration of intracellular cyclic adenosine monophosphate (cAMP). Both LPS treatments significantly induced the mRNA expression of pro-inflammatory cytokines. However, LLLT inhibited the LPS-induced pro-inflammatory cytokine expression and elevated intracellular levels of cAMP. The LLLT inhibitory effect may function by downregulating NF-κB transcriptional activity and by increasing the intracellular levels of cAMP. LLLT might inhibit LPS-induced inflammation in hPDLCs through cAMP/NF-κB regulation. These results should be further studied to improve periodontal therapy.

Enhanced wound healing associated with Sharpey's fiber-like tissue formation around FGF-2-apatite composite layers on percutaneous titanium screws in rabbits

BACKGROUND

Pin-tract infections are the most common complications of external fixation. To solve the problem, we developed a fibroblast growth factor-2 (FGF-2)-apatite composite layer for coating titanium screws. The purpose of this study was to elucidate the mechanism of the improvement in infection resistance associated with FGF-2-apatite composite layers.

METHOD

We analyzed FGF-2 release from the FGF-2-apatite composite layer and the mitogenic activity of the FGF-2-apatite composite layer. We evaluated time-dependent development of macroscopic pin-tract infection around uncoated titanium control screws (n = 10). Screws coated with the apatite layer (n = 16) and FGF-2-apatite composite layer (n = 16) were percutaneously implanted for 4 weeks in the medial proximal tibia in rabbits.

RESULTS

A FGF-2-apatite composite layer coated on the screws led to the retention of the mitogenic activity of FGF-2. FGF-2 was released from the FGF-2-apatite composite layer in vitro for at least 4 days, which corresponds to a period when 30% of pin-tract infections develop macroscopically in the percutaneous implantation of uncoated titanium control screws. The macroscopic infection rate increased with time, reaching a plateau of 80-90% within 12 days. This value remained unchanged until 4 weeks after implantation. The screws coated with an FGF-2-apatite composite layer showed a significantly higher wound healing rate than those coated with an apatite layer (31.25 vs. 6.25%, p < 0.05). The interfacial soft tissue that bonded to the FGF-2-apatite composite layer is a Sharpey's fiber-like tissue, where collagen fibers are inclined at angles from 30 to 40° to the screw surface. The Sharpey's Wber-like tissue is rich in blood vessels and directly bonds to the FGF-2-apatite composite layer via a thin cell monolayer (0.8-1.7 μm thick).

CONCLUSION

It is suggested that the enhanced wound healing associated with the formation of Sharpey's fiber-like tissue triggered by FGF-2 released from the FGF-2-apatite composite layer leads to the reduction in the pin-tract inflammation rate.

An in vivo 3D micro-CT evaluation of tooth movement after the application of different force magnitudes in rat molar

OBJECTIVE

To investigate the precise longitudinal change in the periodontal ligament (PDL) space width and three-dimensional tooth movement with continuous-force magnitudes in living rats.

MATERIALS AND METHODS

Using nickel-titanium closed-coil springs for 28 days, 10-, 25-, 50-, and 100-g mesial force was applied to the maxillary left first molars. Micro-CT was taken in the same rat at 0, 1, 2, 3, 10, 14, and 28 days. The width of the PDL was measured in the pressure and tension sides from 0 to 3 days. Angular and linear measurements were used to evaluate molar position at day 0, 10, 14, and 28. The finite element model (FEM) was constructed to evaluate the initial stress distribution, molar displacement, and center of rotation of the molar.

RESULTS

The initial evaluation of PDL width showed no statistical differences among different force magnitudes. Tooth movement was registered 1 hour after force application and gradually increased with time. From day 10, greater tooth movement was observed when 10 g of force was applied. The FEM showed that the center of rotation in the molar is located in the center of five roots at the apical third of the molar roots.

CONCLUSION

The rat's molar movement mainly consists of mesial tipping, extrusion of distal roots, intrusion of mesial root, palatal inclination, and mesial rotation. Although the initial tooth movement after the application of different force magnitudes until day 3 was not remarkably different, 10 g of force produced more tooth movement compared with heavier forces at day 28.

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.

Effect of mechanical loading on periodontal cells

Mechanical loading is an important regulatory factor in alveolar bone homeostasis, and plays an essential role in maintaining the structure and mass of the alveolar processes throughout lifetime. A better understanding of the cellular and molecular responses of periodontal cells is a prerequisite for further improvements of therapeutic approaches in orthodontics, periodontal and alveolar bone repair and regeneration, implantology, and post-surgical wound healing. The purpose of this review is to provide an insight into some cell culture and animal models used for studying the effects of mechanical loading on periodontal cells, and into the recent developments and utilization of new in vivo animal models. There has been an increased awareness about the need for improvement and development of in vivo models to supplement the widely used cell culture models, and for biological validation of in vitro results, especially in the light of evidence that developmental models may not always reflect bone homeostasis in an adult organism. Due to the limitations of in vivo models, previous studies on mechanical regulation of alveolar bone osteoblasts and cementoblasts mostly focused on proliferative responses, rather than on the stimulation of cell differentiation. To address this problem, we have recently characterized and implemented a mouse osteoinductive tooth movement model for studying mechanically induced regulation of osteoblast- and cementoblast-associated genes. In this model, a defined and reproducible mechanical osteogenic loading is applied during a time course of up to two weeks. Regulation of gene expression in either wild-type or transgenic animals is assessed by a relative quantitative measurement of the level of target mRNAs directly within the subpopulations of periodontal cells. To date, results demonstrate a defined temporal pattern of cell-specific gene regulation in periodontal osteoblasts mechanically stimulated to differentiate and deposit bone matrix. The responses of osteoblast-associated genes to mechanical loading were 10- to 20-fold greater than the increase in the numbers of these cells, indicating that the induction of differentiation and an increase of cell function are the primary responses to osteogenic loading. The progression of the osteoblast phenotype in the intact mouse periodontium was several-fold faster compared with that in cultured cells, suggesting that the mechanical signal may be targeting osteoblast precursors in the state of readiness to respond to an environmental challenge, without the initial proliferative response. An early response of alkaline phosphatase and bone sialoprotein genes was detected after 24 hrs of treatment, followed by a concomitant stimulation of osteocalcin and collagen I between 24 and 48 hrs, and deposition of osteoid after 72 hrs. Although cementoblasts constitutively express biochemical markers similar to those of osteoblasts, distinct responses of osteocalcin, collagen I, and bone sialoprotein genes to mechanical loading were observed in the two cell phenotypes. This finding indicates that differential genetic responses to mechanical loading provide functional markers for distinction of the cementoblast and osteoblast phenotypes.

Effects of cyclosporin A on alveolar bone: an experimental study in the rat

BACKGROUND

There have been several investigations on the role of cyclosporin A (CSA) in gingival hyperplasia in both animals and humans. However, less attention has been given to the drug's effect on alveolar bone. This study used light microscopy to histologically and histometrically evaluate the effects of CSA on alveolar bone in the rat.

METHODS

Sixty, 6-week-old Sprague-Dawley rats were separated into test and control groups. Animals in the test group received CSA in mineral oil (30 mg/kg body weight) daily by gastric feeding over the 6-week study. Control animals received only mineral oil. Ten animals from each group were sacrificed at weeks 2, 4, and 6. After histologic processing, the labial crest of the alveolar bone in the anterior mandible was evaluated by light microscopy.

RESULTS

A distinct pattern of increased osteoclasia and reduced bone formation was observed in the CSA-exposed animals compared to the controls. Increased osteoclasia was observed in periodontal sites and decreased bone formation was observed in symphyseal sites.

CONCLUSIONS

The results suggest that CSA has distinct effects on alveolar bone.

The response of periodontal ligament cells to fibronectin

Fibronectin (fn) is an extracellular matrix (ECM) molecule important in cell adhesion and migration and in wound healing. It is also likely important in periodontal ligament (PDL) cell-ECM interactions, and thus in regenerating periodontal tissues. In this study we characterized PDL cells and their interactions with FN, testing different PDL cell isolates taken from healthy and diseased conditions. PDL cells were characterized by their morphology, integrin profile, motility, and bone nodule formation. Cells were then assayed for adhesion, proliferation, and chemotaxis in response to FN or FN fragments. Cell isolates were morphologically heterogeneous and fibroblastic, had a normal-appearing actin cytoskeleton and a wide range of migration potentials, and formed bone-like nodules in vitro. They expressed alpha5, beta1, alpha v, and alpha4 integrin subunits, known receptors for FN, and in fact they bound FN preferentially at 5 and 10 microg/ml. Intact FN induced greater PDL cell proliferation and chemotaxis than did FN fragments (120-kDa cell-binding, 60-kDa heparin-binding, and 45-kDa collagen-binding). PDL cells harvested from diseased and healthy conditions were no different on the basis of these assays. These data demonstrate that PDL cells are a mixed population of fibroblastic cells, capable of forming a mineralized matrix. They also suggest that maximal proliferation and chemotaxis require specific FN domains that are present on the intact molecule but not its fragments.

Is fibroblast heterogeneity relevant to the health, diseases, and treatments of periodontal tissues?

There are wide variations of gene expression and strikingly different responses to extracellular signals among different fibroblast populations. This has prompted a large number of in vitro studies which suggest that fibroblasts are not homogeneous but instead comprise multiple subpopulations with extensive site-to-site and intra-site variations. Conceivably, either fibroblasts are not all created equal, or, alternatively, discrete subpopulations may emerge in development, inflammatory lesions, or wound healing. While the heterogeneous nature of cultured fibroblasts has been known for some time, are these variations relevant to our understanding of the biology of oral tissues, their involvement in disease, and their response to therapy? Since fibroblasts are the predominant cell type in soft connective tissue matrices, the regulation of their proliferative, synthetic, and degradative behavior is likely to be important in tissue physiology and pathology. In this review, we use the current literature to assess whether fibroblast subpopulations really make a difference in the health and disease of periodontal tissues. We address the following questions: (1) Is fibroblast heterogeneity a real in vivo phenomenon? (2) How can we advance our knowledge of phenotypic variations and the regulation of fibroblast differentiation? (3) Could a knowledge of fibroblast heterogeneity have an impact on the development of new approaches to pathogenesis and the treatment of periodontal tissues?

Fibroblast heterogeneity in the periodontium and other tissues

Fibroblasts are the major resident cells which inhabit the periodontal tissues. As such, they are crucial for maintaining the connective tissues which support and anchor the tooth. Little is known of their origins, synthesis of regulatory cytokines and growth factors in health and disease, and importance in soft tissue regeneration. An emerging concept is that fibroblasts are not homogeneous, but instead consist of subsets of cells which can regulate bone marrow-derived cells such as T lymphocytes. Fibroblasts can be separated into subsets on the basis of morphology, size and expression of intermediate filaments as well as collagen subtypes. Differential surface marker expression has also been a key feature to distinguish fibroblast subsets from many tissues. Antigens such as Thy-1, class II MHC, and C1q are among those surface proteins which have been employed successfully to separate fibroblasts. Importantly, these fibroblast subsets are not only antigenically diverse, but also possess distinct functions. Thy 1+ pulmonary fibroblasts can display class II MHC antigens, synthesize IL-1 and can activate T lymphocytes, whereas the Thy 1+ subset is devoid of these functions. Recently, fibroblasts from the human orbit have also been shown to be separable on the basis of Thy 1 surface marker expression. Fibroblasts derived from human gingiva and periodontal ligament also appear to be composed of subsets with a heritable pattern of surface markers which will permit their separation into functional subpopulations. This paper will review findings of fibroblast heterogeneity in periodontal and other tissues. Evidence will be presented for the use of surface markers to delineate functional subsets. The ability to discriminate subsets of fibroblasts will aid in studies of periodontal disease pathogenesis and wound healing.

Periodontal ligament cell population: the central role of fibroblasts in creating a unique tissue

BACKGROUND

Fibroblasts are the predominant cells of the periodontal ligament (PL) and have important roles in the development, function, and regeneration of the tooth support apparatus. Biological processes initiated during the formation of the PL contribute to the long-lasting homeostasic properties exhibited by PL fibroblast populations.

DEVELOPMENT

The formation of the PL is likely controlled by epithelial-mesenchymal and epithelial hard tissue interactions, but the actual mechanisms that contribute to the development of cellular lineages in the PL are unknown. Fibroblasts in the normally functioning PL migrate through the tissue along collagen fibres to cementum and bone and in an apico-coronal direction during tooth eruption. ADULT TISSUE: Cell kinetic experiments have shown that PL fibroblasts comprise a renewal cell system in steady-state and the progenitors can generate multiple types of more differentiated, specialized cells. Progenitor cell populations of the PL are enriched in locations adjacent to blood vessels and in contiguous endosteal spaces. In normally functioning periodontal tissues, there is a relatively modest turnover of cells in which apoptotic cell death balances proliferation. Large increases of cell formation and cell differentiation occur after application of orthodontic forces or wounding. As PL cells comprise multiple cellular phenotypes, it has been postulated that after wounding, the separate phenotypes repopulating the site will ultimately dictate the tissue form and type.

CONCLUSIONS

PL fibroblasts play an essential role in responses to mechanical force loading of the tooth by remodelling and repairing effete or damaged matrix components. In consideration of the important roles played by fibroblasts in PL homeostasis, they could be described as "the architect, builder, and caretaker" of the periodontal ligament.

Origins and functions of cells essential for periodontal repair: the role of fibroblasts in tissue homeostasis

OBJECTIVE

A review is undertaken of rodent model systems and cell culture studies that address the role of periodontal fibroblasts in tissue homeostasis in both normal function and after wound healing.

RATIONALE

Fibroblasts are the predominant cells of the periodontal ligament (PL) and of healthy gingiva and have important roles in the development, function and regeneration of the tooth support apparatus.

REVIEW

In normally functioning periodontal tissues cell turnover involves generation of new cells by proliferation which in turn is balanced by apoptopic cell death. Consequently PL fibroblasts comprise a renewal cell system in steady-state. PL cell progenitors can generate multiple types of more differentiated, specialized cells including large numbers of fibroblastic cells and more limited numbers of osteogenic or cementogenic cells. However PL fibroblasts constitutively block osteogenesis and thereby maintain the PL width. Proliferating progenitor cell populations of the PL are enriched in locations adjacent to blood vessels and in contiguous endosteal spaces from where they migrate to the body of the PL. Large increases of cell formation and cell differentiation occur after wounding but surprisingly, the cells that repopulate the PL adjacent to the root surface are largely post-mitotic. As PL cell populations comprise multiple lineages, it is likely that after wounding, the separate phenotypes repopulating the wound site will be selected by environmental factors. Further, the specific repopulating lineages will strongly influence the form and function of the nascent tissue. To illustrate the specificity of fibroblast functions, examples of migratory and contractile fibroblast phenotypes are provided which exhibit constitutively different levels of gelsolin and alpha-smooth muscle actin respectively, cytoskeletal proteins which are markers for these cell types.

CONCLUSION

Fibroblasts contribute to PL homeostasis by their abilities to remodel tissues, to repopulate wounds, to influence the metabolism of other cell types and to create a new fibrous attachment.

Dependence of collagen remodelling on alpha-smooth muscle actin expression by fibroblasts

To study the relation between expression of the putative myofibroblast marker alpha-smooth muscle actin and the remodelling of extracellular matrix, immunocytochemical, gel electrophoresis, and collagen gel contraction studies were performed on two human fibroblast subtypes. Double immunolabelling for total actins and alpha-smooth muscle (sm) actin as well as affinity labelling of filamentous and monomeric actins in gingival fibroblasts demonstrated that alpha-sm was colocalized in stress fibres and in regions with high levels of monomeric actin throughout the cytoplasm. alpha-sm comprised up to 14% of total cellular actin as assessed by 2D gel electrophoresis. Thirteen different gingival and seven different periodontal ligament fibroblast lines constitutively expressed on alpha-sm actin. These cells exhibited up to 60% inter-line variations of fluorescence due to alpha-sm actin and up to 70% and 45% inter-line variation in the rate of collagen gel contraction. Quantitative, single cell fluorimetry of alpha-sm actin immunoreactivity demonstrated a linear relation between gel contraction and alpha-sm actin (correlation coefficients of 0.71 for gingival and 0.61 for periodontal ligament cells), but there was no detectable relationship between total actin content and gel contraction. In contrast, flow cytometry demonstrated that 99% of the total gated cells from cell lines exhibiting rapid gel contraction showed alpha-sm actin staining above background fluorescence as compared to only 35% of cells with slow rates of gel contraction. Contracting collagen gels stained with FITC-phalloidin showed cells with well-developed stress fibres that were progressively more compact and elongated during the time of maximal gel contraction. To examine the dependence of gel contraction on assembly of monomeric actin into actin filaments, cells were electroporated in the presence of phalloidin or cytochalasin D. Collagen gels exhibited up to 100% inhibition of gel contraction that was dose-dependent. Gel contraction was inhibited 93% by electroinjection of cells with alpha-sm actin antibody prior to incubation, but the antibody did not inhibit actin assembly after attachment and spreading on substrates. These data indicate that gel contraction is dependent on alpha-sm actin expression and that alpha-sm actin is a functional marker for a fibroblast subtype that rapidly remodels the extracellular matrix.

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.

Cellular colonization of denuded root surfaces in vivo: cell morphology in dentin resorption and cementum repair

Cellular colonization of denuded root surfaces on replanted teeth was examined with scanning electron microscopy. The dentin surface was rapidly colonized by a few macrophage-like cells which resorbed dentin in limited areas. They had well-developed ruffled borders but no clear zones. The resorptive activity was gradually inhibited by the ingrowth of a monolayer of fibroblast-like cells from the periphery of the denuded area. At the advancing front, the ingrowing fibroblast-like cells were long and slender, with long tail-like projections trailing the nuclear domain. The invading cells behind these were well-spread and tightly packed. They had a granulated surface and showed morphological signs of fibre synthesis. After 6 weeks, the entire cavity was covered by a monolayer of these cells. Removal of the cells with sodium hypochlorite revealed that a cementum-like tissue had been formed under the invading cells, indicating that they were cementoblasts. It was concluded that wound repair on the root surface is dependent upon the differentiation and proliferation of pre-cementoblasts in the periphery of the wound and that cells in the main body of the periodontal membrane are of little significance in the healing process on the root surface.