Immunolocalization of transforming growth factor beta in rat molars

Expression of Clu and Tgfb1 during murine tooth development: effects of in-vivo transfection with anti-miR-214

Expression of clusterin (Clu) in the murine first molar tooth germ was markedly increased at postnatal developmental stages. The time-course of expression of this gene paralleled those of other genes encoding proteins involved during the secretory phase of odontogenesis, as described previously. Immunohistochemical studies of clusterin in murine molar tooth germs suggested this protein to be located in outer enamel epithelium, regressing enamel organ, secretory ameloblasts, and the dental epithelium connecting the tooth to the oral epithelium at an early eruptive stage. Immunolabelling of transforming growth factor beta-1 (TGF-β1) revealed it to be located close to clusterin. The levels of expression of Clu and Tgfb1 were markedly decreased following in-vivo transfection with anti-miR-214. In contrast, the expression of several genes associated with regulation of growth and development were increased by this treatment. We suggest that clusterin has functions during secretory odontogenesis and the early eruptive phase. Bioinformatic analysis after treatment with anti-miR-214 suggested that, whilst cellular activities associated with tooth mineralization and eruption were inhibited, activities associated with an alternative developmental activity (i.e. biosynthesis of contractile proteins) appeared to be stimulated. These changes probably occur through regulation mediated by a common cluster of transcription factors and support suggestions that microRNAs (miRNAs) are highly significant as regulators of differentiation during odontogenesis.

Chronology and regulation of gene expression of RANKL in the rat dental follicle

Tooth eruption in the rat requires bone resorption resulting from a major burst of osteoclastogenesis on postnatal day 3 and a minor burst of osteoclastogenesis on postnatal day 10 in the alveolar bone of the first mandibular molar. The dental follicle regulates the major burst on postnatal day 3 by down-regulating its osteoprotegerin (OPG) gene expression to enable osteoclastogenesis to occur. To determine the role of receptor activator of nuclear factor-kappa B ligand (RANKL) in tooth eruption, its gene expression was measured on postnatal days 1-11 in the dental follicle. The results show that RANKL expression was significantly elevated on postnatal days 9-11 in comparison to low expression levels at earlier time-points. As OPG expression is high at this latter time-point, this increase in RANKL expression would be needed for stimulating the minor burst of osteoclastogenesis. Tumor necrosis factor-alpha enhances RANKL gene expression in vitro and it may be responsible for up-regulating RANKL in vivo. Transforming growth factor-beta1 and interleukin-1alpha also enhance RANKL gene expression in vitro but probably have no effect in vivo because they are maximally expressed early. Bone morphogenetic protein-2 acts to down-regulate RANKL expression in vitro and, in vivo, may promote alveolar bone growth in the basal region of the tooth.

In vivo expression of RANKL in the rat dental follicle as determined by laser capture microdissection

Tooth eruption is a localized event in which many of the genes required for eruption are expressed in the dental follicle. A major function of the follicle is to recruit mononuclear cells for osteoclastogenesis such that the alveolar bone can be resorbed. Osteoclastogenesis is primarily regulated by receptor activator of nuclear factor-kappa B ligand (RANKL), colony-stimulating factor-one (CSF-1) and osteoprotegerin (OPG). In the rat first mandibular molar, osteoclastogenesis is maximal at day 3 and CSF-1 is maximally expressed in the follicle at this time whereas OPG expression is reduced. Whether or not RANKL is expressed in vivo in the follicle is controversial, however. It is critical to determine this because others have shown that in partially-rescued mice null for RANKL, teeth do not erupt. This suggests that RANKL should be expressed in the follicle for eruption to occur. Thus, to precisely determine if RANKL is expressed in the follicle in vivo, laser capture microdissection (LCM) was used to excise dental follicle tissue from frozen sections followed by RNA isolation and RT-PCR. The results show that RANKL is expressed in the dental follicle at days 1-9 postnatally. The technique was confirmed by controls showing that LCM isolates of the follicle, and alveolar bone, express OPG. Also, LCM isolates of alveolar bone were positive for RANKL. Thus, RANKL has now been shown to be expressed in the follicle and it is probable that interactions between it, CSF-1 and OPG regulate locally the osteoclastogenesis needed for tooth eruption.

TGF beta 1 signaling and stimulation of osteoadherin in human odontoblasts in vitro

Transforming growth factor beta 1 (TGF beta 1) is generally considered to be a potent inducer of dentin formation. In order to further assess this role, we studied the influence of this factor in human dental pulp cells on the expression of osteoadherin (OSAD), a newly described proteoglycan found in bone and dentin and suspected to play a role in mineralization events. We performed TGF beta 1 stimulation both in cultures of human tooth thick slices including mature odontoblasts and in pulp explant cultures giving rise to early secretory odontoblasts or pulpal fibroblasts. We first showed by immunohistochemistry that molecules involved in TGF beta 1 signal transduction, that is, membrane receptors T beta RI and T beta RII and intracellular proteins SMAD-2, SMAD-3, and SMAD-4, were present in human dental cells in vivo and were all maintained after culture of thick-sliced teeth in cells undergoing TGF beta 1 stimulation. In this culture system, OSAD synthesis was increased in mature odontoblasts close to the TGF beta 1 delivery system. In explant cultures, semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) analysis indicated that the growth factor stimulated OSAD gene expression in early secretory odontoblasts and in pulpal fibroblasts. Taken together, these results indicate that OSAD expression is stimulated by TGF beta 1 in pulpal fibroblasts and in early secretory and mature odontoblasts. We suggest that TGF beta 1 in this way could control the organization and the mineralization of the extracellular matrix deposited by these cells during dentin formation.

Cellular, molecular, and genetic determinants of tooth eruption

Tooth eruption is a complex and tightly regulated process that involves cells of the tooth organ and the surrounding alveolus. Mononuclear cells (osteoclast precursors) must be recruited into the dental follicle prior to the onset of eruption. These cells, in turn, fuse to form osteoclasts that resorb alveolar bone, forming an eruption pathway for the tooth to exit its bony crypt. Some of the molecules possibly involved in the signaling cascades of eruption have been proposed in studies from null mice, osteopetrotic rodents, injections of putative eruption molecules, and cultured dental follicle cells. In particular, recruitment of the mononuclear cells to the follicle may require colony-stimulating factor-one (CSF-1) and/or monocyte chemotactic protein-1 (MCP-1). Osteoclastogenesis is needed for the bone resorption and may involve inhibition of osteoprotegerin transcription and synthesis in the follicle, as well as enhancement of receptor activator of NF kappa B ligand (RANKL), in the adjacent alveolar bone and/or in the follicle. Paracrine signaling by parathyroid-hormone-related protein and interleukin -1 alpha, produced in the stellate reticulum adjacent to the follicle, may also play a role in regulating eruption. Osteoblasts might also influence the process of eruption, the most important physiologic role likely being at the eruptive site, in the formation of osteoclasts through signaling via the RANKL/OPG pathway. Evidence thus far supports a role for an osteoblast-specific transcription factor, Cbfa1 (Runx2), in molecular events that regulate tooth eruption. Cbfa1 is also expressed at high levels by the dental follicle cells. This review concludes with a discussion of the several human conditions that result in a failure of or delay in tooth eruption.

Cellular events at the onset of physiological root resorption in rabbit deciduous teeth

For elucidation of how physiological root resorption of deciduous teeth is initiated, the cellular events that occur surrounding the root of rabbit deciduous teeth before and at the onset of physiological root resorption were observed by means of light and electron microscopy. In addition, the cytodifferentiation of odontoclasts during the initial phase of this root resorption was evaluated by histochemical staining of tartrate-resistant acid phosphatase (TRAP) activity as a marker odontoclasts and their precursors. The present investigation was focused on the physiological root resorption of the deciduous lower second molar of rabbits from Day 0-5 postnatally. At birth, the deciduous molar had not erupted yet, and no TRAP-positive cell could be found surrounding the tissue adjacent to the root of the deciduous tooth. TRAP-positive mononuclear cells were initially detected in the coronal portion of the dental follicle of the permanent tooth at Day 1 postnatally. Ultrastructurally, these mononuclear cells had moderate numbers of mitochondria and short-strand rough endoplasmic reticulum, as well as scattered free ribosomes throughout their cytoplasm. TRAP-positive mononuclear cells then appeared in the cementoblast layer immediately adjacent to the surface of the deciduous roots. These mononuclear cells projected cytoplasmic extensions between the cementoblasts and made contact with the cementum. At that time, cell-cell contact was frequently observed between these mononuclear cells and cementoblasts. During 3-5 days postnatally, the number of TRAP-positive multinucleate odontoclasts on the root surface gradually increased. They had well-developed ruffled borders and made typical resorption lacunae on the root surface of the deciduous tooth. During this early postnatal period, neither inflammatory cells nor necrotic tissue could be observed surrounding the deciduous root. This study demonstrates that the dental follicle of the permanent tooth as well as the connective tissue adjacent to the deciduous root might play important role in site- and time-specific recruitment, development, and activation of odontoclasts before and at the onset of physiological root resorption.

Effects of TGFbeta1 on dental pulp cells in cultured human tooth slices

Transforming growth factor-beta1 (TGF beta1) is a potent modulator of tissue repair in various tissues. To analyze its role during human dental repair, we used thick-sliced teeth cultured as described previously (Magloire et al., 1996). The supply of TGF beta1 to the pulp tissue was accomplished by means of a small tube glued onto the dentin. We show that this device allowed the growth factor to diffuse locally through dentinal tubules and to bind to the cells present in the coronal pulp opposite the TGF beta1-delivery tube. The tube was filled with 20 ng/mL TGF beta1, and slices were cultured for 4 days. Results show a preferential accumulation of cells in the odontoblastic and subodontoblastic layers in the vicinity of the tube. Cell proliferation increased in the subodontoblastic layer and in the underlying pulp, and BrdU-positive cells were abundant around the blood vessels. TGF beta1 induced type I collagen production by the odontoblastic/subodontoblastic/pulp cells in the stimulated zone, as demonstrated by in situ hybridization. These results suggest that TGF beta1 could be directly involved in the regulation of cell proliferation, migration, and extracellular matrix production in the human dental pulp and eventually in the repair process occurring after tooth injury.

Designing new treatment strategies in vital pulp therapy

OBJECTIVES

The development of strategies in vital pulp therapy, which aim to maintain vitality and function of the dentine-pulp complex, represents a major focus of attention. Recent progress in understanding the molecular and cellular changes during tooth development and how they are mimicked during dental tissue repair offers the opportunity to now assess whether this knowledge can be exploited to design new treatment strategies in vital pulp therapy.

DATA SOURCES AND STUDY SELECTION

Current literature on the molecular and cellular basis of tooth development and dental tissue repair has been reviewed in the context of stimulating dentinogenic responses in the tooth together with pertinent published abstracts of relevant conferences and personal communications. Tissue events of direct relevance to clinical application for vital pulp therapy are discussed.

CONCLUSIONS

The involvement of growth factors and extracellular matrix molecules in signalling and regulating dentinogenic events during tooth development has been identified. During dental tissue repair, many of the processes are mimicked leading to responses of focal deposition of tertiary dentine at injury sites. The nature and specificity of these responses are determined in part by the extent of tissue injury. Traditional clinical strategies are capable of exploiting endogenous signalling molecules in the tissues to develop more effective treatment modalities. Application of exogenous signalling molecules offers opportunities for development of new therapies, although a number of delivery considerations must be addressed before these can be introduced into clinical practice.

Cytokine secretion of periodontal ligament fibroblasts derived from human deciduous teeth: effect of mechanical stress on the secretion of transforming growth factor-beta 1 and macrophage colony stimulating factor

The periodontal ligament may play an important role in tooth eruption, root development and resorption. The tissue physiologically receives mechanical force during mastication. We focused on the effects of intermittent mechanical strain on the cytokine synthesis of periodontal ligament (PDL) fibroblasts in vitro. The cells were derived from human periodontal ligament of deciduous teeth (HPLF-Y) and permanent teeth (HPLF). The two kinds of PDL cells and human gingival fibroblasts (HGF) were cultured in flexible bottomed culture plates. The cells were mechanically stretched at 5% elongation, 3-cycles/min for 24 h on d 7 in culture using a Flexercell strain unit. After the stretching, we measured DNA content and alkaline phosphatase activity in the cell layer, transforming growth factor beta 1 (TGF-beta 1) and macrophage colony stimulating factor (M-CSF) contents in the conditioned medium. The TGF-beta 1 level in the conditioned medium of HPLF was significantly higher than that of HPLF-Y and HGF. It was stimulated by mechanical stretching only on HPLF, whereas no significant effect was observed on HPLF-Y and HGF. M-CSF secretion was inhibited by the stretching on all of HPLF, HPLF-Y and HGF. 1 alpha, 25 dihydroxy vitamin D3 (D3) stimulated M-CSF secretion into the culture medium of both HPLF and HPLF-Y, but the stretching inhibited M-CSF secretion and completely blocked the enhancement by D3. These data suggest that periodontal ligament cells synthesize and secrete the molecules as autocrine or paracrine factors that affect bone remodelling and root resorption and the level of those factors change in response to mechanical stress.

Leukocyte adhesion deficiency in a child with severe oral involvement

Leukocyte adhesion deficiency is a rare inherited defect of phagocytic function resulting from a lack of leukocyte cell surface expression of beta2 integrin molecules (CD11 and CD18) that are essential for leukocyte adhesion to endothelial cells and chemotaxis. A small number of patients with leukocyte adhesion deficiency-1 have a milder defect, with residual expression of CD18. These patients tend to survive beyond infancy; they manifest progressive severe periodontitis, alveolar bone loss, periodontal pocket formation, and partial or total premature loss of the primary and permanent dentitions. We report on a 13-year-old boy with moderate leukocyte adhesion deficiency-1 and severe prepubertal periodontitis. This case illustrates the need for the dentist to work closely with the pediatrician in the prevention of premature tooth loss and control of oral infection in these patients.

Expression of transforming growth factor-beta receptors types II and III within various cells in the rat periodontium

This study reports the immunohistochemical localization of TGF-beta receptor type II (T beta R-II) and type III (T beta R-III) in cells of the forming periodontal ligament (PDL) in rat first molar roots. Mandibular periodontium was obtained from 3, 6 and 12-wk-old rats. This represented tissue from the initial, pre-mature and post-mature stages of root and periodontal development, respectively. Mandibular bone chips and molar roots were used to isolate osteoblasts, fibroblasts and cementoblasts. Cells were obtained using a 2-step trypsinization and explant technique, and cultured in Dulbecco's modification of Eagle's medium (DMEM) under routine cell culture conditions. Cells were cultured on coverslips for the purpose of detecting TGF-beta receptors, and compared with whole tissue sections using the same detection method. Cells which stained positively for T beta R-II and T beta R-III on both paraffin sections and cultured cell slides were counted. Both receptors were expressed in the various periodontal tissue compartments. PDL fibroblasts, cementoblasts and osteoblasts were stained positively for T beta R-II and T beta R-III. Endothelial cells were noted to be positive for T beta R-II only. T beta R-II was more widely distributed in cells than T beta R-III, but T beta R-III was extensively localized in the extracellular matrix. Both receptors were expressed on the cell membrane and also localized in the cytoplasm. The findings for paraffin sections were consistent with the immunohistochemical staining of cultured cells. The percentage of cells which stained positively for T beta R-II was greater (approximately 85%) than that for T beta R-III (approximately 60%) in all major types of the PDL cells on both paraffin sections and cultured cell slides. Extensive location of TGF-beta receptors in both cells and extracellular matrix suggests that several binding sites are available for TGF-beta s to interact with target cells during development and following maturation of the periodontium.

Stimulation of the dentine-pulp complex of rat incisor teeth by transforming growth factor-beta isoforms 1-3 in vitro

Expression of transforming growth factor-beta (TGF-beta) isoforms by odontoblasts leads to their sequestration within dentine matrix. TGF-beta1 and -beta3 stimulate matrix secretion and also initiate odontoblast cytodifferentiation in vitro and in vivo. Using a recently established organ-culture model, the aim here was to examine the effects of TGF-beta isoforms on the response of the dentine-pulp complex during culture. Agarose beads were soaked in isoforms 1-3, and placed on the odontoblast area of slices of 28-day-old rat incisor tooth. The slices were maintained in Trowel-type cultures for 7 days. Both TGF-beta1 and -beta3 stimulated a local increase in predentine secretion at the site of the bead application when compared to control cultures. Mitogenic effects on the cells of the subodontoblast layer were also seen and occasionally small foci of newly differentiated odontoblast-like cells could be observed a little distant from the application site of TGF-beta3. TGF-beta2 had a minimal effect on the cultured tissues. These results demonstrate that TGF-beta1 and -beta3 can stimulate secretion of extracellular matrix by odontoblasts, are mitogenic to pulp cells, and that TGF-beta3 may have inductive effects on pulpal cells. Such activities might be important during reparative processes in the dentine-pulp complex after tissue injury.

Expression of transforming growth factor-beta 1 (TGF-beta1) in the developing periodontium of rats

Transforming growth factor-beta 1 (TGF-beta1) has been reported to be expressed within several tissue compartments of developing molar crowns and therefore is implicated in tooth development. Additionally, TGF-beta1 may also play a crucial role in tissue repair and regeneration. The aim of this study was to determine the distribution of TGF-beta1 in the developing periodontal attachment apparatus (cementum, periodontal ligament, and alveolar bone) in Lewis rats. Animals aged 3, 6, and 12 wks were killed, their mandibles removed, fixed, demineralized, and processed in paraffin. The localization of TGF-beta1 in tissues was detected by polyclonal goat antibodies against human TGF-beta1 by means of immunoperoxidase techniques. TGF-beta1 messenger RNA was detected by in situ hybridization with a cocktail oligonucleotide probe. Cell counts were determined for analysis of the percentage of cells stained positive for TGF-beta1. Results revealed that TGF-beta1 was expressed in the developing alveolar bone, periodontal ligament, and cementum at all stages of tissue development studied. Staining was stronger at sites of cementum and alveolar bone compared with the periodontal ligament. Intensity of the positive staining, based on 3 grades, indicated a similarity between the tissues obtained from different ages, but varied between several cell types. Cementoblasts and osteoblasts stained more strongly than fibroblasts. Large numbers (approximately 90%) of the osteocytes in developing bone expressed TGF-beta1; however, in mature bone, fewer osteocytes stained for TGF-beta1. The percentages of positively stained cementoblasts, osteoblasts, and fibroblasts in the periodontal space were greater at the apical portion than at the cervical portion of the root. TGF-beta1 mRNA was expressed in osteoblasts, some bone marrow cells, cementoblasts, and fibroblasts. This study indicates that TGF-beta1 may play an important role in the modulation of tissue formation and development of the periodontium.

Tooth eruption molecules enhance MCP-1 gene expression in the dental follicle of the rat

Tooth eruption is a localized developmental event that requires the presence of the dental follicle, a loose connective tissue sac that surrounds each tooth. Early postnatally in the first mandibular molar of the rat there is an influx into the follicle of mononuclear cells (monocytes) which, in turn, fuse to form osteoclasts that resorb the bone to form an eruption pathway. The chemoattractant that may attract the mononuclear cells to the follicle to initiate the cellular events of eruption is monocyte chemotactic protein-one (MCP-1). MCP-1 is secreted by the dental follicle cells and its gene is expressed maximally at an early postnatal age, correlating with the monocyte influx into the follicle. In this study, we show that other potential tooth eruption molecules--EGF, IL-1alpha, TGF-beta1 and CSF-1--all enhance the expression of the MCP-1 gene in the cultured dental follicle cells. In vivo, injections of IL-1alpha or EGF also enhance the gene expression of MCP-1 in the follicle with maximal enhancement occurring in the early postnatal days. Thus, there appears to be a redundant function of the different tooth eruption genes to ensure that the MCP-1 gene is expressed. In turn, expression of MCP-1 may be critical for recruiting the monocytes to the dental follicle to initiate the cellular events of tooth eruption.

Effects of recombinant basic fibroblast growth factor, insulin-like growth factor-II and transforming growth factor-beta 1 on dog dental pulp cells in vivo

The effects of recombinant basic fibroblast growth factor (bFGF), insulin-like growth factor (IGF)-II and transforming growth factor (TGF)-beta 1 on dental pulp cells were investigated by light and transmission electron microscopy after their implantation for 1 and 3 weeks at central sites of mechanically exposed pulps in dog molar and canine teeth. The implants were Millipore filters that have been soaked with solutions containing 100 or 500 ng/ml of bFGF or IGF-II or 100 ng/ml of TGF-beta 1. Control filters were soaked with dog albumin. No changes in cell organization or matrix synthesis were seen after implantation of control filters. Groups of columnar, polarized cells with numerous mitochondria and Golgi elements or elongated cells unassociated with any matrix deposition were demonstrated after 1 or 3 weeks, respectively, in close proximity to the filters that had been soaked with bFGF solution; at a distance from these implants enhanced formation of an osteotypic matrix was seen beneath the exposure site. No particular response was found in close proximity to the filters that had been soaked with IGF-II solution after 1 or 3 weeks implantation but thick zones of osteodentine were found beneath the exposure site and at adjacent circumferential dentine sites. Numerous elongated, polarized cells with long cytoplasmic extensions invading the filter pores were consistently seen after 1 week in close proximity to the filters that had been soaked with TGF-beta 1 solution. After 3 weeks implantation of these filters, deposition of a tubular matrix surrounding the implants was seen in association with the highly elongated odontoblast-like cells, while enhancement of circumferential dentine formation was also found at adjacent peripheral sites. These experiments demonstrate that TGF-beta 1 when implanted for short term periods at central pulp sites exerted dentine-specific effects, inducing differentiation of odontoblast-like cells and stimulating primary odontoblasts. Implantation of bFGF and IGF-II did not result in reparative dentine formation, but did stimulate osteotypical matrix deposition at a distance from the implants.

Immunostaining and transcriptional enhancement of interleukin-1 receptor type I in the rat dental follicle

Interleukin-1alpha (IL-1alpha) enhances the gene expression of colony-stimulating factor-one (CSF-1) in dental follicle cells. In turn, CSF-1 appears to be a critical molecule in stimulating the cellular events of eruption that require the presence of the follicle. Chronologically, the maximal transcription and translation of CSF-1 in the follicle occurs early postnatally, followed by a decline later. Thus, in this study, immunostaining for the interleukin-1 receptor type I (IL-1RI) was used to determine if it paralleled the CSF-1 localization and chronology. The results showed that IL-1RI is primarily localized in the dental follicle, with maximal immunostaining early postnatally and a greatly reduced staining by day 10. In conjunction with this, molecules that enhance the gene expression of IL-1alpha epidermal growth factor (EGF) and transforming growth factor-beta1 (TGF-beta1) were also shown to enhance the expression of IL-1RI, but IL-1alpha did not increase the gene expression of IL-1RI. After injections of EGF at different times postnatally the mRNA of IL-1RI increased over comparable controls. Between days 2 and 5 the IL-1RI mRNA in the follicle decreased. In combination the results suggest that, as the expression of IL-1alpha is enhanced in the stellate reticulum either by EGF or TGF-beta1, these two molecules could also enhance the expression of IL-1RI in the dental follicle such that more receptors would be available to respond to the increased IL-1alpha secreted. The maximal presence of the receptors (IL-1RI) in the dental follicle early postnatally, followed by their subsequent decline, parallels the rise and fall of CSF-1 in the follicle. Thus, regulation of the IL-1RI and IL-1RI gene expression might be a means of regulating changes in CSF-1 in the follicle.

The genetic control of early tooth development

Most vertebrate organs begin their initial formation by a common, developmentally conserved pattern of inductive tissue interactions between two tissues. The developing tooth germ is a prototype for such inductive tissue interactions and provides a powerful experimental system for elucidation of the genetic pathways involved in organogenesis. Members of the Msx homeobox gene family are expressed at sites of epithelial-mesenchymal interaction during embryogenesis, including the tooth. The important role that Msx genes play in tooth development is exemplified by mice lacking Msx gene function. Msxl-deficient mice exhibit an arrest in tooth development at the bud stage, while Msx2-deficient mice exhibit late defects in tooth development. The co-expression of Msx, Bmp, Lefl, and Activin beta A genes and the coincidence of tooth phenotypes in the various knockout mice suggest that these genes reside within a common genetic pathway. Results summarized here indicate that Msxl is required for the transmission of Bmp4 expression from dental epithelium to mesenchyme and also for Lefl expression. In addition, we consider the role of other signaling molecules in the epithelial-mesenchymal interactions leading to tooth formation, the role that transcription factors such as Msx play in the propagation of inductive signals, and the role of extracellular matrix. Last, as a unifying mechanism to explain the disparate tooth phenotypes in Msxl- and Msx2-deficient mice, we propose that later steps in tooth morphogenesis molecularly resemble those in early tooth development.

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.

Tooth eruption: theories and facts

The mechanisms of tooth eruption (i.e., the answer to the question of how and why teeth erupt) has been a matter of long historical debate. This review focuses on human and other mammalian teeth with a time- and spacewise limited period of eruption and analyzes recent observations and experimental data on dogs, rats, primates, and humans in a framework of basic biological parameters to formulate a guiding theory of tooth eruption. Acknowledging basic parameters (i.e., that teeth move in three-dimensional space, erupt with varying speed, and arrive at a functional position that in inheritable) eliminates a number of previously held theories and favors those that accommodate basic parameters, such as alveolar bone remodeling in association with root elongation, with possible correction factors in the form of cementum apposition and periodontal ligament formation. We have critically analyzed, summarized, and integrated recent findings associated with preeruptive movements of developing teeth, the intraosseous stage of premolar eruption in dogs, molar eruption in rodents, and premolar and molar eruption in primates. The variable speeds of eruption are particularly important. We conclude with basic principles of tooth eruption--that is, the type of signals generated by the dental follicle proper, the conditions under which teeth are moved and the clinical understanding to be derived from this knowledge.

Transcription and translation of CSF-1 in the dental follicle

The dental follicle, a loose connective tissue sac which surrounds the unerupted tooth, is required for eruption to occur. Injection of colony-stimulating factor-1 (CSF-1) will accelerate molar eruption in rats, as well as stimulate tooth eruption in osteopetrotic rats. Utilizing in situ hybridization and reverse- transcription polymerase chain-reaction techniques, we show here that CSF-1 mRNA is present in vivo in the dental follicle of the first mandibular molar of the rat. Analysis of the molars from day 0 through day 10 post-natally demonstrates that the maximal expression of CSF-1 mRNA is at day 3 post-natally. Immunostaining also reveals that the CSF-1 mRNA is translated, with immunostaining for the CSF-1 itself, being heavy in early post-natal days and absent by day 9 postnatally. In view of the fact that there is a maximal influx of mononuclear cells (monocytes) into the dental follicle at day 3 post-natally--an influx which increases the numbers of osteoclasts needed to form a tooth eruption pathway--it is probable that the maximal expression of CSF-1 mRNA by day 3 post-natally contributes to this monocyte influx. Thus, this study establishes a relationship among a molecule (CSF-1), cell (monocyte), and tissue (dental follicle) that appear to play a major role in tooth eruption.

A new biological approach to vital pulp therapy

Molecular biology is providing opportunities to develop new strategies or agents for the treatment of a wide variety of diseases. The availability of large amounts of highly purified proteins produced by recombinant DNA techniques is an obvious example. Recent evidence has implicated proteins belonging to the bone morphogenetic protein (BMP) subgroup of the transforming growth factor beta supergene family in tooth formation and dentinogenesis. It has long been known that bone and dentin contain bone morphogenetic protein activity. Recently, recombinant human BMP-2, -4, and -7 (also known as OP-1), have been shown to induce reparative dentin formation in experimental models of large direct pulp exposures in permanent teeth. The manner in which these agents act appears unique. New reparative dentin replaces the stimulating agents applied directly to the partially amputated pulp. Hence, the new tissue forms contiguous with, largely superficial to, and not at the expense of the remaining vital pulp tissue. This suggests a therapeutic approach permitting the induction of a predetermined and controlled amount of reparative dentin. Additionally, OP-1 has been associated with the formation of reparative dentin after application to a freshly cut but intact layer of dentin. These findings may provide future clinicians with additional options for the treatment of substantially damaged or diseased vital teeth.

Immunolocalization of interleukin-1 alpha in rat mandibular molars and its enhancement after in vivo injection of epidermal growth factor

Immunolocalization of interleukin-1 alpha in the first mandibular molars of rats from day 0-12 postnatally showed that the protein was localized in the epithelial stellate reticulum adjacent to the dental follicle. Staining of the stellate reticulum was most prominent in the early days postnatally and was absent by postnatal day 11. Injection of epidermal growth factor into rats at day 0 greatly increased the intensity of the staining for interleukin-1 alpha in the stellate reticulum. Epidermal growth factor (EGF) enhanced the gene expression of interleukin-1 alpha in stellate reticulum cells in vitro, and this study suggests there is enhanced translation of interleukin-1 alpha messenger RNA in the stellate reticulum following EGF injection. In turn, the interleukin-1 alpha may exert its effect on the dental follicle cells adjacent to the stellate reticulum because EGF also enhanced expression of the interleukin-1 receptor type I messenger RNA in cultured dental follicle cells as well as enhancing its expression in vivo. In view of the fact that injection of EGF will stimulate precocious eruption of teeth, its stimulus of interleukin-1 alpha synthesis in the stellate reticulum may be the mechanism by which EGF initiates a cascade of molecular events to signal the onset of tooth eruption.

The basic and applied biology of tooth eruption

The dentition and the alveolar process of each jaw develop simultaneously so that, by the time the crown is completed and eruption begins, the crown is enclosed in a crypt within alveolar bone. Thus, the eruption of a tooth to its functional position involves discretely localized, bilaterally symmetrical bone resorption to produce an eruption pathway and bone formation to fill in the space previously occupied by the crown and growing roots. Studies of crypt surfaces during eruption confirm this polarization of alveolar bone metabolism around a tooth with respect to both bone cells and mineralized surface topography. Experimental studies of tooth eruption have shown that the dental follicle, the dense connective tissue investment of the tooth, is necessary for eruption and that neither bone resorption nor bone formation occur without the adjacent part of the dental follicle. Early in eruption the coronal part of the follicle accumulates mononuclear cells which have cytochemical and ultrastructural features of osteoclasts and the apical part of the follicle, a site of intense cell proliferation, binds epidermal growth factor (EGF). The dental follicle contains a variety of proteins and the concentration of several change during eruption. Prominent among them are a reduction in matrix metalloproteinases and an increase in protoglycans as eruption proceeds. The contribution of these changes to those in cell proliferation, migration and differentiation during tooth eruption present experimental opportunities for developmental biologists. The rate-limiting factor of the earliest (intraosseous) stage of tooth eruption is bone resorption and eruption can be accelerated or retarded by the local delivery of factors which increase or decrease the activity of osteoclasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of dentine in amputated pulp of dogs by recombinant human bone morphogenetic proteins-2 and -4 with collagen matrix

Recombinant human bone morphogenetic protein (BMP)-2, BMP-4 and transforming growth factor (TGF)-beta 1 combined with collagen matrix as a carrier were examined for their effects on pulp regeneration and dentine formation. Seventy days after implantation of 2 micrograms of BMP-2, mineralized osteodentine-like tissue containing embedded osteodentinocytes was seen in the cavity. Unmineralized fibrous tissue and pulp-like loose connective tissue were also found in the same cavity. In teeth implanted with 660 ng of BMP-2 only unmineralized fibrous and pulp tissues were seen. In teeth with 220 ng of BMP-2 or collagen alone, pulp tissue was seen. It is therefore likely that the cavity fills with pulp tissue and that spindle-shaped cells elaborate extracellular matrix that mineralizes to be osteodentine in a dose-dependent manner. Similar osteodentine was seen in teeth implanted with 4 micrograms of BMP-4 and collagen. No distinct tubular dentine was formed, unlike an earlier experiment in which BMP-2 or -4 was implanted with enriched, inactivated dentine matrix. These findings suggest that both BMP-2 and -4 induce osteodentine formation if combined with collagen matrix; some other matrix component present in inactivated dentine matrix might be essential for further differentiation into odontoblasts. In teeth implanted with TGF-beta 1, the carrier collagen remained in the cavity and little pulp tissue proliferation was seen, suggesting a possible inhibitory effect of TGF-beta 1 in pulp regeneration. It is likely that the response to growth and differentiation factors is dependent on the state of differentiation of pulp cells.

The effect of removing the true dental follicle on premolar eruption in the dog

Eruption is a highly localized process during which the bone resorption and formation that occur on opposite sides of the tooth are dependent upon the surrounding soft tissues, the true dental follicle externally and the enamel organ internally. To examine the ability of the enamel organ to cause eruption the external layer (dental follicle) was removed just prior to and up to 4 weeks before eruption in 13 mandibular premolars in dogs and eruption followed clinically, radiographically and histologically. None of the teeth without dental follicles erupted but three teeth from which the follicle was separated then replaced did erupt. These data indicate that the enamel organ without the dental follicle cannot support tooth eruption and provide indirect evidence for the central role of the dental follicle, alone or in combination with the enamel organ, in eruption.

Effect of epidermal growth factor on expression of transforming growth factor-beta 1 mRNA in stellate reticulum cells of rat mandibular molars

Cultured stellate reticulum cells isolated from rat mandibular molars respond to incubation in EGF by increasing their level of expression of TGF-beta 1 mRNA. Northern blots showed that incubation in EGF for 6 hours stimulated over a two-fold increase in TGF-beta 1 mRNA in the cells. In contrast, incubating the cells in TGF-beta 1 did not enhance the expression of TGF-beta 1 mRNA in the cells, indicating that TGF-beta 1 does not have an autocrine effect on these cells. Immunocytochemistry showed that EGF receptor was present on the surface of many but not all of the cultured stellate reticulum cells. Because EGF does stimulate premature eruption of teeth, it is possible that its effect on the stellate reticulum region of the enamel organ would be to stimulate synthesis of TGF-beta 1 mRNA which, in turn, could lead to increased synthesis of TGF-beta 1 by these cells. The cells do contain the TGF-beta 1 protein as revealed by immunocytostaining. The newly synthesized TGF-beta 1 may exert its effect on the adjacent dental follicle to either initiate the onset of the cellular events of tooth eruption or to increase the secretion of extracellular matrix proteins by the follicle for formation of the periodontal ligament.

Effects of transforming growth factor-beta 1 on cultured dental follicle cells from rat mandibular molars

Analysis of the total proteins secreted by cultured dental follicle cells revealed that transforming growth factor-beta 1 (TGF-beta 1) stimulated them to secrete more extracellular matrix proteins into a serum-free medium than did follicle cells not exposed to the growth factor. Electrophoresis and scanning densitometry showed that secretion of all the major proteins was increased by exposure to the growth factor but the amounts ranged from a 66% increase for one of the procollagen chains to a 7% increase for fibronectin. Immunofluorescence using anti-type I collagen and anti-fibronectin showed that the intracellular concentration and intracellular localization of the antibodies was not changed by incubating the cells with the growth factor. The growth factor did not cause an increase in cell number but did modify the association of the cells in the culture, causing them to aggregate into clusters whereas the control cells formed a confluent monolayer. These results suggest that TGF-beta 1 may signal the fibroblasts of the dental follicle to secrete the extracellular matrix needed for its development into a periodontal ligament.

Current concepts of the biology of tooth eruption

Tooth eruption is defined as the movement of a tooth from its site of development within the jaws to its position of function within the oral cavity. We present a critical review of evidence for the mechanisms and regulation of the intraosseous and supraosseous phases of eruption, with an emphasis upon the canine premolar model studied by the authors. Analyses at different stages of premolar eruption indicate that selective fragmentation of dental follicle protein DF-95 correlates with the presence of elevated levels of follicular collagenase and stromelysin, and with the onset of premolar movement. A dramatic decrease in these metalloproteinases followed initiation of movement. A biochemical and cell biological model for regulation of tooth eruption is proposed based upon these new and existing data.

Culture and characterization of dental follicle cells from rat molars

Because the dental follicle is necessary for the eruption of teeth of limited eruption, it was the objective of this study to determine if the cells of the follicle could be cultured in vitro. To achieve this, dental follicles and associated enamel organs were dissected from the first and second mandibular molars of 6-7-day-old rats (secretory stage of amelogenesis), and then cultured in a medium that promotes fibroblast growth--the predominant cell type of the dental follicle. The cultured cells grew to confluency and were kept through 3 passages before experimentation. The cultured cells were fibroblastic in shape, elongate with processes, and transmission electron microscopy revealed that they contained an abundant rough endoplasmic reticulum, but did not form desmosomes. Immunofluorescent staining for anti-vimentin showed that all the cells stained and electron-microscopic immunogold labeling indicated that the antibody was associated with intermediate filaments. As revealed by SDS-polyacrylamide gel electrophoresis and Western blotting, the cultured cells synthesized and secreted the extracellular matrix molecules fibronectin and procollagens. Subsequent immunofluorescence staining of permeabilized and non-permeabilized cells confirmed the presence of fibronectin and type I collagen both intra- and extracellularly. Thus, based on all the above characteristics, the cultured cells appeared to be fibroblasts derived from the dental follicle, although a few of the fibroblasts may be derived from undifferentiated mesenchymal cells interposed between the alveolar bone and follicle. Experiments now can be conducted to determine how these cultured cells respond directly to growth factors that alter the rates of tooth eruption.