Formation of acellular root cementum in relation to dental and non-dental hard tissues in the rat

In vitro effects of photobiomodulation on cell migration and gene expression of ALP, COL-1, RUNX-2, and osterix in cementoblasts

The aim of this study was to evaluate the effects of photobiomodulation (PBM) on cell migration and alkaline phosphatase (ALP), type I collagen (Col-1), runt-related transcription factor 2 (RUNX-2), and Osterix (OSX) gene expression in a cementoblast culture (OCCM-30), in a microenvironment mimicking an injury on the cementoblast layer, such as it occurs during root resorption. For this, OCCM-30 cells were cultured in 6-well plates and the following parameters were assayed: (1) migration by scratch assay and ALP, Col-1, Runx2, and Osx by real-time PCR. PBM was performed in two protocols using a LED device emitting light at 660 nm (± 30 nm). OCCM-30 cementoblasts were grown and divided into four groups: (1) negative control; (2) positive control (scratch); (3) scratch + PBM with a total energy of 36 J and energy density 1.6 J/cm²; and (4) scratch + PBM with a total energy of 72 J and energy density of 3.2 J/cm². Data were statistically analyzed, with the level of significance set at 5%. Cementoblasts migrated from the edge of the scratch toward the center, and the wound closed after 24 h, with the PBM3.2J/cm² group showing the higher cell migration compared with the other groups at 2 h, 6 h, 8 h, and 13 h (p < 0.05). The control and PBM1.6J/cm² groups showed similar levels of cell migration, with no significant differences (p > 0.05). PBM3.2J/cm² group exhibited greater ALP, Col-1, OSX, and RUNX2 in comparison with the other experimental groups (p < 0.05). Similar levels of all genes evaluated were observed between the PBM1.6J/cm² group and the positive control group (p > 0.05). In conclusion, our findings support the effectiveness of photobiomodulation on cementoblast migration and gene expression, which may contribute to the formation of a new cementum layer.

The prostacyclin analogue iloprost promotes cementum formation and collagen reattachment of replanted molars and upregulates mineralization by human periodontal ligament cells

OBJECTIVES

This study evaluated the use of the prostacyclin analogue, iloprost, as a root surface treatment agent in promoting acellular cementum(AC) formation and collagen reattachment following tooth replantation in vivo. In addition, its effect on human periodontal ligament cell(hPDLC) mineralization was assessed in vitro.

METHODS

First molars of 8-weeks-old Wistar rats were extracted. In one group, the root surfaces were treated with Hank's Balanced Salt Solution(HBSS) and the other group's root surfaces were treated with 10^-6 M iloprost before re-plantation. At day 30, maxillae were prepared for microCT and histomorphometric analysis. The effect of iloprost on mineralization by hPDLCs were analyzed by mineralized nodule formation and qPCR at 7 and 14 days.

RESULTS

MicroCT demonstrated a significant higher bone-volume in the iloprost-groups, whereas the HBSS-groups had extensive bone and root resorption. Histologic analysis revealed deposition of a thick AC layer along the root in iloprost-group with well-organized PDL fibers inserted into the cementum. The HBSS-group demonstrated more osteoclasts than the iloprost-group. In vitro, iloprost-treated hPDLCs had a significantly increased RUNX2, OSX, BSP, and ALP gene expression that coincided with an increased deposition of mineralized nodules. These effects were abrogated by a PGI₂-receptor inhibitor.

CONCLUSION

Our results revealed that iloprost promoted PDL regeneration in replanted molars. Furthermore, resorption of the roots was decreased, whereas AC deposition was stimulated. Iloprost-treatment increased hPDLC mineralization and was mediated by PGI₂-receptor activation. These observations indicate that iloprost may be a promising root surface treatment agent.

Are Cementoblasts a Subpopulation of Osteoblasts or a Unique Phenotype?

Experimental studies have shown a great potential for periodontal regeneration. The limitations of periodontal regeneration largely depend on the regenerative potential at the root surface. Cellular intrinsic fiber cementum (CIFC), so-called bone-like tissue, may form instead of the desired acellular extrinsic fiber cementum (AEFC), and the interfacial tissue bonding may be weak. The periodontal ligament harbors progenitor cells that can differentiate into periodontal ligament fibroblasts, osteoblasts, and cementoblasts, but their precise location is unknown. It is also not known whether osteoblasts and cementoblasts arise from a common precursor cell line, or whether distinct precursor cell lines exist. Thus, there is limited knowledge about how cell diversity evolves in the space between the developing root and the alveolar bone. This review supports the hypothesis that AEFC is a unique tissue, while CIFC and bone share some similarities. Morphologically, functionally, and biochemically, however, CIFC is distinctly different from any bone type. There are several lines of evidence to propose that cementoblasts that produce both AEFC and CIFC are unique phenotypes that are unrelated to osteoblasts. Cementum attachment protein appears to be cementum-specific, and the expression of two proteoglycans, fibromodulin and lumican, appears to be stronger in CIFC than in bone. A theory is presented that may help explain how cell diversity evolves in the periodontal ligament. It proposes that Hertwig’s epithelial root sheath and cells derived from it play an essential role in the development and maintenance of the periodontium. The role of enamel matrix proteins in cementoblast and osteoblast differentiation and their potential use for tissue engineering are discussed.

Characterization of rat apical tissues in different root development stage

In this study, we try to compare the histological characteristics and the odontogenic capability of apical tissues (AT) at different root development stages of rat molar teeth. AT of mandibular first molars from 8-day-old, 21-day-old, and 35-day-old Sprague-Dawley rats were selected as being representative of root-initiating, root-forming, and root-completing stages, respectively. Cell counting, flow cytometry assays, alkaline phosphatase activity, alizarin red staining, and reverse transcription polymerase chain reaction were performed to assess the proliferation and mineralization potential of apical tissue cells at different stages of root development in vitro. In vivo transplantation of apical tissue cells combined with ceramic bovine bone was used to characterize the differentiation capacity. It was shown that there was a structurally and functionally dynamic change in the apical tissue of developing tooth root of rats, of which the unique developmental potential will reduce gradually with the ending up of root development. The AT of root-initiating and root-forming stage exhibited much higher proliferation and tissue-regenerative capacity than those of root-completing stage. Our present results indicate that the apical tissue, with the sustainable developmental ability throughout almost the whole process of tooth development, can yet be regarded as a competent candidate source for root/periodontal tissues regeneration.

Physiology and pathology of collagen receptors

Just before the transition from pre-genomic to the post-genomic era, the two latest members of the mammalian integrin family were identified. These integrins, which were named alpha10beta1 and alpha11beta1, are both collagen receptors and are related. Rather than being twins, they can be regarded as close cousins. They both belong to the subfamily of integrins that contain an I-domain in the alpha subunit. This domain is also the part that endows these integrins with the capacity to bind the GFOGER sequence in collagens. In the current review, we summarize and update the current knowledge about the in vitro and in vivo functions of these integrins.

Alpha11 beta1 integrin-dependent regulation of periodontal ligament function in the erupting mouse incisor

The fibroblast integrin alpha11beta1 is a key receptor for fibrillar collagens. To study the potential function of alpha11 in vivo, we generated a null allele of the alpha11 gene. Integrin alpha11(-/-) mice are viable and fertile but display dwarfism with increased mortality, most probably due to severely defective incisors. Mutant incisors are characterized by disorganized periodontal ligaments, whereas molar ligaments appear normal. The primary defect in the incisor ligament leads to halted tooth eruption. alpha11beta1-defective embryonic fibroblasts displayed severe defects in vitro, characterized by (i) greatly reduced cell adhesion and spreading on collagen I, (ii) reduced ability to retract collagen lattices, and (iii) reduced cell proliferation. Analysis of matrix metalloproteinase in vitro and in vivo revealed disturbed MMP13 and MMP14 synthesis in alpha11(-/-) cells. We show that alpha11beta1 is the major receptor for collagen I on mouse embryonic fibroblasts and suggest that alpha11beta1 integrin is specifically required on periodontal ligament fibroblasts for cell migration and collagen reorganization to help generate the forces needed for axial tooth movement. Our data show a unique role for alpha11beta1 integrin during tooth eruption.

Formation of acellular cementum-like layers, with and without extrinsic fiber insertion, along inert bone surfaces of aging c-Src gene knockout mice

To investigate the long-term effects of c-src deficiency on skeletal and dental tissues, we examined the lower jaws and long bones of c-src gene knockout (c-src KO) mice by histological and histochemical methods. Numerous multinucleated osteoclasts were distributed throughout the mandible in 5-wk-old c-src KO mice, but by 14 wk they had almost completely disappeared from the alveolar bone, leaving tartrate-resistant acid phosphatase (TRAP)-positive layers along the bone surface. Deposition of osteopontin-positive mineralized tissue, reminiscent of acellular afibrillar cementum (AAC), was confirmed along the TRAP-positive bone surface at 14 wk. The layer progressively thickened up to 21 months. A comparable mineralized layer was noted along the trabeculae of long bones as thickened cement lines. In the periostin-rich areas of jaw bones, but not in the long bones, portions of AAC-like mineralized layers were often replaced with and/or covered by acellular extrinsic fiber cementum (AEFC)-like tissue. These data suggest that the deposition of AAC-like mineralized tissue is a general phenomenon that may occur along inert or slowly remodeling bone surfaces under conditions characterized by reduced bone-resorbing activity, whereas the induction of AEFC-like tissue seems to be associated with the expression of certain molecules that are particularly abundant in the microenvironment of the periodontal ligament.

Use of microarrays to find novel regulators of periodontal ligament fibroblast differentiation

Periodontal regeneration requires the coordinated movement and differentiation of several cell types in order to re-establish the cementum, periodontal ligament (PDL), and alveolar bone. Cells in culture are often used as model systems for mature tissues, although they may represent expanded progenitor cell populations. Comparison of transcript expression between fresh PDL tissue and PDL cell isolates by MicroArray analysis has revealed numerous molecular differences. Several transcripts (including alkaline phosphatase, bone sialoprotein, periostin, and fibromodulin) are expressed at higher levels in fresh PDL than in cultured PDL cells. In contrast, PDL cells in culture selectively express a variety of growth factors. Several of these growth factors alter PDL fibroblast behavior. Two members of the transforming growth factor beta family of growth factors, namely, bone morphogenic protein-7 (BMP7) and growth differentiation factor-5 (GDF5), reduce cell proliferation and Stro-1 expression (a bone marrow stromal stem cell marker), whereas only BMP7 induces alkaline phosphatase activity. In contrast, fibroblast growth factor-5 induces enhanced cell proliferation and Stro-1 expression, while repressing alkaline phosphatase activity. The stimulation of PDL cells to differentiate (either by BMP7 or GDF5) inhibits cell motility. Thus, PDL cells in culture are regulated by several factors that differentially stimulate a mineralized (cementoblast-like) fate, a non-mineralized fate (mature fibroblasts), or the propagation of a more naive phenotype (potential progenitors).

Effects of bone morphogenetic protein-6 on periodontal wound healing in a fenestration defect of rats

BACKGROUND

Bone morphogenetic proteins (BMPs) may play significant roles in bone formation. The ability of BMP-6 to promote wound healing has been chosen as the subject of this investigation. In this study, a synthetic rat BMP-6 polypeptide was applied to a periodontal fenestration defect in rats to elucidate the effects of BMP-6 on periodontal wound healing.

MATERIAL AND METHODS

Following surgery to create a bony window on the buccal aspects of mandibular molar roots, 24 male Sprague Dawley rats were divided into four groups according to BMP application (0, 1, 3 and 10 microg, respectively). Animals were killed after 28 days and the mandible taken for histological examination. Histometric measurements were performed on sections selected from three levels (coronal, middle and apical levels; with 240 microm apart from the central) of the defect. New bone and cementum formation (including area and thickness) were analyzed and compared.

RESULTS

In general, minimal new bone was observed on the surgically created defects in the non-BMP group, whereas a complete osseous healing occurred in all BMP-6 treated animals. New bone formation (both in area and thickness) was significantly influenced by both the dosage and the examining level, whereas new cementum formation was affected by dosage only. An increase in bone and cementum formation was noted in all three BMP groups when compared with the control group at all examined levels. Among the BMP groups, greatest new bone and cementum formation were noted in the 3 microg group. New cementum thickness increased on the cementum surfaces of the defects compared with the dentinal surfaces in all study groups.

CONCLUSION

An increase in new bone and cementum formation was noted after applying a synthetic BMP-6 polypeptide to a periodontal fenestration defect in rats. Therefore, we suggest that BMP-6 may play a certain role in periodontal regeneration.

Factors that modulate the effects of bone morphogenetic protein-induced periodontal regeneration: a critical review

The healing process initiated by a single molecular species of bone morphogenetic protein (BMP) such as BMP-2 or BMP-7 sets in motion a cascade of cellular events resulting in differentiation of progenitor cells into phenotypes involved in periodontal regeneration. For example, animal studies show that a single dose of recombinant human (rh) BMP-2 increases the rate of normal intramembranous bone formation and enhanced cementum formation during periodontal wound healing. However, the optimal effects of BMPs are modulated by a range of factors that need careful evaluation in clinical studies. These factors include the influence of root conditioning, occlusal loading, BMP dose, and the release characteristics of the carrier as well as the suitability of the model to evaluate the efficacy of BMPs. Each of these factors may affect the rate of BMP-induced osteogenesis and cementogenesis and subsequent periodontal ligament (PDL) formation during the early and late stages of periodontal wound healing. Although BMP-2 initiates stem cells along an osteogenic pathway, the dose may have to be of sufficient concentration to ensure other growth and differentiation factors do not redirect or retard the osteogenic potential of the cell. Understanding when to manipulate the cell's differentiation pathway with the application of single or multiple doses of BMPs at the appropriate concentration is required to optimize the effect of BMPs in periodontal wound healing. Therefore, different release profiles from the same carrier may be particularly important in tissues with mixed cell populations such as in the periodontium, where similar tissues like bone and cementum grow at different rates. Furthermore, treatment of intrabony defects with BMPs are likely to not only require appropriate temporal release of the BMP(s), but also a carrier that can serve as a template for new tissue formation providing space maintenance and supporting the mucoperiosteal flap. Many of these issues have not been adequately addressed from a periodontal standpoint; therefore the purpose of this review is to clarify our current understanding of the factors that are likely to modulate the effects of BMP-induced periodontal regeneration. Moreover, assessing the importance of these factors is essential prior to conducting expensive human clinical trials.

Tooth auto-transplantation with double periodontal ligament stimulation to replace periodontally compromised teeth

BACKGROUND

The healing process of autotransplantation puts 2 different tissues in competition: the ligament on the root surface and the bone tissue of the alveolus. This study shows the effects of a protocol with 2 surgical stages, which promote ligament repair, inhibit adhesion between bone and dental root, and reduce the occurrence of the ankylosis-root resorption phenomenon.

METHODS

Forty-three patients, 33 to 73 years old, received 47 transplantations of mature teeth (including retained teeth) during a 5-year period. During the first surgical step, the transplanted tooth is extracted, measured, immediately replaced in its origin site, and maintained with an original suture technique. The alveolus to which the tooth will be transplanted is adapted after extraction of the periodontally compromised tooth. The second surgery occurs at day 14, when regeneration of periodontal ligament (PDL) is at a maximum (first stimulation). The tooth is transplanted in its new alveolus and retained using the same suture technique to avoid a rigid splint and to create mechanical stimulation of the PDL (second stimulation).

RESULTS

The results were 95.75% positive with normal PDL, with a 4.25% failure rate (transplant loss) and no ankylosis. Mean probing reduction was 8.37 +/- 3.0 mm. Mean radiographic bone gain was 7.73 +/- 4.32 mm.

CONCLUSIONS

This study suggests that auto-transplantation with double PDL stimulation can be a viable treatment in clinical practice, especially to replace teeth with large periodontal lesions, deep furcation defects, and/or root fractures. This study shows the high potential of stimulated PDL to regenerate alveolar bone and periodontal structures in severe destruction sites.

N-cadherin expression during periodontal ligament cell differentiation in vitro

BACKGROUND

When confluent periodontal ligament (PDL) cells were cultured in the presence of dexamethasone (Dex), ascorbic acid (AA), and beta-glycerophosphate (GP), they underwent sequential differentiation, demonstrating distinct morphological characteristics. At 1 week, localized cell proliferation led to the formation of multilayers of cells. As cell differentiation progressed, they formed nodules by deposition of matrix in the clusters of cells at 2 weeks, and mineralized the nodules at 3 weeks. These changes implicate extensive cell-to-cell interactions. Cadherins are known to play an important role in establishing cell contacts during tissue formation.

METHODS

To determine whether cadherins are involved in PDL cell differentiation, and the formation and mineralization of nodules by the cells in vitro, we investigated the expression of N-cadherin using immunofluorescence labeling and Northern blot analysis.

RESULTS

Immunolabeling showed that N-cadherin was expressed in PDL cells in the stages of nodule formation and mineralization. Northern blot analysis demonstrated a 3-fold increase in the expression of N-cadherin mRNA in the stages. However, neither E-cadherin nor P-cadherin was expressed.

CONCLUSIONS

Our data suggest that N-cadherin may play an important role in PDL cell differentiation and the formation of mineralized nodules by PDL cells.

Transmission and scanning electron microscopic analysis of mineralized loci formed by human periodontal ligament cells in vitro

Fibroblasts isolated from human periodontal ligament (PDL) were cultured under a medium supplemented with ascorbic acid, beta-glycerophosphate and dexamethasone. The cultures were assessed for their ability to elaborate a mineralized matrix. Cell cultures stained positive when analyzed for alkaline phosphatase activity throughout the culture period. After about 3 weeks in culture, the cells produced a calcified matrix. Light microscopy showed formation of clusters of different shapes and sizes. Von Kossa staining revealed mineral deposits as amorphous brown-black precipitates. Transmission electron microscopy showed cells in multilayers and mineralized formations in close association with a dense network of collagen fibers. Scanning electron microscopy revealed smooth formations rising over the cultures with an abundant fiber matrix. We conclude that human PDL fibroblasts can be induced to form a mineralized matrix which shares features with bone mineralized matrix but most likely represents a more immature type of in vitro mineralization. Moreover, the present study further supports the osteoblastic potential of these cells.

The structure and function of periodontal ligament cells in acellular cementum in rat molars

To elucidate the structure and function of periodontal ligament cells at the periodontal ligament-cementum interface in advanced acellular cementogenesis, the cervical regions of molars in rats aged 6 weeks were observed by light and electron microscopy. The light and transmission electron microscopy showed the periodontal ligament cells to be elongated between dense, well-developed principal fibers. The transmission and scanning electron microscopy showed that these cells extended wing-like projections from the lateral surface, forming cylindrical compartments surrounding the principal fibers. In addition, finger-like projections extended toward the cementum from the cementum-facing ends. The main results suggest the following: at the periodontal ligament-cementum interface, the periodontal ligament cells maintain the architecture of the principal fibers by means of extracellular compartments. The arrangement of finger-like projections results in the formation of acellular cementum containing only Sharpey's fibers as a fibrous component.

Immunolocalization of glycosaminoglycans in ageing, healthy and periodontally diseased human cementum

The distribution of glycosaminoglycans in the extracellular matrix of human cementum was investigated in periodontally involved and periodontal disease-free teeth separated into eight different age groups (from 12 to 90 years), to investigate possible changes in the distribution of glycosaminoglycan species associated with ageing and periodontal disease. A standard indirect immunoperoxidase technique was used, with a panel of monoclonal antibodies, 2B6, 3B3, 5D4, and 7D4, that recognize epitopes in chondroitin-4-sulphate/dermatan sulphate (C-4S/DS), chondroitin-6-sulphate (C-6S), keratan sulphate (KS) and a novel sulphated chondroitin sulphate (CS) epitope, respectively. Intense positive staining for C4-S/DS was observed at the margins and lumina of almost all the lacunae and canaliculi in cellular cementum in all sections. Immunoreactivity to C6-S, KS and novel CS epitopes was limited to a proportion of lacunae and canaliculi in all sections, although C6-S and the novel CS epitopes were more widely distributed than KS. In acellular cementum, there was no demonstrable staining for any of the glycosaminoglycans except where periodontal ligament (Sharpey's) fibres insert; periodontal ligament fibres inserting in cellular cementum also demonstrated positive immunoreactivity. In addition, the cementoblasts on the outer root surface, as well as the pericellular areas around a proportion of these cells, demonstrated positive immunoreactivity. These results indicate that glycosaminoglycan species present in human cementum include C4-S, DS, C6-S, and novel sulphated CS epitopes. KS is also present in cementum but is limited to a more restricted proportion of lacunae and canaliculi. Regional differences in the distribution of glycosaminoglycans exist between the two cementum types, but no qualitative differences in that distribution were observed between the various age groups or between periodontally involved and periodontal disease-free teeth. The immunoreactivity observed in a proportion of lacunae after staining for C6-S, KS, and novel sulphated CS epitopes could suggest the existence of different cementocyte subpopulations.

Developmental appearance and distribution of bone sialoprotein and osteopontin in human and rat cementum

BACKGROUND

Bone sialoprotein (BSP) and osteopontin (OPN), two major noncollagenous proteins (NCPs) in collagen-based mineralized tissues, have been implicated in mineral deposition and cell- and matrix-matrix interactions during root development. However, their role in cementogenesis is still a subject of debate. Since distribution of proteins is indicative of function, we have analyzed their temporo-spatial appearance in relation to that of cementum collagen.

METHODS

Human premolars and rat molars at various stages of root development characterized by differing rates of formation were fixed in aldehyde and embedded in epoxy and LR White resin. Sections were processed for ultrastructural analysis and postembedding colloidal gold (immuno)cytochemistry.

RESULTS

Incubations with antibodies against BSP and OPN and with lectins recognizing prominent sugars in these proteins generally revealed similar labeling patterns in both human and rat teeth, with gold particles accumulating mainly in the interfibrillar spaces. The lectin Helix pomatia, specific for N-acetyl-D-galactosamine, was distinctive in that it consistently reacted with human cementum, but only sporadically labeled rat cementum. Regardless of both the species and the stage of root development, mineralization initiated in mantle predentin in association with distinct foci immunoreactive for BSP and OPN. In human teeth, the deposition of cementum collagen began before the start of dentin mineralization and thus prior to any detectable labeling for BSP and OPN. However, at early stages of root formation in the rat, cementum collagen appeared after BSP and OPN accumulated on the root surface, whereas at advanced stages the deposition of cementum collagen, BSP and OPN coincided.

CONCLUSIONS

The temporo-spatial differences in the appearance of BSP and OPN relative to cementum collagen correlate well with known differences in the speed of root elongation and explain the variable appearance of the dentino-cemental junction. The data reveal no causal relationship between BSP and OPN and the differentiation of cementoprogenitor cells and indicate that the distribution of collagen fibrils ultimately determines the amount and pattern of accumulation of these NCPs. There also is no consistent planar accumulation of BSP and OPN between dentin and cementum such as the cement lines found between "old" and "new" bone. It is concluded that the interlacement of collagen fibrils at the dentino-cemental junction, across which mineralization spreads, represents the primary attachment mechanism between cementum and dentin.

Recombinant human bone morphogenetic protein-2 promotes wound healing in rat periodontal fenestration defects

Although there is considerable interest in the use of bone morphogenetic protein (BMP) to promote periodontal regeneration, little is known of its effects on the early stages of wound healing. The aim of this study was to investigate the effects of recombinant human bone morphogenetic protein 2 (rhBMP-2) on an early stage of post-operative wound healing and following complete healing (10 and 38 days, respectively) in a rat model of periodontal regeneration. The buccal aspects of molar roots were carefully denuded of their periodontal ligament through a bony window created in the mandibles of Wistar rats under general anesthesia. After the root surfaces were acid-conditioned, a 10-microL quantity of 50 microg/mL rhBMP-2 in a collagen gel solution was placed into the surgically created defect in test animals; in controls, either a 10-microL quantity of only collagen gel was received, or the defect was untreated. Animals were killed 10 days or 38 days after surgery and the tissues processed for histological examination. Transverse 5-microm sections were stained for the identification of new bone, cementum, and collagen fiber formation. In the 10-day study groups, new bone formation over the second molar and beyond the defect was significantly increased in the test group (p < 0.02), although there was no evidence of increased ankylosis. RhBMP-2 stimulated more than twice the area of cementum growth coronally compared with controls (712 +/- 286 microm2 and 258 +/- 57 microm2, respectively). Connective tissue attachment, including the number and width of collagen bundles, was similar in both test and controls. Complete healing without any evidence of ankylosis had occurred in all animals 38 days post-operatively, and no significant differences were observed between test and control groups. In conclusion, a single dose of rhBMP-2 increased the rate of normal intramembranous bone formation and selectively enhanced cementum formation coronally during early wound healing. However, the finding that rhBMP-2 induced bone formation at some distance from the defect suggests the importance of developing a suitable delivery system to maintain the concentration of BMP-2 at the site of implantation for potential therapeutic use.

Ultimate tensile strength of PDL of molars in rats after 1-hydroxyethylidene-1,1-bisphosphonate injections

OBJECTIVES AND METHODS

Administration during root formation of a bisphosphonate, 1-hydroxyethylidene-1,1-bisphosphonate (HEBP), at a dose corresponding to 10 mg P/kg body weight, has been found to interfere with the formation of acellular cementum in rats. The purpose of this study was to measure the force required to extract a tooth lacking normal acellular cementum, and to correlate this force and the ultimate periodontal strength with the morphology of the periodontal tissues at different time intervals after single or multiple injections of HEBP.

RESULTS

A single injection of HEBP given during root formation inhibited the formation of acellular cementum and resulted in a temporary reduction of the extraction force and the ultimate tensile strength. Ninety days after the injection of HEBP, both parameters were the same as in the controls. The increase in extraction force and ultimate tensile strength was associated with the onset of occlusal contact of the teeth. The organization of periodontal ligaments was improved after the teeth reached the occlusal level. After daily injections of HEBP for 3 days, there was a permanent reduction in root length and dento-alveolar ankylosis developed in the furcation area.

CONCLUSION

(1) A single or three injections of HEBP changed the formation of acellular cementum to that of an atypical hyperplastic cementum which increased the resorption risk at this site. (2) The ultimate tensile strength was markedly reduced in teeth lacking acellular cementum.

Growth and differentiation of periodontal ligament-derived cells in serum-free defined culture

We have developed a serum-free medium for the growth and differentiation of periodontal ligament-derived cells (PLC). In addition, the expression of both fibroblast growth factor (FGF) and FGF receptor (FGFR) in the PLC was investigated by immunohistochemical examination, heparin affinity chromatography (HAC), and reverse transcription-polymerase chain reaction (RT-PCR) analysis. Optimal growth of the cells was achieved in Iscove's modified Dulbecco's medium supplemented with insulin, transferrin, 2-mercaptoethanol, 2-ethanolamine, sodium selenite, and oleic acid in type-I collagen-coated dishes. Both FGF-1 and FGF-2 stimulated cell growth and inhibited differentiation as measured by inhibition of alkaline phosphatase activity of the cells. An immunohistochemical analysis of FGF-1 and FGF-2 revealed that immunoreactive FGF-1 and FGF-2 were detected predominantly in the cytoplasm of growing cells. In addition, perinuclear FGF-1 staining and nuclear FGF-2 staining were observed in the same growing cells. In contrast, a faint diffuse staining of FGF-1 and FGF-2 was detected in cytoplasm of the confluent differentiated cells. The 2.15 M NaCl eluate from HAC of the cell extracts exhibited growth-promoting activities for the PLC, and it also stimulated the growth of human umbilical vein-derived endothelial cells and inhibited binding of [125I]-FGF to its receptors, indicating the cells produced FGFs or FGF-like growth factors. RT-PCR analysis revealed that the cells expressed FGFR-1 mRNA but not mRNAs for FGFR-2, FGFR-3 and FGFR-4 mRNA. These results suggest that the FGF-FGFR-1 system plays an important role in the growth and differentiation of periodontal ligament-derived cells.

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.

Root surface defects in rat molar induced by 1-hydroxyethylidene-1,1-bisphosphonate

Cementum surface alterations induced by 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) in the maxillary first molars in rats were studied by means of scanning electron microscopy. Single or triple injections of HEBP inhibit the formation of acellular extrinsic fiber cementum and delay the formation of cellular mixed-fiber cementum. The results indicate the importance of acellular extrinsic fiber cementum as a protection barrier against root resorption and the different mechanisms underlying the formation of the two cementum varieties.

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.

Cell-bound and extracellular matrix-associated alkaline phosphatase activity in rat periodontal ligament. Experimental Oral Biology Group

In previous studies it was noted that alkaline phosphatase (ALP) activity in periodontal ligament does not only seem to be related to cells but may also be associated with the extracellular matrix. In an attempt to clarify this we studied the distribution of the enzyme at the electron microscopic level. In addition, ALP-activity was assessed biochemically following extraction of the ligament with (i) agents dissolving the membrane or splitting the phosphatidylinositol anchor (Triton X-100 or phosphatidylinositol-phospholipase C, respectively), and (ii) a matrix-degrading enzyme cocktail (collagenase, hyaluronidase and elastase). Histochemical observations revealed (a) a heterogeneous distribution of ALP-activity, with highest activity adjacent to the alveolar bone and (b) two pools of activity; one bound to cells and one associated with the collagenous extracellular matrix. In line with this were the biochemical data indicating that approximately 10% of the enzyme activity was firmly bound to the extracellular matrix and 90% to plasma membranes. Isoelectric focusing did not reveal differences between the two fractions, both samples yielding a single broad band corresponding with an isoelectric point of about 4.4.

Alkaline phosphatase activity in the periodontal ligament and gingiva of the rat molar: its relation to cementum formation

Alkaline phosphatase (ALP) is a glycoprotein thought to be involved in processes leading to mineral formation in tissues like bone and cementum. In the rat molar periodontium, several regions are associated with the formation of cementum (periodontal ligament, inner part of the gingiva), whereas other areas are not (e.g., the outer part of the gingiva just beneath the outer oral epithelium). In an attempt to establish how the spatial distribution of ALP activity relates to cementum formation, we assessed the activity of the enzyme quantitatively in the periodontium of the rat maxillary molars, by using the indoxyl-tetrazolium salt method. It appeared that the distribution of enzyme activity in the ligament was heterogeneous, indicating local variations in the phosphate household. Highest activity was found in areas related to mineralization, adjacent to the alveolar bone and cementum. Enzyme activity was higher adjacent to cellular cementum than to acellular cementum. With respect to acellular cementum, a highly significant positive correlation was found between ALP activity and cementum thickness, which indicates a close relationship between local phosphate production and cementum formation rate. An interesting observation in the connective tissue of the gingiva mesial to the first molar was a sharp demarcation between an ALP-positive inner part, adjacent to the tooth, and an ALP-negative outer part, underneath the outer oral epithelium. In the interdental gingiva, the entire connective tissue proved positive for the enzyme, suggesting that this region consists of the combined inner gingival parts of two adjacent teeth.

DNA content and alkaline phosphatase expression in cells of different gingival overgrowths

The present study compared the alkaline phosphatase (ALPase) expression and DNA content at specific periods in cultured cells derived from non-inflamed enlarged gingivae of idiopathic gingivofibromatosis (IGF) and phenytoin-induced hyperplasia (PHG). Cultured cells from healthy gingiva or periodontal ligament (PDL) were used as controls. The DNA assay, ALPase assay and cytochemical staining for ALPase in cultured cells were performed at four, seven, and nine days. The presence of intense ALPase activity was a prominent feature in cultured IGF cells, whereas very low ALPase activity was detected in PHG cells. The cell lines tested showed no significant differences in DNA content. The expression of ALPase in these cells was population density-dependent. The observation that cells isolated from both types of gingival overgrowth exhibited a different ALPase profile at variance with normal gingival fibroblasts suggested that a distinct pathogenic mechanism may be involved in each type of gingival overgrowth.

Formation of afibrillar acellular cementum-like layers induced by alkaline phosphatase activity from periodontal ligament explants maintained in vitro

Fibroblasts of the periodontal ligament, by their alkaline phosphatase (ALP) activity, are considered to play a role in the formation of acellular cementum. As a means of exploring this hypothesis, periodontal ligament explants from rat incisors were cultured in direct contact with bovine dentin slices in the presence of 10 mmol/L beta-glycerophosphate. Periosteal and pericardial tissue explants were maintained under similar conditions. After two weeks, the slices were harvested and processed for electron microscopic examination. Controls included periodontal ligament explants to which the ALP-inhibitor levamisole was added. The results suggest that only ALP-positive cultures from periodontal ligament and periosteum form mineralized layers along the dentin. After demineralization, layers consisted of fine filamentous or granular material of moderate electron-density and resembled afibrillar acellular cementum. Our findings support the hypothesis that periodontal ligament fibroblasts, by means of their ALP activity, play a pivotal role in the formation of acellular cementum.

Disturbances of cementum formation induced by single injection of 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) in rats: light and scanning electron microscopic studies

With the rat molar as a model, evidence is presented that dentin mineralization influences formation of acellular cementum. Formation of acellular cementum did not occur on the surface of experimentally induced unmineralized dentin. Instead, an atypical hyperplastic cementum was formed. The disturbance in acellular cementum formation was permanent.

Ultrastructure of cementogenesis as affected by growth hormone in the molar periodontium of the hypophysectomized rat

To document the effect of hypophysectomy and growth hormone replacement on the ultrastructure of cementogenesis in the developing rat third molar, 12 female Wistar rats were randomly allocated to normal control, hypophysectomized or hypophysectomized plus human growth hormone (for 10 days) treatment groups. The results of this study by electron and light microscopy and morphometry have shown that qualitative and quantitative changes occur in the organelles of cementoblasts forming cellular cementum as a result of hypophysectomy and growth hormone replacement. After hypophysectomy, the changes of less prominent nucleoli and nuclear pores, less prominent Golgi apparatuses and decreased endoplasmic reticulum can be interpreted as diminished cementum matrix biosynthesis--an interpretation that can be confirmed morphometrically by less cellular cementum formation. Growth hormone replacement for 10 days reactivates protein synthesis and cementogenesis as evidenced by ultrastructural changes in cementoblasts and a greater production of cementum.

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

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

Evidence for up-regulation of epidermal growth-factor receptors on rat periodontal ligament fibroblastic cells associated with stabilization of phenotype in vitro

This study sought to understand the role of epidermal growth factor receptor (EGF-R) in periodontal ligament (PDL) fibroblasts. Rat PDL fibroblastic cells and ROS 17/2.8 cells (highly differentiated osteoblastic osteosarcoma cells) were cultured and treated with transforming growth factor-alpha (TGF-alpha), EGF, dexamethasone (Dex) or a combination of EGF and Dex. Alkaline phosphatase (ALP) activity, an early differentiation marker for mineralized tissue-forming cells, was measured using p-nitrophenylphosphate as a substrate. For Scatchard analysis of [125I]-EGF binding, cells were incubated in Dulbecco's modified Eagle's medium containing 0.2% bovine serum albumin and 0-64 ng/ml of [125I]-EGF for 4 h at 4 degrees C. Also, the synthesis of EGF-R protein and the expression of mRNA for EGF-R were measured by immunoprecipitation and Northern blot analysis, respectively. Untreated PDL fibroblastic cells showed a gradual increase in spontaneous ALP activity from 32.4 U/10(6) cells at 2 days to 49.6 U/10(6) cells at 7 days of culture. ALP activity was further increased to 70.8 U/10(6) cells at 7 days after treatment with Dex, whereas EGF treatment reduced it to 19.4 U/10(6) cells. Culture of PDL fibroblastic cells in the presence of a combination of Dex and EGF decreased the Dex-induced ALP activity from 70.8 U to 41.8 U/10(6) cells at 7 days. A similar inhibitory effect on ALP activity was found after treatment with TGF-alpha. In contrast, ROS cells maintained a high ALP activity (1748 U/10(6) cells) throughout culture, unaffected by EGF. Scatchard analysis demonstrated that PDL fibroblastic cells have both high- and low-affinity forms of EGF-R, while ROS cells did not have any detectable EGF-R. Treatment of PDL cells with Dex for 2 days decreased the synthesis of EGF-R protein, the expression of EGF-R mRNA and the number of EGF-R. In contrast, EGF treatment increased the expression of EGF-R mRNA. These data suggest that PDL fibroblastic cells express numerous EGF-R, but the number decreases during their differentiation into mineralized tissue-forming cells under the influence of Dex. Thus, EGF-R may function in the stabilization of phenotype in PDL fibroblastic cells.

In vitro formation of mineralized nodules by periodontal ligament cells from the rat

The purposes of this study were to determine whether periodontal ligament (PDL) cells are capable of producing mineralized nodules in vitro and to analyze ultrastructural features of the nodules. Rat PDL cells were obtained from coagulum in the socket at 2 days after tooth extraction and cultured at confluence in standard medium containing Dulbecco's Modified Eagle's Medium supplemented with 10% FBS and antibiotics. To test mineralized nodule formation, cells were further cultured for an additional 3 weeks in the standard medium containing (1) ascorbic acid (50 micrograms/ml) and sodium beta-glycerophosphate (10 mM), (2) ascorbic acid, sodium beta-glycerophosphate, and dexamethasone (5 microM), or (3) ascorbic acid alone. Cells were then fixed in 2.5% glutaraldehyde, postfixed in 1% OsO4, and prepared for light and electron microscopy. Three-dimensional nodules containing mineralized matrices were formed only when the cells were cultured in the presence of ascorbic acid and dexamethasone. They were composed of multilayered fibroblasts (up to 13 layers), and highly organized collagen fibrils with 64 nm cross-banding patterns between the cell layers. The fibroblasts in the nodules exhibited an elongated shape with a high degree of cytoplasmic polarity throughout the nodule, and have the morphological features of PDL fibroblasts as seen in vivo. Mineral deposition with needle-like crystals was initiated on collagen fibrils located in intercellular spaces of the upper cell layers and became increasingly heavier towards the bottom half of the nodules. X-ray microanalysis and electron diffraction analysis confirmed that mineral deposition contained calcium and phosphate in the form of immature hydroxyapatite.(ABSTRACT TRUNCATED AT 250 WORDS)

Initial formation of cellular intrinsic fiber cementum in developing human teeth. A light- and electron-microscopic study

The present study describes the formative process of the initiation of cellular intrinsic fiber cementum (CIFC) in still growing human teeth. From 29 premolars and molars with incomplete roots developed to 60-90% of their final length, 8 premolars (with roots formed to three quarters of their final length) were selected for electron-microscopic investigation. All teeth were clinically intact and prefixed in Karnovsky's fixative immediately after extraction. Most of them were decalcified in ethylene diaminetetraacetic acid (EDTA), and the apical part of the roots was divided axially into mesial and distal portions that were subdivided in about 5 slices each. Following osmication and embedding in Epon, these blocks were cut for light- and electron-microscopic examination. In addition, 5 teeth with incomplete roots were freed from organic material and processed for scanning electron microscopy. It was found that CIFC-initiation commenced very close to the advancing root edge and resulted in a rapid cementum thickening. Thereafter, appositional growth continued on the already established cementum surface. Large, basophilic and rough endoplasmic reticulum-rich cementoblasts, some of which became cementocytes, were responsible for both fast and slow CIFC-formation. The CIFC-matrix was free of Sharpey's fibers and composed of more or less organized intrinsic collagen fibrils, in part fibril bundles, that ran roughly parallel to the root surface. Initially, the cementum fibrils intermingled with those of the dentinal collagen fibrils, which were not yet mineralized. This boundary subsequently underwent calcification. The development of collagen fibril bundles and their extracellular arrangement were associated with cytoplasmic processes probably involved in fibril formation and fibril assembly. Many cementoblasts contained intracytoplasmic, membrane-bounded collagen fibrils, which probably were related to fibril formation rather than degradation.

Bundle formation of principal fibers in rat molars

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

[Light, scanning and electron microscopic studies of the substrate dependence in replacement cementum formation]

The cement genesis obviously does not depend on surface parameters such as the exposition of dentin collagen. It can be reproduced on different surfaces of ceramics and is not bound to substrate. The classification primarily depends on the extent of contact of the cement with the substrate surface. However the integration of Interpore 200 seems to be more physiological compared to the integration of pure synthetic ceramics Osprovit. Yet both porous ceramics show no resorption stability and are therefore replaced by substances of cement. The size of the synthetic hydroxylapatite crystals is important to the bonding stability.

Alkaline phosphatase induces the deposition of calcified layers in relation to dentin: an in vitro study to mimic the formation of afibrillar acellular cementum

An attempt was made to test the hypothesis that alkaline phosphatase, an enzyme which is abundant in periodontal ligament, plays a role in the formation of acellular root cementum. Thin slices of bovine dentin were incubated in Iscove Modified Dulbecco's Medium supplemented with 10% normal rabbit serum and 10 mmol/L beta-glycerophosphate (beta-GP) or folded into pericardial explants. Intestinal bovine alkaline phosphatase (APase), covalently linked to agarose beads, was added to the cultures. In the presence of the enzyme, the dentin slices were covered with thin layers of mineralized material. Such layers were not observed in cultures not provided with APase-beads or beta-GP. They also did not form in relation to demineralized dentin. The layers of calcified material appeared to consist of crystallites embedded in a granular matrix of moderate electron density, which often exhibited the presence of incremental lines and resembled the matrix of afibrillar acellular cementum formed under in vivo conditions. When pericardial explants were interposed between the enzyme-containing beads and the dentin, mineral deposition in relation to the dentin was retarded. This finding lends support to the view that soft connective tissues interfere with the free diffusion of phosphate.

Formation of acellular root cementum in relation to dental and non-dental hard tissues in the rat

After the periodontium of the rat was wounded, the formation of acellular extrinsic fiber cementum (AEFC) did not appear to be restricted to the hard dental tissues (pre-existing cementum, dentin, and enamel). Layers resembling AEFC were also deposited along the inner wall of the alveolar bone. At the time of observation (six weeks after being wounded), cells other than fibroblast-like cells could not be distinguished close to the newly formed AEFC-like layers. We suggest that the deposition of this material was related to the activity of periodontal ligament fibroblasts.