Role of fibroblast subpopulations in periodontal physiology and pathology

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.

Tissue engineering, morphogenesis, and regeneration of the periodontal tissues by bone morphogenetic proteins

Tissue engineering is the emerging field of science developing techniques for fabrication of new tissues for replacement based on principles of cell and developmental biology and biomaterials. Morphogenesis is the cascade of pattern formation and the attainment of form of the various organs and the organism as a whole. The periodontium consist of the periodontal ligament, cementum, and alveolar bone. Bone has considerable potential for regeneration and therefore is a prototypic model for tissue engineering. The three main ingredients for tissue engineering are regulatory signals, responding stem cells, and extracellular matrix. Recent advances in molecular biology of the bone morphogenetic proteins (BMPs) have set the stage for tissue engineering of bone and related tissues, including the periodontium. Bone-derived BMPs, with a collagenous matrix as carrier, induced cementum and alveolar bone regeneration in surgically created furcation defects in the primate. It is noteworthy that there was morphogenesis of periodontal ligament and a faithful insertion of Sharpey's fibers into cementum. In the same furcation model, recombinant human osteogenic protein-1 (rhOP-1, also known as BMP-7), in conjunction with the collagenous carrier, induced extensive cementogenesis with insertion of Sharpey's fibers into the newly formed cementum. The observation that BMPs induce cementogenesis and periodontal ligament formation indicates that these proteins may have multiple functions in vivo not limited to cartilage and bone induction. The rapid advances in the molecular biology of BMPs and their receptors bode well for novel strategies to engineer the regeneration of the periodontal tissues.

The effect of lipopolysaccharide on growth factor-induced mitogenesis in human gingival fibroblasts

Quiescent and non-quiescent human gingival fibroblasts (HGF) were incubated for 24 hours with Actinobacillus actinomycetemcomitans lipopolysaccharide (LPS) and/or growth factors (interleukin-1 beta [IL-1 beta], insulin, epidermal growth factor [EGF], platelet-derived growth factor [PDGF], fibroblast growth factor [FGF], and transforming growth factor-beta [TGF-beta]) to examine the ability of LPS to modify HGF proliferation in response to these autocrine and paracrine growth factors. A. actinomycetemcomitans LPS at high concentrations (> or = 9 micrograms/well) generally resulted in a reduction in DNA synthesis in quiescent and non-quiescent fibroblasts; however, LPS at low concentrations (< 9 micrograms/well) showed a minimal enhancement of DNA synthesis (40 to 60%) in quiescent and non-quiescent cells. HGF co-incubated with mitogenic agents and LPS (9 micrograms/well) exhibited suppression of growth factor-induced 3H-Tdr uptake compared to growth factor-stimulated controls. In contrast, 3H-Tdr uptake was slightly elevated with addition of LPS at low concentrations (0.09 microgram/well). These trends were seen with all growth factors tested. Non-quiescent cells, in general, were more responsive to the growth factors and LPS/growth factor combinations when compared to the quiescent HGF. HGF were further tested for the ability of LPS to alter growth factor responsiveness by pretreating the cells with LPS prior to incubation of the growth factor, as well as, subsequent addition of LPS to growth factor-pretreated cells. Similar patterns were observed as above, except IL-1 beta-pretreated quiescent and non-quiescent HGF followed by LPS addition demonstrated a marked elevation in proliferation when compared to IL-1 beta stimulated controls. These findings suggest that LPS may potentially modulate the proliferative rate of connective tissue undergoing inflammatory or growth factor-induced reparative processes in periodontal lesions.

Albumin gene expression during mouse odontogenesis

Albumin protein is present in developing teeth of several species. Oligomer primers and cRNA probes specific for albumin were designed to perform RT-PCR, and for in situ hybridization, respectively. In situ hybridization failed to reveal albumin expression in any tooth cells, however, albumin PCR products were amplified from tissues adhering to the roots of developing teeth from four-week-old mice. It is concluded that this source is not the primary source of albumin protein found in developing enamel, because of the location and level of expression of albumin mRNA in periodontal tissue.

Human granulation-tissue fibroblasts show enhanced proteoglycan gene expression and altered response to TGF-beta 1

Granulation-tissue fibroblasts are phenotypically unique cells that play an important role in wound repair and the development of chronic inflammatory lesions in connective tissue. In the present study, we compared proteoglycan, type I, and type III procollagen gene expression by granulation-tissue fibroblasts from wound and chronically inflamed tissues with normal gingival fibroblasts. We also analyzed the effect of TGF-beta 1 on proteoglycan mRNA levels and macromolecule production by these cells. One granulation-tissue fibroblast strain that was composed exclusively of alpha-smooth-muscle actin-positive cells (myofibroblasts) expressed strongly elevated basal levels of biglycan, fibromodulin, and versican (the large chondroitin sulphate proteoglycan), as well as type I and III procollagen mRNA. TGF-beta 1 enhanced more potently the expression of types I and III procollagen, biglycan, and versican mRNA by these cells as compared with normal fibroblasts. Other granulation-tissue fibroblast strains, in which about half of the cells expressed alpha-smooth-muscle actin, also showed enhanced proteoglycan and types I and III procollagen expression as compared with normal fibroblasts. These results suggest that alterations in matrix composition during inflammation and wound healing are regulated partly by altered phenotypes of the cells that produce the matrix, and partly by altered responses of these cells to TGF-beta 1.

Biochemical alterations in inflammatory periodontal diseases I. Poly (ADP-ribose) synthetase activity in gingiva and gingival fibroblasts from humans with periodontitis

Periodontal diseases are characterized in part by generation of oxygen free radicals, which can cause breaks in cellular DNA strands. Repair of damaged DNA is dependent upon the synthesis of poly (ADP-ribose)(PADPR) catalyzed by PADPR synthetase, an enzyme known to be activated by the broken ends of DNA strands. We measured the activities of PADPR synthetase and of PADPR glycohydrolase, which degrades PADPRS, in gingival biopsy specimens from 16 sites with adult periodontitis and 12 clinically healthy control sites. The results indicated that sites with periodontitis displayed markedly reduced PADPR synthetase activity compared with healthy control sites, whereas PADPR glycohydrolase activity was not changed. The mean specific activity of PADPR synthetase for the diseased specimens was one-sixth of that of the healthy specimens (p < 0.001). The PADPR synthetase activity was negatively correlated with the Gingival Index (rs = -0.60), pocket depth (rs = -0.70) and bleeding upon probing (rs = -0.72). Cultured fibroblasts derived from clinically characterized healthy and diseased gingival sites reflected similar patterns of enzyme activity. The mean specific activity of PADPR synthetase for the diseased-site cultures (n = 9) was 56 +/- 7% (p < 0.001) of the cultures from healthy control sites (n = 6). These results suggest that a reduced level of PADPR synthetase activity is associated with increased inflammation and periodontal destruction, and that the ability to synthesize PADPR is compromised in adult periodontitis.

Prostaglandin E2 potentiates interleukin-1 beta induced interleukin-6 production by human gingival fibroblasts

Increased levels of cytokines and prostanoids have been detected in inflamed gingival tissue and may play an important role in periodontal pathogenesis. Recent studies suggest that monocytic products, such as interleukin (IL)-1 beta, could stimulate IL-6 production by human gingival fibroblasts (HGF). In this context, the production of local cytokines and inflammatory mediators could regulate the secretory capacity of resident gingival fibroblasts. Therefore, the purpose of this study was to determine if PGE2 induced by IL-1 beta could potentiate the IL-6 response by HGF. Utilizing an ELISA, it was determined that maximal IL-6 occurred when HGF were stimulated with 0.10-10 nM IL-1 beta. These concentrations of IL-1 beta also induced a small, but significant increase in PGE2 production by HGF. Interestingly, the combination of IL gamma beta and PGE2 induced a synergistic rise in IL-6 production by HGF. Moreover, inclusion of indomethacin caused a 20% reduction in IL-6 production and totally eliminated PGE2 production. These findings provide additional rationale for the clinical use of NSAIDs in the management of periodontal disease due to their ability to attenuate production of both PGE2, and IL-6. These results suggest the endogenous PGE2 induced by IL-1 beta plays an important regulatory role in IL 6 production by HGF. Moreover, they support the concept that elevated PGE2induced during inflammation can regulate HGF secretory function.

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.

Cementogenesis reviewed: a comparison between human premolars and rodent molars

BACKGROUND

Cementum continues to be the least-known mineralized tissue. Although recent advances in the field of molecular biology have contributed to an understanding of the involvement of molecular factors in cementum formation during development and regeneration, cementogenesis on a cell biological basis is still poorly understood. Virtually nothing is known about cementoblast origin, differentiation, and the cell dynamics during normal development, repair, and regeneration. This review describes the recent findings of cementogenesis on roots of human premolars and opposes them to those of teeth from other mammals, particularly the rodent molar.

METHODS

Using light and electron microscopy, light microscopic radioautography, and various measurements, a comprehensive insight into the development and repair of cementum during and after root formation and tooth eruption has been achieved for human premolars.

RESULTS

Cementum is a highly responsive mineralized tissue. This biological activity is necessary for root integrity and for bringing and maintaining the tooth in its proper position. With regard to cementum formation and periodontal fiber attachment, considerable species-particularities exist that are mainly based on differences in growth rates and tooth sizes. Since root development and initial cementogenesis last on the average 5-7 years in human premolars, cementum formation in these teeth is characterized by along-lasting phase of prefunctional development, with occurs independent of principal periodontal fiber attachment to the root and which may take 5 years or more. The first molar of the rat, however, is in functional occlusion 3 1/2 weeks after the onset of root formation. Since initial cementum formation and periodontal fiber attachment to the root occur almost at the same time in this tooth, the distinction between cells associated with one or the other process is very difficult to achieve, and cementogenesis cannot be described independent of periodontal fiber attachment to the root. Therefore, the determination of cementoblast origin in the rodent molar may be intricate.

CONCLUSIONS

Taking into account these species differences, the current description on the origin and differentiation of cementoblasts is inconsistent and the description of cementogenesis is still incomplete. This review calls into question the currently held concept of cementogenesis and offers a possible alternative.

Wound healing following myocardial infarction

A wound-healing response that eventuates in fibrous tissue formation appears at the site of myocardial infarction (MI) in the affected ventricle. Fibrosis can likewise appear remote to the MI and cause an extensive structural remodeling of the myocardium of infarcted and noninfarcted ventricles. Substances involved in promoting healing at and remote to MI are of considerable interest and an important clinical issue, given that the healing response is subject to pharmacologic intervention. Angiotensin-converting enzyme (ACE) is expressed by wound-healing fibroblast-like cells; it likely serves to regulate local concentrations of angiotensin II and bradykinin involved in healing and matrix remodeling. Wound healing following MI and its regulation are addressed in this review.

Immunolocalization of basic fibroblast growth factor (bFGF) in human periodontal ligament (PDL) tissue

The distribution of basic fibroblast growth factor (bFGF) in periodontal ligament (PDL) tissue was investigated in samples which were obtained from freshly extracted human teeth. The PDL tissue was collected by scraping, and bFGF was identified and localized by immunohistochemistry. Fibroblasts, endothelial cells, some fibrocytes and extracellular matrix (ECM) stained positively for bFGF. It was observed that cells from healthy PDL stained more intensely than those from PDL of teeth associated with chronic periodontitis; histological cell counts revealed that the numbers of fibroblasts was greater (p < or = 0.0005) in healthy PDL than in diseased PDL tissue. The results of this study show that bFGF is produced primarily by PDL fibroblasts and endothelial cells in the PDL and that bFGF levels may be decreased in tissue associated with chronic periodontal lesions.

Isolation of alkaline phosphatase-positive gingival fibroblasts from patients with chronic inflammatory periodontal disease

We have reported recently that increased expression of membrane alkaline phosphatase (ALP) activity is a phenotypical characteristic of gingival fibroblasts located in chronic inflammatory periodontal lesions. To understand the cellular properties of these cells, we isolated ALP-positive gingival fibroblasts from patients with adult periodontitis and evaluated their proliferative potential. Using an enzymatic digestion procedure, we prepared gingival cell suspensions containing ALP-positive fibroblasts without affecting their ALP activities. These cell suspensions were then subjected to 1 g sedimentation, followed by allowing cells to adhere to substrata. Using this procedure, 71.9% of isolated cells were ALP-positive. Dissociation of ALP-positive fibroblasts and contamination by non-fibroblastic cells were examined by cytochemical and immunocytochemical analyses. The proliferative capacity of ALP-positive fibroblasts in culture was assessed by monitoring the proportion of ALP-positive cells after repeated subculture passages and by labelling DNA-synthesizing cells with bromodeoxyuridine (BrdU). The proportion of ALP-positive fibroblasts decreased during cell culture passages without an apparent change in the ALP-positive phenotype. The percentage of BrdU-positive cells was significantly lower among ALP-positive than among ALP-negative fibroblasts. These results indicate that ALP-positive fibroblasts in chronic inflammatory periodontal lesions have low growth potential. We suggest that their reduced capacity to grow in vitro reflects a more differentiated state induced under inflammatory conditions in vivo.

Growth factor modulation of fibroblasts in simulated wound healing

Growth factors are potent bioactive molecules responsible for the co-ordination of many cells functions and interactions. Of these agents PDGF and IGF have shown particular promise as agents which may be used to stimulate periodontal regeneration. In order to further understand the mechanisms by which growth factors may work, a simple model of in vitro wound healing has been utilized to assess the effects of PDGF on human periodontal ligament fibroblasts and its potential to stimulate wound healing. Human periodontal ligament fibroblasts were plated into 24-well plates and upon reaching confluence were wounded by creating uniform discoid lesions stripped of cells. The influence of various concentrations of PDGF on cell proliferation, cell migration and extracellular matrix synthesis was monitored. The results of this study indicated that in the presence of 10 ng/ml PDGF on 0.2% fetal calf serum, both cell proliferation and cell migration were significantly stimulated. In the wounded cultures, PDGF appeared to cause a moderate stimulation of proteoglycan synthesis compared to unwounded cultures. In conclusion, the model system tested appears to be useful for studying fundamental cellular and biochemical events associated with wound healing. The effects of PDGF in this system confirm that it is capable of modulating fibroblasts in a manner compatible with the events associated with wound repair.

Functional characteristics of gingival and periodontal ligament fibroblasts

In periodontal surgery, healing after guided tissue regeneration (GTR) may be explained by differences in functional activities of gingival and periodontal ligament fibroblasts (GF and PDLF). Several studies in vitro have supported this hypothesis, but much remains to be defined. In the present work, gingival and periodontal ligament fibroblasts derived from five healthy subjects were isolated and compared in vitro. The morphology of the cells was observed under scanning electron microscopy (SEM). Several extracellular matrix components (ECM) were studied to compare the effects on fibroblast attachment, proliferation, and protein synthesis. Several biochemical markers were examined in both cellular extract (CE) and conditioned medium (CM). We also examined the muscle differentiation markers alpha-smooth muscle actin, desmin, and smooth-muscle myosin. Finally, we studied the effects of epithelial cells on the proliferation and protein synthesis of the two types of fibroblasts. GF and PDLF appeared identical under the SEM. All ECM components enhanced attachment; however, while collagen types I and IV promoted the attachment of GF, gelatin, laminin, and vitronectin promoted that of PDLF. Most ECM components increased the proliferation rate of GF and the biosynthetic activity of PDLF. The biochemical markers were similarly distributed between the two cell types, except for alkaline phosphatase, which was detected only in the CE of PDLF. Both GF and PDLF strongly expressed alpha-smooth-muscle actin and were negative for desmin; only PDLF were positive for smooth-muscle myosin. Epithelial cells increased the proliferation of both GF and PDLF but had no effect on their biosynthetic activity. These in vitro results may better explain the in vivo functional differences between GF and PDLF.

Loss of attachment in the marginal periodontium of the rat incisor under non-inflammatory conditions. Expression of alkaline phosphatase activity. Experimental Oral Biology Group

Alkaline phosphatase (ALP) has been suggested to play a role in acellular cementum formation and maintenance of periodontal attachment. In an attempt to determine whether changes in attachment level are associated with altered expression of ALP-activity in the periodontium we induced natural loss of attachment in rats by pinning the lower incisor to the jaw bone. Previous studies have shown that this procedure results in regressive changes in the marginal periodontium without any inflammatory response. Six months after blockage of eruption the attachment level on the experimental (right) side had shifted about 700 microm in the apical direction. On the control (left) side the apical termination of the junctional epithelium had remained stationary with respect to the alveolar crest. Our observations have shown that during the first few weeks of the experiment loss of attachment is accompanied by considerable reduction of ALP-activity in the supracrestal part of the periodontium. At later time intervals, however, no distinct relation was found between apical migration of junctional epithelium and loss of ALP-activity in the supracrestal region, indicating that the two phenomena are not directly related to each other. The domain of the ALP-positive fibroblasts in the supracrestal extension of the periodontal ligament decreased in size and was replaced by ALP-negative connective tissue cells probably coming from the outer gingival domain. Since at all time intervals a distinct demarcation could be observed between the ALP-positive and ALP-negative areas, we interpret our data as indicating that ligament and gingival cells do not mix.

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.

Human periodontal ligament and gingival fibroblast response to TGF-beta 1 stimulation

The purpose of this study was to measure the time-sequence response of RNA and protein synthesis to transforming growth factor-beta 1 (TGF-beta 1) by human periodontal ligament (HPDLF) and gingival (HGF) fibroblasts in culture. HPDLF and HGF were cultured from explants of healthy gingival tissue and freshly extracted teeth. Cultures of 8 x 10(4) cells/ml were exposed to medium containing 3H-uridine and 35S-methionine with TGF-beta 1 at concentrations from 10(-9) M to 10(-21) M, or control medium, for up to 60 hours in order to assess RNA and protein synthesis. Protein concentrations of comparable cultures were also assayed colorimetrically. Results were reported as specific activity (CPM/microgram protein). The results indicate that 10(-9) M TGF-beta 1 treated cultures showed a significant increase in RNA synthesis by HPDLF and HGF over time, as compared to the control cultures. HPDLF showed a significant increase in protein synthesis over time while that by HGF was not significant as compared to the control cultures. Lower concentrations of TGF-beta 1 demonstrated no significant differences from control. Results suggest that the effects of TGF-beta 1 on HPDLF and HGF are both time and dose dependent, with 10(-9) M TGF-beta 1 providing the best response of those concentrations tested. These findings support the concept that TGF-beta 1 may play a role in periodontal regeneration due to its ability to promote fibroblast RNA and protein synthesis. The results also demonstrate that although these two cells types appear morphologically similar, they exhibit distinct biological responses to growth factors such as TGF-beta 1.

Migration and proliferation of progenitor cells in the connective tissue of rat gingival papilla

The aim of this investigation was to determine whether gingival fibroblast progenitor cells undergo division and whether there is evidence of migration of their progeny from the paravascular sites within the interdental gingival septum (IGS). 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 transverse Historesin sections of the papilla between second and third mandibular molars in 29 specimens, taken from equidistant intervals between the col and alveolar bone crest. Median grain counts in the whole papilla were greatest within 10 microns of blood vessels at 3 h (p < 0.02) and decreased throughout the papilla with time (p < 0.0001). The median grain counts at level 5, nearest the periodontal ligament, were greatest at all times (p < 0.01). The distance of labelled nuclei from blood vessel walls increased with time, most notably at level 5 (p < 0.001). The mean Clustering Index increased (p < 0.01) from 4.6% at 3 h to 12.3% and 11.6% at 75 h and 171 h respectively. At 75 h, the median grain count of clustered nuclei was smaller (p < 0.01) than that of non-clustered nuclei. These data are consistent with the occurrence of clonal division and migration within the IGS and the presence of a slowly cycling cell population near to the periodontal ligament.

Dexamethasone enhances the effects of parathyroid hormone on human periodontal ligament cells in vitro

Periodontal ligament cells (PDL) are thought to play a major role in promoting periodontal regeneration. Recent studies, focused on characterizing PDL cells, have been directed at establishing their osteoblast-like properties and determining biological mediators and/or factors that induce osteoblastic cell populations in the PDL. The glucocorticoid, dexamethasone (Dex), has been shown to selectively stimulate osteoprogenitor cell proliferation and to induce osteoblastic cell differentiation in many cell systems. In the present study the ability of Dex to modulate parathyroid hormone (PTH)-stimulated cAMP synthesis in cultured human PDL cells was examined. PDL cells, obtained from premolar teeth extracted for orthodontic reasons, were cultured with Dex (0-1000 nM) for 7 days prior to PTH (1-34) stimulation. The exposure of PDL cells to Dex resulted in a dose-dependent increase in cAMP production in response to PTH stimulation. This response was seen in cells obtained from three different patients. The first significant Dex effect was seen on day 7 when compared to day 1 for 100 nM Dex. PTH (1-34) stimulation caused a dose-dependent increase in cAMP synthesis after Dex (1000 nM) treatment for 7 days. Conversely, stimulation of the cells with PTH (7-34) (0-1000 nM) did not increase cAMP production in PDL cells after Dex treatment. Forskolin- (1 microM) and isoproterenol- (1 microM) stimulated cAMP synthesis was not augmented by Dex treatment. Dex treatment did not alter calcitonin-(1 microM) stimulated cAMP production in PDL cells. Glucocorticoid enhancement of PTH-stimulated cAMP synthesis in these cells supports the presence of an osteoblast-like population in the PDL, in vitro.

Immunohistochemical characterisation of odontogenic cysts with mesenchymal and myofilament markers

The aim of the present investigation was to analyse the fibroblast phenotype in the main groups of odontogenic cysts. We have examined the muscle differentiation markers alpha-smooth muscle actin (ASMA) and desmin and the presence of vimentin using immunocytochemistry. Fibroblasts of other cyst types, hyperplastic fibrous lesions, and from several normal sites were used as controls. All fibroblasts stained for vimentin, but a variable proportion of cyst wall fibroblasts were also positive for the antibody to ASMA. Their distribution was not random and two positive zones could be distinguished, an inner subepithelial layer and an outer region adjacent to the bone-facing surface. This staining pattern was observed for all the types of odontogenic cysts studied, although the subepithelial staining was more frequent in the presence of an inflammatory infiltrate. The contractile nature of these cells was supported by their ultrastructural appearance. Some non-odontogenic cysts showed subepithelial fibroblasts strongly labelled with ASMA; in the control tissues ASMA was negative except for pericytes and some groups of cells in supracrestal gingiva. These results demonstrate a distinct cell population with a myofibroblast phenotype in the odontogenic cyst wall and with a distribution which may enable them to contribute to wall elasticity constraining cyst expansion. Their presence in all types of odontogenic cyst studied would suggest that myofibroblast formation is independent of the pattern of cyst growth.

Differentiation of periodontal ligament fibroblasts into osteoblasts during socket healing after tooth extraction in the rat

BACKGROUND

The entire socket after tooth extraction is filled with new bone formed by osteoblasts (Obs), but the origin of these Obs remains unknown. Thus, the proliferation and migration of paravascular and endosteal fibroblastic cells and periodontal ligament (PDL) fibroblasts (Fbs) and their differentiation into Obs during socket healing after extraction of the first maxillary molars of the rat were investigated.

METHODS

The proliferative activity and migration of these cells in the sockets after tooth extraction were studied using radioautography and immunohistochemistry after injection of 3H-thymidine and 5-bromo-2'-deoxyuridine (BrdU), respectively. Their morphological changes during differentiation was investigated by transmission electron microscopy.

RESULTS

One day after tooth extraction, PDL Fbs were the major cell type in the PDL remnant of the socket. Proliferation was low (labeling index (LI) = approximately 2%) until 16 h after tooth extraction but dramatically increased to a maximum level 1 day postextraction (LI = 23%). Between 1 and 2 days, numerous PDL Fbs in the PDL remnant actively migrated into the coagulum and continued to proliferate. On the basis of the high proliferative activity and small number of cellular organelles responsible for procollagen synthesis, these cells appear immature. At 3 days, Fbs contained more cellular organelles and deposited more collagen fibers as they replaced the coagulum with dense connective tissue and the LI declined. At 4 and 5 days, some of the Fbs began to differentiate into Obs, and the proliferation of Fbs dramatically decreased to baseline values. The migration of PDL Fbs and their differentiation into Obs were investigated by labeling with 3H-thymidine or BrdU 1 day after tooth extraction. Heavily labeled Fbs were observed in the PDL remnant at 1 day, in the coagulum at 2 days, and in the dense connective tissue at 3 days. Labeled Obs associated with new bone were seen 4 days after injection. Endosteal and paravascular Fbs also proliferated, but at a lower level and at later time periods than the PDL Fbs. Surprisingly, endosteal and paravascular Fbs contributed only a small population of Fbs to socket healing.

CONCLUSIONS

These results indicate that PDL Fbs after tooth extraction actively proliferative, migrate into the coagulum, form dense connective tissue, and differentiate into Obs which form new bone during socket healing.

Alterations of fibroblast metabolism in early ligature-induced periodontitis in the cynomolgus monkey

Cellular and biochemical observations were made of fibroblasts harvested from ligature-induced periodontitis and treated gingivitis areas in four adult female cynomolgus monkeys (Macaca fascicularis) to define the changes that occur in the early periodontitis lesion. Compared with fibroblasts from the treated sites, fibroblasts from the diseased areas had a significantly higher rate of proliferation, produced about two-thirds the amount of total protein and collagen, and failed to respond to TGF-beta, which normally stimulates extracellular matrix formation in mesenchymal cells. The diseased cells were also deficient in the activity of poly(ADP-ribose) synthetase, an enzyme involved in the repair of DNA breaks such as occur from the insults of superoxide and other active radicals present in inflamed areas. Although the precise nature of these biochemical defects is not fully elucidated, they may have an important bearing on chronic periodontitis cases with a "downhill" course.

Periodontal regeneration: potential role of bone morphogenetic proteins

Initiation of osteogenesis and cementogenesis is a problem central to periodontal regeneration. A major advance in the understanding of bone formation has been the identification of an entirely new family of protein initiators, the bone morphogenetic proteins, that regulate cartilage and bone differentiation in vivo. The purification, genetic cloning and expression of recombinant human bone morphogenetic proteins (BMPs) have laid the foundation for the cellular and molecular dissection of bone development and regeneration. The striking evolutionary conservation of the BMP genes indicates that they are critical in the normal development and function of animals. In addition to postfetal osteogenesis, the BMPs may play multiple roles in embryonic development and organogenesis, including skeletogenesis and the development of craniofacial and dental tissues. The availability of recombinant human BMPs provides several challenges and opportunities to gain insights into the mechanisms regulating the regeneration of bone and cementum for optimal outcome in the periodontal patient.

Expression of membrane alkaline phosphatase activity on gingival fibroblasts in chronic inflammatory periodontal disease

We investigated the expression of membrane alkaline phosphatase (ALP) activity on fibroblasts in inflamed gingiva from 7 patients with adult periodontitis. ALP activity was ultrahistochemically detected by a cerium-based capture method. The degree of ALP activity was estimated by morphometric analysis of the percentage of the perimeter on which ALP reaction product was deposited. Fibroblasts in the non-inflammatory connective tissue were surrounded by bundles of collagen fibrils, and the majority of these fibroblasts showed ALP-negative or weakly ALP-positive reaction. By contrast, fibroblasts in the inflammatory connective tissue were either surrounded by a non-collagenous substance or in contact with inflammatory cells, and the majority of these fibroblasts showed a strong ALP-positive reaction. These results suggest that the expression of membrane ALP activity on gingival fibroblasts is induced by microenvironmental changes associated with the loss of contact between the cells and the extracellular collagenous matrix during inflammatory reactions.

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.

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.

Effects of hormones and cytokines on stimulation of adenylate cyclase and intracellular calcium concentration in human and canine periodontal-ligament fibroblasts

Adenylate cyclase was stimulated by prostaglandin E2 (PGE2) and parathyroid hormone-related protein (PTHrP) in both these types of fibroblast and by calcitonin gene-related protein (CGRP) in the human fibroblasts in vitro. PGE2 (1 microM), CGRP (1 microM), and PTHrP (1 microM) stimulated adenylate cyclase up to 50-fold, 10-fold and 9-fold, respectively. Calcitonin (CT), substance P (SP), interleukin-1 beta (IL-1 beta), and transforming growth factor-beta 1 (TGF beta 1) had no effect on adenylate cyclase in either fibroblast. Intracellular Ca2+ (iCa2+) was measured in individual fibroblasts from the periodontal ligament using Indo-1 and an adherent cell analysis and sorting interactive laser cytometer. Ionomycin (3 microM) caused a transient rise of iCa2+ in all human and canine fibroblasts tested. The mean percentage increase in iCa2+ in response to ionomycin was 820 and 840% for human and canine fibroblasts, respectively. The human fibroblasts responded to PGE2 (1 microM) by an increased iCa2+ concentration; the mean percentage increase in iCa2+ was 187%. SP caused a less pronounced increase in iCa2+ in the human fibroblasts (56%). CGRP and SP caused a similar response in the canine fibroblasts. The mean percentage increase in iCa2+ in response to SP and CGRP was 95 and 78%, respectively. PTH, PTHrP, platelet-activating factor, CT, and IL-1 beta had no effect on iCa2+ in either type of fibroblast. The data indicate that cAMP and calcium have roles as intracellular secondary messengers in the action of PGE2, SP, CGRP, and PTHrP in fibroblasts of human and canine periodontal ligament.

Deficiencies in collagen phagocytosis by human fibroblasts in vitro: a mechanism for fibrosis?

Degradation of collagen by fibroblast phagocytosis is an important pathway for physiological remodelling of soft connective tissues. Perturbations of this pathway may provide a mechanism for the development of fibrotic lesions. As collagen phagocytosis may be regulated by either a change of the proportions or the activity of phagocytic cells, we quantified phagocytosis with an in vitro model system. Collagen-coated fluorescent latex beads were incubated with human gingival fibroblasts and the fluorescence associated with internalized beads was measured by flow cytometry. Cells from normal tissues that had been incubated with beads for 3 hours contained a mean of 64% phagocytic cells; however, a small subpopulation (10% of phagocytic cells) contained more than threefold higher numbers of beads per cell than the mean. In contrast, cells from fibrotic lesions exhibited a large reduction of the proportions of phagocytic cells (mean = 13.8%) and there were no cells with high numbers of beads. On the basis of 3H-Tdr labeling, cells from fibrotic lesions that had internalized beads failed to proliferate, in contrast to phagocytic cells from normal tissues, which underwent repeated cell divisions. This result was not due to variations of cell cycle phase as there was no preferential internalization of beads during different phases of the cell cycle. The low phagocytic rate of cells from fibrotic lesions was also not due to asymmetric partitioning of phagosomes at mitosis as videocinemicrography of bead-labeled phagosomes in single, pre-mitotic cells demonstrated that > 90% of phagocytic cells equally partitioned beads to daughter cells. To investigate if inhibition of phagocytosis could be replicated in vitro, cells were incubated with the fibrosis-inducing drugs nifedipine or dilantin. These cultures exhibited marked (15-75%), dose-dependent reductions in the proportions of phagocytic cells, but there was no reduction in bead number per cell. Fibrotic lesions appear to contain fibroblasts with marked deficiencies in phagocytosis and the reduced phagocytic activity of these cells may contribute to unbalanced degradation and fibrosis.

Disturbances of gingival fibroblast population homeostasis due to experimentally induced inflammation in the cynomolgus monkey (Macaca fascicularis): potential mechanism of disease progression

We have studied the relationship between perturbations of fibroblast turnover in inflamed gingiva of different severities. To perform detailed spatial analyses of gingival fibroblast progenitor cells, inflammatory cell infiltrates and blood vessels, 3 Cynomolgus monkeys with healthy periodontium and 2 with naturally occurring gingivitis and ligature-induced periodontitis were pulse-labeled with 3H-thymidine. Morphometric analyses of radioautographs from mid-sagittal supra-alveolar gingival connective tissues of incisors were performed in sites subjacent to junctional sulcular and oral epithelium, in the body of the lamina propria and just superior to the alveolar crest. The percentage of fibroblasts incorporating 3H-thymidine label, expressed as the labeling index (LI), was higher subjacent to the sulcular epithelium in periodontitis (1.73 +/- 0.37) than in healthy sites (1.06 +/- 0.22). This was not statistically significant (0.05 < p < 0.1) due to the small number of animals used. The sites subjacent to the sulcular epithelium also exhibited the largest increase in lymphocyte density from health to gingivitis (p < 0.01). In contrast, the LI of fibroblasts subjacent to the oral epithelium was 5-fold higher in healthy (0.82 +/- 0.17) compared to periodontitis sites (0.13 +/- 0.09; p < 0.05). Labeled fibroblasts were found close to blood vessels in all compartments and in all disease states; distance to blood vessels was reduced in inflamed sites (p < 0.10). There were increased numbers of blood vessels per unit area in the lamina propria of gingivitis compared to healthy sites. However, there were no regional differences with respect to blood vessel numbers or area in sites subjacent to junctional epithelium with different disease states. The results indicate that: 1) experimentally-induced inflammation in the gingiva of Cynomolgus monkeys is associated with site-specific perturbations of cell turnover; 2) fibroblast progenitors are preferentially situated adjacent to blood vessels as in the periodontal ligament; 3) the vascular response to inflammation is a generalized increase in blood vessel numbers, but not their size; 4) reactive proliferation of fibroblasts may compensate for cell death in the lamina propria but is not detectable at the site of connective tissue attachment loss subjacent to the junctional epithelium. Failure to maintain the fibroblast progenitor population may be an important component of attachment loss in progressive periodontitis lesions.

Expression and function of gingival fibroblast C1q receptors are upregulated by interleukin-1 beta and transforming growth factor-beta

In injury and inflammation, complement (C) component C1q, in addition to its central role in initiation of classical pathway of complement activation, modulates diverse cellular functions by binding to specific cell surface receptors. Interaction of substrate-bound C1q with receptors for the collagen-like domain of C1q (C1qRC) of human gingival fibroblasts (HGF) promotes cell attachment. We investigated modulation of the adhesive function and expression of C1qRC by interleukin-1 beta (IL-1 beta) and transforming growth factor-beta (TGF-beta). Confluent fibroblast monolayers were incubated under standard culture conditions with or without cytokines. C1qRC function was measured by attachment assays. IL-1 beta and TGF-beta increased fibroblast adhesion to C1q to 146% and 131% of controls, respectively. Cytokine enhancement of HGF adhesion was concentration-dependent, saturable (20 ng/ml IL-1 beta; 1 ng/ml TGF-beta) and time-dependent (IL-1 beta 12-hr peak; TGF-beta 24-hr peak). Effect of IL-1 beta and TGF-beta on C1qRC expression was assessed by flow cytometry measurements of fluorescence intensity of cells stained with C1q and FITC anti-C1q antibody, and by binding studies with 125I-C1q. Cells treated with cytokines displayed a two- to four-fold increased fluorescence of cell-bound C1q compared to controls. Binding studies indicated the increased fluorescence correlated with increase in number of C1qRC in both IL-1 beta (4.7 x 10(6)/cell) and TGF-beta (3.9 x 10(6)/cell)-treated cells, compared to control (3.0 x 10(6)/cell), but had no effect on binding affinity. Rates of internalization of receptor-bound C1q were similar in cytokine-treated cells and controls. We propose from these data that IL-1 beta and TGF-beta have the ability to upregulate C1qRC expression, and this effect contributes to increased adhesion of HGF to substrate-bound C1q.

Bone-like nodules formed in vitro by rat periodontal ligament cells

The periodontal ligament has been shown to possess the ability to regenerate both new cementum and alveolar bone as well as a self-regenerative capacity; however, the source of cementoblasts and osteoblasts is not still clear. We investigated the development of bone-like tissue in vitro by periodontal ligament cells, in order to determine whether the periodontal ligament contains osteoprogenitor cells. Periodontal ligament cells were obtained from periodontal ligament tissue attached to the maxillary incisors of 6-week-old WKA rats by means of the explant technique. Cells at passage #3 were cultured for long term in alpha-minimum essential medium containing 10% fetal bovine serum, antibiotics, and 50 micrograms/ml ascorbic acid, and were then examined using phase-contrast microscopy, histochemistry, transmission electron microscopy, X-ray microanalysis, and electron diffraction. Nodules were formed in the cultures, and when 10 mM Na-beta-glycerophosphate was added, these nodules became mineralized. The mineralized nodules were identified as bone-like elements in view of the presence of osteoblast-like and osteocyte-like cells, collagenous matrix, a mineral composed of hydroxyapatite, and intense alkaline phosphatase activity. The results show that the periodontal ligament contains osteoprogenitor cells, which differentiate into osteoblasts and produce bone-like tissue.

Mineralized tissue-formation in periodontal wound healing

The purpose of the present study was to examine and compare the different mineralized tissues that are found on and around the dental root following treatment of different periodontal pathosis. The material has been compiled from previously published experimental studies on periodontal therapies and trauma treatments. 4 distinctly different appearances of the mineralized tissue layers on the marginal dentin surfaces were described; new cementum, non-attached bone-like tissue, partly attached bone-like tissue and ankylosis preceded by root resorption. It was concluded that, healing in the periodontal/root interface following periodontal therapy may yield different mineralized tissues, depending on a number of host-specific and external factors. The temporal pattern of such healing processes is schematically represented.

Endodontic infection and calcium hydroxide-treatment. Effects on periodontal healing in mature and immature replanted monkey teeth

Large experimental defects with marginal communication, and small isolated experimental defects were created on the root surfaces of extracted monkey lateral incisors with either open or closed apices. The pulp tissue was either infected or removed, and calcium hydroxide placed in the root canal. The teeth were then replanted and the healing pattern evaluated histomorphometrically after 20 weeks. From the results, it was concluded that (1) an intrapulpal infection promotes marginal epithelial down-growth on a denuded dentin surface irrespective of tooth developmental stage, and that (2) the periodontal healing potential after calcium hydroxide-treatment appears to be higher in teeth with open apices compared with teeth with closed apices, where ankylosis was promoted as opposed to teeth with open apices where significantly more reparative cementum was found.

Alpha-smooth muscle actin is expressed in a subpopulation of cultured and cloned fibroblasts and is modulated by gamma-interferon

Clinical and experimental investigations have shown that, during wound healing and fibrocontractive diseases, fibroblasts acquire, more or less permanently according to the situation, morphological and biochemical features of smooth muscle (SM) cells including the expression of alpha-SM actin. Primary and passaged cultures of rat and human fibroblasts contain a subpopulation of cells expressing alpha-SM actin. These cells could derive from SM cells and/or pericytes present in the tissue from which cultures have been produced or represent bona fide fibroblasts. We have investigated the presence of alpha-SM actin in fibroblast cultures, clones, and subclones. In all cases the fibroblastic populations studied showed a proportion of alpha-SM actin expressing cells. Even after cloning, we never obtained populations negative for alpha-SM actin. We conclude that alpha-SM actin expression in fibroblastic cultures is not due to contaminant cells but is a feature of fibroblasts themselves. Our results support the view that fibroblastic cells are a heterogeneous population. It has been previously shown that gamma-interferon (gamma-IFN) decreases alpha-SM actin expression in SM cells. In rat and human fibroblasts, gamma-IFN decreases alpha-SM actin protein and mRNA expression as well as proliferation. The properties of this cytokine make it a good candidate for exerting an anti-fibrotic activity in vivo.