Expression of type I and XII collagen during development of the periodontal ligament in the mouse

Characterization of a clonal human periodontal ligament stem cell line exposed to methacrylate resin-, bioactive glass-, or silicon-based root canal sealers

The aim of this study was to characterize a clonal human periodontal ligament (PDL) stem cell line (line 2-23 cells) cultured with root canal sealers based on methacrylate resin (SuperBond sealer; SB), bioactive glass (Nishika Canal Sealer BG; BG), or silicon (GuttaFlow 2; GF). The sealers were set in rubber molds to form sealer discs. Line 2-23 cells were cultured with or without the discs for 3 days. The cell viability was evaluated by direct cell counting and MTT assay. Inflammation-, PDL-, collagen-, and cell cycle-related gene expression was investigated by real-time RT-PCR. Collagen production was analyzed by Picro Sirius Red staining. Calcium ion concentration in the culture was measured by a QuantiChrom calcium assay kit. Line 2-23 cells survived when cultured with GF discs, but decreased cell viability was observed with SB and BG discs. The expression of inflammation-related genes was higher in cells cultured with SB discs, and expression of PDL-related genes was lower in cells exposed to SB and BG discs. These discs also down-regulated collagen production in line 2-23 cells. BG discs increased calcium ion concentration in the culture medium. Cells exposed to GF discs exhibited the same inflammation-, PDL-, collagen-, and cell cycle-related gene expression and collagen production as untreated cells. These results suggested that the characteristics of line 2-23 cells cultured with GF discs was highly resemble to untreated cells throughout the 3 days of the culture model.

Membrane proteome characterization of periodontal ligament cell sets from deciduous and permanent teeth

BACKGROUND

Physiological roles for the periodontal ligament (PDL) include tooth eruption and anchorage, force absorption, and provision of proprioceptive information. Despite the advances in understanding the biology of PDL cells, there is a lack of information regarding the molecular signature of deciduous (DecPDL) and permanent (PermPDL) PDL tissues. Thus, the present study was designed to characterize the membrane proteome of DecPDL and PermPDL cells.

METHODS

Primary PDL cells were obtained (n = 6) and a label-free quantitative proteome of cell membrane-enriched components was performed. Proteome findings were validated by quantitative polymerase chain reaction and Western blot assays in fresh human tissues (n = 8) and primary cell cultures (n = 6). In addition, confocal microscopy was used to verify the expression of target factors in the PDL cell cultures.

RESULTS

Comparative gene ontology enrichment analysis evidenced that most stickling differences involved "endomembrane system" (PICALM, STX4, and LRP10), "hydrolase activity" (NCSTN and XRCC6), "protein binding" (PICALM, STX4, GPNMB, VASP, extended-synaptotagmin 2 [ESYT2], and leucine-rich repeat containing 15 [LRRC15]), and "isomerase activity" (FKBP8). Data are available via ProteomeXchange with identifier PXD010226. At the transcript level, high PICALM in DecPDL and ESYT2 and LRRC15 in PermPDL were confirmed in fresh PDL tissues. Furthermore, Western blot analysis confirmed increased levels of PICALM, LRRC15, and ESYT2 in cells and/or fresh tissues, and confocal microscopy confirmed the trends for PICALM and LRRC15 expression in PDL cells.

CONCLUSION

We report the first comprehensive characterization of the membrane protein machinery of DecPDL and PermPDL cells, and together, we identified a distinct molecular signature for these cell populations, including unique proteins for DecPDL and PermPDL.

Osteopontin regulates dentin and alveolar bone development and mineralization

The periodontal complex is essential for tooth attachment and function and includes the mineralized tissues, cementum and alveolar bone, separated by the unmineralized periodontal ligament (PDL). To gain insights into factors regulating cementum-PDL and bone-PDL borders and protecting against ectopic calcification within the PDL, we employed a proteomic approach to analyze PDL tissue from progressive ankylosis knock-out (Ank^-/-) mice, featuring reduced PP_i, rapid cementogenesis, and excessive acellular cementum. Using this approach, we identified the matrix protein osteopontin (Spp1/OPN) as an elevated factor of interest in Ank^-/- mouse molar PDL. We studied the role of OPN in dental and periodontal development and function. During tooth development in wild-type (WT) mice, Spp1 mRNA was transiently expressed by cementoblasts and strongly by alveolar bone osteoblasts. Developmental analysis from 14 to 240days postnatal (dpn) indicated normal histological structures in Spp1^-/- comparable to WT control mice. Microcomputed tomography (micro-CT) analysis at 30 and 90dpn revealed significantly increased volumes and tissue mineral densities of Spp1^-/- mouse dentin and alveolar bone, while pulp and PDL volumes were decreased and tissue densities were increased. However, acellular cementum growth was unaltered in Spp1^-/- mice. Quantitative PCR of periodontal-derived mRNA failed to identify potential local compensators influencing cementum in Spp1^-/- vs. WT mice at 26dpn. We genetically deleted Spp1 on the Ank^-/- mouse background to determine whether increased Spp1/OPN was regulating periodontal tissues when the PDL space is challenged by hypercementosis in Ank^-/- mice. Ank^-/-; Spp1^-/- double deficient mice did not exhibit greater hypercementosis than that in Ank^-/- mice. Based on these data, we conclude that OPN has a non-redundant role regulating formation and mineralization of dentin and bone, influences tissue properties of PDL and pulp, but does not control acellular cementum apposition. These findings may inform therapies targeted at controlling soft tissue calcification.

Extracellular Matrix from Periodontal Ligament Cells Could Induce the Differentiation of Induced Pluripotent Stem Cells to Periodontal Ligament Stem Cell-Like Cells

The periodontal ligament (PDL) plays an important role in anchoring teeth in the bone socket. Damage to the PDL, such as after severe inflammation, can be treated with a therapeutic strategy that uses stem cells derived from PDL tissue (PDLSCs), a strategy that has received intense scrutiny over the past decade. However, there is an insufficient number of PDLSCs within the PDL for treating such damage. Therefore, we sought to induce the differentiation of induced pluripotent stem (iPS) cells into PDLSCs as an initial step toward PDL therapy. To this end, we first induced iPS cells into neural crest (NC)-like cells. We then captured the p75 neurotrophic receptor-positive cells (iPS-NC cells) and cultured them on an extracellular matrix (ECM) produced by human PDL cells (iPS-NC-PDL cells). These iPS-NC-PDL cells showed reduced expression of embryonic stem cell and NC cell markers as compared with iPS and iPS-NC cells, and enrichment of mesenchymal stem cell markers. The cells also had a higher proliferative capacity, multipotency, and elevated expression of PDL-related markers than iPS-NC cells cultured on fibronectin and laminin (iPS-NC-FL cells) or ECM produced by human skin fibroblast cells (iPS-NC-SF cells). Overall, we present a culture method to produce high number of PDLSC-like cells from iPS cells as a first step toward a strategy for PDL regeneration.

Regulators of Collagen Fibrillogenesis during Molar Development in the Mouse

Development of mammalian teeth and surrounding tissues includes time-space changes in the extracellular matrix composition and organization. This requires complex control mechanisms to regulate its synthesis and remodeling. Fibril-associated collagens with interrupted triple helices (FACITs) and a group of small leucine-rich proteoglycans (SLRPs) are involved in the regulation of collagen fibrillogenesis. Recently, collagen type XII and collagen type XIV, members of the FACITs family, were found in the peridental mesenchyme contributing to alveolar bone formation. This study was designed to follow temporospatial expression of collagen types XIIa and XIVa in mouse first molar and adjacent tissues from embryonic day 13, when the alveolar bone becomes morphologically apparent around the molar tooth bud, until postnatal day 22, as the posteruption stage. The patterns of decorin, biglycan, and fibromodulin, all members of the SLRPs family and interacting with collagens XIIa and XIVa, were investigated simultaneously. The situation in the tooth was related to what happens in the alveolar bone, and both were compared to the periodontal ligament. The investigation provided a complex localization of the five antigens in soft tissues, the dental pulp, and periodontal ligaments; in the mineralized tissues, predentin/dentin and alveolar bone; and junction between soft and hard tissues. The results illustrated developmentally regulated and tissue-specific changes in the balance of the two FACITs and three SLRPs.

Multiple essential MT1-MMP functions in tooth root formation, dentinogenesis, and tooth eruption

Membrane-type matrix metalloproteinase 1 (MT1-MMP) is a transmembrane zinc-endopeptidase that breaks down extracellular matrix components, including several collagens, during tissue development and physiological remodeling. MT1-MMP-deficient mice (MT1-MMP(-/-)) feature severe defects in connective tissues, such as impaired growth, osteopenia, fibrosis, and conspicuous loss of molar tooth eruption and root formation. In order to define the functions of MT1-MMP during root formation and tooth eruption, we analyzed the development of teeth and surrounding tissues in the absence of MT1-MMP. In situ hybridization showed that MT1-MMP was widely expressed in cells associated with teeth and surrounding connective tissues during development. Multiple defects in dentoalveolar tissues were associated with loss of MT1-MMP. Root formation was inhibited by defective structure and function of Hertwig's epithelial root sheath (HERS). However, no defect was found in creation of the eruption pathway, suggesting that tooth eruption was hampered by lack of alveolar bone modeling/remodeling coincident with reduced periodontal ligament (PDL) formation and integration with the alveolar bone. Additionally, we identified a significant defect in dentin formation and mineralization associated with the loss of MT1-MMP. To segregate these multiple defects and trace their cellular origin, conditional ablation of MT1-MMP was performed in epithelia and mesenchyme. Mice featuring selective loss of MT1-MMP activity in the epithelium were indistinguishable from wild type mice, and importantly, featured a normal HERS structure and molar eruption. In contrast, selective knock-out of MT1-MMP in Osterix-expressing mesenchymal cells, including osteoblasts and odontoblasts, recapitulated major defects from the global knock-out including altered HERS structure, short roots, defective dentin formation and mineralization, and reduced alveolar bone formation, although molars were able to erupt. These data indicate that MT1-MMP activity in the dental mesenchyme, and not in epithelial-derived HERS, is essential for proper tooth root formation and eruption. In summary, our studies point to an indispensable role for MT1-MMP-mediated matrix remodeling in tooth eruption through effects on bone formation, soft tissue remodeling and organization of the follicle/PDL region.

Microscopic analysis of molar--incisor malformation

OBJECTIVE

Molar-incisor malformation (MIM) is a newly discovered type of dental anomaly that involves a characteristic root malformation of the permanent first molars. The aim of this study was to reveal the microstructure of MIM teeth in order to determine their origin.

STUDY DESIGN

Four MIM teeth were extracted from a 9-year-old girl due to severe mobility. The detailed microstructure of the teeth was determined by examinations with micro-computed tomography (micro-CT), hematoxylin and eosin (H&E) staining, immunohistochemical staining, and scanning electron microscopy to reveal the detailed microstructure.

RESULTS

Micro-CT and H&E staining revealed the pulpal floor comprising three layers: upper, middle, and lower. Amorphous hard tissues and hyperactive cells were observed in the middle layer of the pulpal floor, and the cells stained positively for dentin sialoprotein and osteocalcin, but not for collagen XII.

CONCLUSION

The results of the present study imply that MIM-affected molars probably result from inappropriate differentiation of the apical pulp and dental follicle.

Mechano-regulation of collagen biosynthesis in periodontal ligament

Periodontal ligament (PDL) plays critical roles in the development and maintenance of periodontium such as tooth eruption and dissipation of masticatory force. The mechanical properties of PDL are mainly derived from fibrillar type I collagen, the most abundant extracellular component. The biosynthesis of type I collagen is a long, complex process including a number of intra- and extracellular post-translational modifications. The final modification step is the formation of covalent intra- and intermolecular cross-links that provide collagen fibrils with stability and connectivity. It is now clear that collagen post-translational modifications are regulated by groups of specific enzymes and associated molecules in a tissue-specific manner; and these modifications appear to change in response to mechanical force. This review focuses on the effect of mechanical loading on collagen biosynthesis and fibrillogenesis in PDL with emphasis on the post-translational modifications of collagens, which is an important molecular aspect to understand in the field of prosthetic dentistry.

Comparative gene expression analysis of the human periodontal ligament in deciduous and permanent teeth

There are histological and functional differences between human deciduous and permanent periodontal ligament (PDL) tissues. The aim of this study was to determine the differences between these two types of tissue at the molecular level by comparing their gene expression patterns. PDL samples were obtained from permanent premolars (n = 38) and anterior deciduous teeth (n = 31) extracted from 40 healthy persons. Comparative cDNA microarray analysis revealed several differences in gene expression between the deciduous and permanent PDL tissues. These findings were verified by qRT-PCR (quantitative reverse-transcription-polymerase chain reaction) analysis, and the areas where genes are expressed were revealed by immunohistochemical staining. The expressions of 21 genes were up-regulated in deciduous relative to PDL tissues, and those of 30 genes were up-regulated in permanent relative to deciduous PDL tissues. The genes that were up-regulated in deciduous PDL tissues were those involved in the formation of the extracellular matrix (LAMC2, LAMB3, and COMP), tissue development (IGF2BP, MAB21L2, and PAX3), and inflammatory or immune reactions leading to tissue degradation (IL1A, CCL21, and CCL18). The up-regulated genes in permanent PDL tissues were related to tissue degradation (IL6 and ADAMTS18), myocontraction (PDE3B, CASQ2, and MYH10), and neurological responses (FOS, NCAM2, SYT1, SLC22A3, DOCK3, LRRTM1, LRRTM3, PRSS12, and ARPP21). The analysis of differential gene expressions between deciduous and permanent PDL tissues aids our understanding of histological and functional differences between them at the molecular level.

Implications of cultured periodontal ligament cells for the clinical and experimental setting: a review

The periodontal ligament (PDL) is a key contributor to the process of regeneration of the periodontium. The heterogeneous nature of the PDL tissue, its development during early adulthood, and the different conditions to which the PDL tissue is exposed to in vivo impart on the PDL unique characteristics that may be of consequence during its cultivation in vitro. Several factors affecting the in vivo setting influence the behaviour of PDL fibroblasts in culture. The purpose of this review is to address distinct factors that influence the behaviour of PDL fibroblasts in culture -in vivo-in vitro transitions, cell identification/isolation markers, primary PDL cultures and cell lines, tooth-specific factors, and donor-specific factors. Based on the reviewed studies, the authors recommendations include the use of several identification markers to confirm cell identity, use of primary cultures at early passage to maintain unique PDL heterogeneic characteristics, and noting donor conditions such as age, systemic health status, and tooth health status. Continued efforts will expand our understanding of the in vitro and in vivo behaviour of cells, with the goal of orchestrating optimal periodontal regeneration. This understanding will lead to improved evidence-based rationales for more individualized and predictable periodontal regenerative therapies.

Dilated capillaries, disorganized collagen fibers and differential gene expression in periodontal ligaments of hypomorphic fibrillin-1 mice

The periodontal ligaments (PDLs) are soft connective tissue between the cementum covering the tooth root surface and alveolar bone. PDLs are composed of collagen and elastic system fibers, blood vessels, nerves, and various types of cells. Elastic system fibers are generally formed by elastin and microfibrils, but PDLs are mainly composed of the latter. Compared with the well-known function of collagen fibers to support teeth, little is known about the role of elastic system fibers in PDLs. To clarify their role, we examined PDLs of mice under-expressing fibrillin-1 (mgR mice), which is one of the major microfibrillar proteins. The PDLs of homozygous mgR mice showed one-quarter of the elastic system fibers of wild-type (WT) mice. A close association between the elastic system fibers and the capillaries was noted in WT, homozygous and heterozygous mgR mice. Interestingly, capillaries in PDLs of homozygous mice were dilated or enlarged compared with those of WT mice. A comparable level of type I collagen, which is the major collagen in PDLs, was expressed in PDL-cells of mice with three genotypes. However, multi-oriented collagen fiber bundles with a thinner appearance were noted in homozygous mice, whereas well-organized collagen fiber bundles were seen in WT mice. Moreover, there was a marked decrease in periostin expression, which is known to regulate the fibrillogenesis and crosslinking of collagen. These observations suggest that the microfibrillar protein, fibrillin-1, is indispensable for normal tissue architecture and gene expression of PDLs.

The KK-Periome database for transcripts of periodontal ligament development

The periodontal ligament (PDL) is a strong connective tissue that surrounds the tooth root, absorbs occlusal forces, and functions as a sense organ. PDL originated from dental follicle (DF), which possessed mesenchymal progenitors in the developing tooth germ. However, as specific marker genes for PDL and DF are currently unavailable, the molecular mechanisms of PDL development are yet to be clarified. To facilitate the identification of such genes, we have previously established a transcriptome database of the human PDL (the KK-Periome database) and screened for specific genes expressed during PDL development. Initial screening of the database revealed two marker genes for distinguishing DF and PDL. The KK-Periome database thus appears to offer a useful resource for investigating genes involved in PDL development.

Downregulation of matrix metalloproteinases in hyperplastic dental follicles results in abnormal tooth eruption

In this study, we compared the gene expression profiles of non-syndromic hyperplastic dental follicle (HDF) fibroblasts and normal dental follicle (NDF) fibroblasts using cDNA microarrays, quantitative PCR, and immunohistochemical staining. Microarray analysis showed that several collagens genes were upregulated in the HDFos, including collagen types I, IV, VIII, and XI and TIMP-1, -3, and -4 (fold ratio > 2.0). In contrast, the expression of MMP-1, -3, -10, and -16 together with IL-8 was more than two fold downregulated. The differential expression of the genes encoding alkaline phosphatase, MMP-1, -3, -8, and IL-8 was confirmed by quantitative RT-PCR, while that of 24 HDFs and 18 NDFs was confirmed by immunohistochemical analysis. However, HDFs showed stronger expression of MMP-3 than NDFs (P < 0.001). Collectively, these results indicate that defective regulation of MMPs mediating connective tissue remodeling may be responsible for abnormal tooth eruption.

Establishment of immortalized dental follicle cells for generating periodontal ligament in vivo

The dental follicle is a mesenchymal tissue that surrounds the developing tooth germ. During tooth root formation, periodontal components, viz., cementum, periodontal ligament (PDL), and alveolar bone, are created by dental follicle progenitors. Here, we report the presence of PDL progenitors in mouse dental follicle (MDF) cells. MDF cells were obtained from mouse incisor tooth germs and immortalized by the expression of a mutant human papilloma virus type 16 E6 gene lacking the PDZ-domain-binding motif. MDF cells expressing the mutant E6 gene (MDF( E6-EGFP ) cells) had an extended life span, beyond 150 population doublings (PD). In contrast, normal MDF cells failed to proliferate beyond 10 PD. MDF( E6-EGFP ) cells expressed tendon/ligament phenotype-related genes such as Scleraxis (Scx), growth and differentiation factor-5, EphA4, Six-1, and type I collagen. In addition, the expression of periostin was observed. To elucidate the differentiation capacity of MDF( E6-EGFP ) cells in vivo, the cells were transplanted into severe combined immunodeficiency mice. At 4 weeks, MDF( E6-EGFP ) cell transplants had the capacity to generate a PDL-like tissue that expressed periostin, Scx, and type XII collagen and the fibrillar assembly of type I collagen. Our findings suggest that MDF( E6-EGFP ) cells can act as PDL progenitors, and that these cells may be a useful research tool for studying PDL formation and for developing regeneration therapies.

The epithelial cell rests of Malassez--a role in periodontal regeneration?

This article reviews general aspects about the epithelial cell rests of Malassez (ERM). The historical and general morphological features of the ERM are briefly described. The embryological derivation of the ERM is presented as an important consideration in understanding the events associated with their origin and possible functional roles within the periodontal ligament. The ultrastructural description of the ERM is also included to complement the morphological characteristics which distinguish these cells as the unique epithelial element of the periodontal ligament. The unique ability of these cells to synthesize and secrete a number of proteins usually associated with cells of mesenchymal origin, rather than ectodermal origin, is discussed in light of their role in cementum repair and regeneration. Such considerations lead to our hypothesis that one of the functional roles of the ERM may lie not only their role in maintaining and contributing to the normal periodontal cellular elements and function but also contributing, in a significant manner, to periodontal regeneration.

Establishing and characterizing human periodontal ligament fibroblasts immortalized by SV40T-antigen and hTERT gene transfer

The periodontal ligament (PDL) is a highly specialized tissue connecting the cementum with the tooth socket bone and affects the life span of the tooth. However, little is known about the precise characteristics and regenerative mechanism of PDL cells because of the absence of specific markers and cell lines. Therefore, we aimed to establish three immortalized human PDL fibroblast cell lines by using simian virus40 T-antigen (SV40T-Ag) and human telomerase reverse transcriptase (hTERT) transfection, expecting these cells to have the characteristics of primary cells. The transfected cells were named STPLF. The expression of SV40T-Ag and hTERT in all STPLF lines was verified by using the semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) method, stretch PCR analysis, or Western blotting analysis. All STPLF showed stable proliferation at more than 120 population doublings (PD), whereas primary human PDL fibroblasts (HPLF) stopped at 10-20 PD. Characterization by RT-PCR analysis revealed that all STPLF genes mimicked the expression of their respective original HPLF genes. STPLF expressed runt-related transcription factor-2, osterix, alkaline phosphatase, osteopontin, osteocalcin, periostin, receptor activator of NF-kappa B ligand, osteoprotegerin, epidermal growth factor receptor, alpha-smooth muscle actin, and type XII collagen. STPLF stimulated with 50 micro g/ml ascorbic acid and 2 mM beta-glycerophosphate for 4 weeks produced more calcified deposits than did HPLF cultured with the same reagents. These results suggest that each STPLF line retained the characteristics of the respective original HPLF, that STPLF gained increased calcification activity, and that STPLF are helpful tools for studying the biology and regenerative mechanisms of human PDL.

Position Paper: Periodontal Regeneration

Untreated periodontal disease leads to tooth loss through destruction of the attachment apparatus and tooth-supporting structures. The goals of periodontal therapy include not only the arrest of periodontal disease progression,but also the regeneration of structures lost to disease where appropriate. Conventional surgical approaches (e.g., flap debridement) continue to offer time-tested and reliable methods to access root surfaces,reduce periodontal pockets, and attain improved periodontal form/architecture. However, these techniques offer only limited potential towards recovering tissues destroyed during earlier disease phases. Recently, surgical procedures aimed at greater and more predictable regeneration of periodontal tissues and functional attachment close to their original level have been developed, analyzed, and employed in clinical practice. This paper provides a review of the current understanding of the mechanisms, cells, and factors required for regeneration of the periodontium and of procedures used to restore periodontal tissues around natural teeth. Targeted audiences for this paper are periodontists and/or researchers with an interest in improving the predictability of regenerative procedures. This paper replaces the version published in 1993.

Isolation of precursor cells (PCs) from human dental follicle of wisdom teeth

The dental follicle is an ectomesenchymal tissue surrounding the developing tooth germ. It is believed that this tissue contains stem cells and lineage committed progenitor cells or precursor cells (PCs) for cementoblasts, periodontal ligament cells, and osteoblasts. In this study, we report the isolation of PCs derived from dental follicle of human third molar teeth. These fibroblast-like, colony forming and plastic adherent cells expressed putative stem cell markers Notch-1 and Nestin. We compared gene expressions of PCs, human mesenchymal stem cells (hMSCs), periodontal ligament cells (PDL-cells) and osteoblasts (MG63) for delimitation of PCs. Interestingly, PCs expressed higher amounts of insulin-like growth factor-2 (IGF-2) transcripts than hMSCs. Differentiation capacity was demonstrated under in vitro conditions for PCs. Long-term cultures with dexamethasone produced compact calcified nodules or appeared as plain membrane structures of different dimensions consisting of a connective tissue like matrix encapsulated by a mesothelium-like cellular structure. PCs differentially express osteocalcin (OCN) and bone sialoprotein (BS) after transplantation in immunocompromised mice but without any sign of cementum or bone formation. Therefore, our results demonstrate that cultured PCs are unique undifferentiated lineage committed cells residing in the periodontium prior or during tooth eruption.

Preferential expression of L-type amino acid transporter 1 in ameloblasts during rat tooth development

Certain amino acid transport systems play an important role in supplying organic nutrients to each cell and for cell proliferation during tooth development. However, the mechanisms responsible for such actions are unclear. This study demonstrated for the first time that LAT1 and 4F2hc are expressed during tooth development in prenatal and postnatal rats, and that the transporters show cell-specific expression in ameloblasts, which are the epithelium-derived dental cells. LAT1 and 4F2hc expression was not observed in other dental cells of the developing teeth such as odontoblasts and cementoblasts. Overall, these results suggest that LAT1 and 4F2hc might play an important role in enamel formation.

New Insight into Progenitor/Stem Cells in Dental Pulp Using Col1a1-GFP Transgenes

In recent years there has been increasing progress in identifying stem cells from adult tissues and their potential application in tissue engineering. These advances provide a promising future for tooth replacement/regeneration. Essential for this approach is the identification of donor stem cells for various components of the teeth. Our studies show that pOBCol3.6GFPtpz and pOBCol2.3GFPemd transgenic animals provide a unique model to gain insight into stem cells in the dental pulp. Our in vivo studies of the developing teeth of these transgenic lines show both Col1a1-GFP transgenes are expressed in functional and fully differentiated odontoblasts. The patterns of expression of Col1a1-GFP transgenes during odontoblast differentiation correlates with the expression of DSPP. In the developing craniofacial bones both Col1a1-GFP transgenes are also expressed in osteoblasts and osteocytes of alveolar and calvarial bones. In the alveolar bones, the expression of Col1a1-GFP in osteocytes correlates with the expression of DMP1. Col1a1-3.6-GFP is expressed in the entire layer of the periosteum and in suture mesenchyme containing osteoprogenitor cells. On the other hand, Col1a1-2.3- GFP expression was limited to the osteoblastic layer of the periosteum and was not detected in the fibroblastic layer of the periosteum or in the suture mesenchyme. These observations indicate that Col1a1-3.6-GFP and Col1a1-2.3-GFP transgenes identify different subpopulations of cells during intramembranous ossification. By using the coronal portion of dental pulps isolated from postnatal transgenic mice our observations also provide direct evidence that the dental pulp contains progenitor/stem cells capable of giving rise to a new generation of odontoblast-like cells, as well as osteoblast-like cells.

Expression of MMP-8 and MMP-13 mRNAs in rat periodontium during tooth eruption

The present study was designed to investigate mRNA expression of matrix metalloproteinase-8 (MMP-8) and MMP-13 in forming periodontium during tooth eruption in the rat. RT-PCR for the decalcified paraffin sections indicated expression of MMP-8 and MMP-13 in the periodontal tissues. In situ hydridization demonstrated expression of MMP-8 in osteoblasts, osteocytes, periodontal ligament cells, cementoblasts, and cementocytes along with collagen types I and III. In contrast, transcripts of MMP-13 were confined to a small population of osteoblasts and osteocytes in alveolar bone. The results suggested that MMP-8 may be involved in remodeling the periodontium during tooth eruption, and its expression may be coordinated with that of collagen types I and III, whereas the participation of MMP-13 may be rather limited.

Developmental distribution of collagen type XII in cartilage: association with articular cartilage and the growth plate

Collagen type XII is a member of the fibril-associated collagens and is characterized by a short triple-helical domain with three extended noncollagenous NC3 domains. Previous studies suggested that collagen XII is a component of cartilage but little is known about its spatial-temporal distribution. This study uses a polyclonal antibody to the purified NC3 domain to investigate its developmental distribution in rat forelimb. Collagen XII was present at the joint interzone on embryonic day 16 (E16d) and restricted to the presumptive articular cartilage by E18d. Labeling of the articular surface intensified as development progressed postnatally (day 1 [1d] to 28d) and extended approximately six cell diameters deep. In juvenile rats, collagen XII antibodies also labeled the longitudinal and transverse septa of stacked chondrocytes in the growth plate. However, collagen XII was not associated at any developmental stage with the cartilaginous secondary ossification center and was only weakly expressed in epiphyseal cartilage. Ultrastructural localization of the NC3 domain epitope showed labeling of the surface of collagen II fibrils both in tissue and in isolated fibrils. The results presented provide further evidence that articular cartilage differs substantially from the underlying epiphyseal cartilage and that different chondrocytic developmental fates are reflected in the composition of their extracellular matrix starting early in development. In addition, collagen XII was distributed in areas of cartilage with more organized fibril orientation and may have a role in promoting alignment or stabilizing such an organization, thereby creating a matrix capable of withstanding load-bearing forces.

Collagen XII mutation disrupts matrix structure of periodontal ligament and skin

Collagen XII has been postulated to organize the extracellular matrix (ECM) architecture of dense connective tissues such as the periodontal ligament (PDL) and skin. The objective of this study was to test this hypothesis in transgenic mice carrying a dominant interference mutation of collagen XII. The truncated alpha1(XII) collagen minigene construct MXIINC3(-), driven by the mouse alpha2(I) collagen promoter, was prepared and used to generate transgenic mouse lines. The PDL matrix fibers of molar teeth lost the ordered architecture characteristic of ligament tissue without noticeable inflammation. Cellular cement appeared to be disrupted at the PDL insertion. By confocal laser scanning microscopy, the PDL of transgenic mice demonstrated swollen and irregularly arranged collagen fibers associated with internal porosity. The skin of transgenic mice revealed the lack of matrix fiber structure in the papillary dermis. These results indicated that the dominant interference mutation of collagen XII disorganized the ECM architecture of PDL and skin.

Employing a transgenic animal model to obtain cementoblasts in vitro

BACKGROUND

Proper formation of cementum, a mineralized tissue lining the tooth root surface, is required for development of a functional periodontal ligament. Further, the presence of healthy cementum is considered to be an important criterion for predictable restoration of periodontal tissues lost as a consequence of disease. Despite the significance of cementum to general oral health, the mechanisms controlling development and regeneration of this tissue are not well understood and research has been hampered by the lack of adequate in vitro experimental models.

METHODS

In an effort to establish cementoblast cell populations, without the trappings of a heterogeneous population containing periodontal ligament (PDL) cells, cells were obtained from the root surface of first mandibular molars of OC-TAg transgenic mice. These mice contain the SV40 large T-antigen (TAg) under control of the osteocalcin (OC) promoter. Therefore, only cells that express OC also express TAg and are immortalized in vitro. Based on results of prior in situ studies, OC is expressed by cementoblasts during root development, but not by cells within the PDL. Consequently, when populations are isolated from developing molars using collagenase/trypsin digestion, only cementoblasts, not PDL cells, are immortalized and thus, will survive in culture.

RESULTS

The resulting immortalized cementoblast population (OC/CM) expressed bone sialoprotein (BSP), osteopontin (OPN), and OC, markers selective to cells lining the root surface. These cells also expressed type I and XII collagen and type I PTH/PTHrP receptor (PTH1R). In addition to expression of genes associated with cementoblasts, OC/CM cells promoted mineral nodule formation and exhibited a PTHrP mediated cAMP response.

CONCLUSIONS

This approach for establishing cementoblasts in vitro provides a model to study cementogenesis as required to enhance our knowledge of the mechanisms controlling development, maintenance, and regeneration of periodontal tissues.

Evolution of periodontal regeneration: from the roots' point of view

Tissues lost as a consequence of periodontal diseases, i.e. bone, cementum and a functional periodontal ligament (PDL), can be restored to some degree. Nevertheless, results are often disappointing. There is a need to develop new paradigms for regenerating periodontal tissues that are based on an understanding of the cellular and molecular mechanisms regulating the development and regeneration of periodontal tissues. As one approach we have developed strategies for maintaining cementoblasts in culture by first determining the gene profile for these cells in situ. Next, cells were immortalized in vitro using SV 40 large T antigen (SV40 Tag) or by using mice containing transgenes enabling cellular immortality in vitro. Cementoblasts in vitro retained expression of genes associated with mineralized tissues, bone sialoprotein and osteocalcin, that were not linked with periodontal fibroblasts either in situ or in vitro. Further, cementoblasts promoted mineralization in vitro as measured by von Kossa and ex vivo using a severely compromised immunodeficient (SCID) mouse model. These cells responded to growth factors by eliciting changes in gene profile and mitogenesis and to osteotropic hormones by evoking changes in gene profile and ability to induce mineral nodule formation in vitro. The ultimate goal of these studies is to provide the knowledge base required for designing improved modalities for use in periodontal regenerative therapies.