The periodontal ligament: a unique, multifunctional connective tissue

Prostaglandin E2 (PGE2) downregulates interleukin (IL)-1alpha-induced IL-6 production via EP2/EP4 subtypes of PGE2 receptors in human periodontal ligament cells

OBJECTIVES

Prostaglandin E2 (PGE2) exerts its biological actions via EP receptors, which are divided into four subtypes, EP1, EP2, EP3 and EP4. In the present study, we examined whether PGE2 regulated interleukin (IL)-1alpha-induced IL-6 production in human periodontal ligament (PDL) cells and if so, which subtypes of PGE2 receptors were involved.

METHODS

PDL cells were stimulated with vehicle or IL-1alpha in the presence or absence of indomethacin (a cylooxygenase inhibitor), PGE2 or various EP agonists. IL-6 and PGE2 levels were measured by enzyme-linked immunosorbent assay. EP receptor mRNA expression was examined by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Indomethacin significantly enhanced IL-1alpha-induced IL-6 production by PDL cells, although it completely inhibited IL-1alpha-induced PGE2 production. Exogenous PGE2 significantly suppressed IL-1alpha-induced IL-6 production. Butaprost, a selective EP2 agonist, and ONO-AE1-329, a selective EP4 agonist, significantly inhibited IL-1alpha-induced IL-6 production, although 17-phenyl-omega-trinor PGE2, an EP1 agonist, and ONO-AP-324, an EP3 agonist, did not affect it. RT-PCR analysis showed that EP2 and EP4 mRNA was expressed in PDL cells.

CONCLUSIONS

We suggest that PGE2 downregulates IL-1alpha-induced IL-6 production via EP2/EP4 receptors in human PDL cells.

Lipoxin signaling in neutrophils and their role in periodontal disease

Endogenous molecules involved in counterregulation of inflammatory responses provide an opportunity to explore new therapeutic approaches based on manipulation of new pathways that may reduce the possibility of unwanted toxic side effects. Lipoxins (LX) are trihydroxytetraene-containing eicosanoids that are generated within the vascular lumen during cell-cell interactions or at mucosa through leukocyte-epithelial cell interactions. Transcellular biosynthetic pathways are the major lipoxin biosynthetic routes where LX are formed in vivo during inflammation and serve as "stop signals" that regulate key steps in leukocyte trafficking. In this review, recent findings in lipoxin generation, impact on the resolution of acute inflammation, and organ protection from neutrophil-mediated injury are presented. Periodontitis, specifically localized aggressive periodontitis, which is recognized as an example of neutrophil-mediated tissue injury, is discussed as a disease model where LX and other endogenous pro-resolution pathway mediators could have potential value.

Distinctive localization and function for lumican, fibromodulin and decorin to regulate collagen fibril organization in periodontal tissues

BACKGROUND

Small leucine-rich proteoglycans (SLRPs) decorin, biglycan, fibromodulin and lumican are secreted extracellular matrix molecules that associate with fibrillar collagens and regulate collagen fibrillogenesis. Collagens are the major extracellular matrix components of periodontal connective tissues where they provide mechanical attachment of the tooth to the bone and gingiva and mediate signals that regulate cell functions, including remodeling of the periodontal ligament and bone. Structural organization of collagen may also be important for the defense against periodontal disease, because in certain conditions abnormal collagen fibrils associate with increased susceptibility to periodontal disease.

OBJECTIVES

The purpose of this study was to find out the role of SLRPs to regulate collagen fibril and fibril bundle formation in periodontal tissues.

METHODS

The localization of SLRPs in human and mouse periodontal tissues was studied using immunohistochemical methods. To assess the function of SLRPs we studied periodontal tissues of mice harboring targeted deletions of decorin, fibromodulin or lumican genes and lumican and fibromodulin double knockout mice using histological and electronmicroscopical methods.

RESULTS

The SLRPs were coexpressed in human and mouse gingival and periodontal ligament connective tissues where they colocalized with collagen fibril bundles. Teeth in the knockout animals were fully erupted and showed normal gross morphology. Targeted deletion of decorin, fibromodulin, lumican or both lumican and fibromodulin resulted in abnormal collagen fibril and fibril bundle morphology that was most evident in the periodontal ligament. Each of the gene deletions resulted in a unique fibril and fibril bundle phenotype.

CONCLUSIONS

These findings indicate that decorin, fibromodulin and lumican coordinately regulate the fibrillar and suprafibrillar organization of collagen in the periodontal ligament.

Transcript Profiling of Periodontal Fibroblasts and Osteoblasts

BACKGROUND

Fibroblasts are critical to the establishment and maintenance of the periodontal attachment apparatus (cementum, periodontal ligament [PDL], and bone). In order to characterize the cellular changes that accompany periodontal regeneration, better tools are necessary to distinguish periodontal ligament fibroblasts (PDLF), gingival fibroblasts, and osteoblasts. Our goal is to identify gene markers to better characterize and identify these cell types.

METHODS

We chose to examine and compare the expression of numerous gene transcripts by semiquantitative reverse transcriptase-polymerase chain reaction using primers specific for 44 different gene transcripts in order to better characterize the identity of these cells.

RESULTS

Several transcripts were cell-type specific. Specifically, fibromodulin was expressed only in PDL fibroblasts, while osteopontin was expressed only in dermal fibroblasts. In addition, lumican was expressed by all three types of fibroblasts (PDL, gingival, and dermal), while alkaline phosphatase was expressed by osteoblasts as well as PDL and gingival fibroblasts.

CONCLUSIONS

Our results indicate that PDL fibroblasts are distinct from either gingival or dermal fibroblasts or osteoblasts. In general, PDL and gingival fibroblasts displayed greater similarity to each other than either displayed toward dermal fibroblasts. Furthermore, both gingival and PDL fibroblasts displayed greater similarity to osteoblasts than to dermal fibroblasts, possibly reflecting their common origin (the neural crest).

Human Periodontal Ligament Cell Sheets Can Regenerate Periodontal Ligament Tissue in an Athymic Rat Model

Conventional periodontal regeneration methods remain insufficient to attain complete and reliable clinical regeneration of periodontal tissues. We have developed a new method of cell transplantation using cell sheet engineering and have applied it to this problem. The purpose of this study was to investigate the characteristics of human periodontal ligament (HPDL) cell sheets retrieved from culture on unique temperature-responsive culture dishes, and to examine whether these cell sheets can regenerate periodontal tissues. The HPDL cell sheets were examined histologically and biochemically, and also were transplanted into a mesial dehiscence model in athymic rats. HPDL cells were harvested from culture dishes as a contiguous cell sheet with abundant extracellular matrix and retained intact integrins that are susceptible to trypsin-EDTA treatment. In the animal study, periodontal ligament-like tissues that include an acellular cementum-like layer and fibrils anchoring into this layer were identified in all the athymic rats transplanted with HPDL cell sheets. This fibril anchoring highly resembles native periodontal ligament fibers; such regeneration was not observed in nontransplanted controls. These results suggest that this technique, based on the concept of cell sheet engineering, can be useful for periodontal tissue regeneration.

Periostin is expressed within the developing teeth at the sites of epithelial-mesenchymal interaction

Periostin was originally isolated as an osteoblast-specific factor that functions as a cell adhesion molecule for preosteoblasts and is thought to be involved in osteoblast recruitment, attachment, and spreading. The protein was renamed "periostin" because of its expression in the periosteum and periodontal ligament, indicating a potential role in bone and maintenance of tooth structure. Periostin has structural similarity to insect fasciclin-I and can be induced by TGF-beta and Bmp2. Because tooth and periodontium development is a well-described genetic model for organogenesis governed by a reciprocal set of epithelial-mesenchymal interactions, thought to be controlled by various TGF-beta superfamily members, we investigated whether periostin is present during tooth morphogenesis. Both periostin mRNA and protein expression were analyzed throughout normal tooth development (embryonic day [E] 9.5-newborn) and within both Bmp4- and Msx2-null embryos. Periostin mRNA is initially present within the E9.5 first branchial arch epithelium and then shifts to underlying ectomesenchyme. Both mRNA and protein are asymmetrically localized to the lingual/palatal and buccal side during the early epithelial-mesenchymal interactions. Periostin is also present in dental papilla cells and within the trans-differentiating odontoblasts during the bell and hard tissue formation stages of tooth development. We suggest that periostin plays multiple roles as a primary responder molecule during tooth development and may be linked to deposition and organization of other extracellular matrix adhesion molecules during maintenance of the adult tooth, particularly at the sites of hard-soft tissue interface.

Investigation of multipotent postnatal stem cells from human periodontal ligament

BACKGROUND

Periodontal diseases that lead to the destruction of periodontal tissues--including periodontal ligament (PDL), cementum, and bone--are a major cause of tooth loss in adults and are a substantial public-health burden worldwide. PDL is a specialised connective tissue that connects cementum and alveolar bone to maintain and support teeth in situ and preserve tissue homoeostasis. We investigated the notion that human PDL contains stem cells that could be used to regenerate periodontal tissue.

METHODS

PDL tissue was obtained from 25 surgically extracted human third molars and used to isolate PDL stem cells (PDLSCs) by single-colony selection and magnetic activated cell sorting. Immunohistochemical staining, RT-PCR, and northern and western blot analyses were used to identify putative stem-cell markers. Human PDLSCs were transplanted into immunocompromised mice (n=12) and rats (n=6) to assess capacity for tissue regeneration and periodontal repair. Findings PDLSCs expressed the mesenchymal stem-cell markers STRO-1 and CD146/MUC18. Under defined culture conditions, PDLSCs differentiated into cementoblast-like cells, adipocytes, and collagen-forming cells. When transplanted into immunocompromised rodents, PDLSCs showed the capacity to generate a cementum/PDL-like structure and contribute to periodontal tissue repair.

INTERPRETATION

Our findings suggest that PDL contains stem cells that have the potential to generate cementum/PDL-like tissue in vivo. Transplantation of these cells, which can be obtained from an easily accessible tissue resource and expanded ex vivo, might hold promise as a therapeutic approach for reconstruction of tissues destroyed by periodontal diseases.

Molecular dissection of the fibroblast-traction machinery

Motile fibroblasts generate forces that can be expressed as cell migration or as traction, the drawing-in of extracellular matrix. Traction by cultured fibroblasts can induce a rapid concerted reorganization of collagen gel, creating a pattern of collagen alignment similar to that seen in tendons and ligaments. In such fibrous connective tissues, after pattern morphogenesis is complete, ongoing traction may be responsible for the maintenance of proper form and function. The molecules that generate and transmit forces have been catalogued; however, how these nanometer-scale molecules contribute to millimeter-scale patterns has not been directly tested. Here, we placed pairs of explants of human periodontal ligament fibroblasts in collagen gels, where ligament-like straps of anisotropic collagen formed on the axes between them. We scrutinized the traction apparatus using electron microscopy, video microscopy, and computer-based pattern analysis, augmented with pharmacologic inhibitors of cytoskeletal function. Patterning was marked by the co-alignment of collagen, fibroblasts, and their actin cytoskeletons, all parallel to the axis between explants. The pattern was diminished by depolymerizing actin filaments or by blocking myosin activity, but was accentuated by depolymerizing microtubules. The plasma membrane also seems to contribute to the traction force. These molecular components combine to exert a sub-maximal traction force on the matrix, suggesting that the force may be regulated to ensure tissue tensional homeostasis.

Oxytalan fibres in the periodontal ligament of equine molar cheek teeth

The distribution and arrangement of oxytalan fibres were examined in periodontal specimens of cheek teeth from seven horses. Oxidation prior to aldehyde fuchsin exposition permitted a selective staining of the oxytalan fibres, which are a distinct component of the elastic fibre system. On three horizontal levels of the periodontium--(a) subgingival, (b) middle third and (c) apical--two oxytalan fibre groups were shown histologically: 'blood vessel-related' and 'independent' oxytalan fibres. In levels a and b, both groups were arranged in a typical occluso-apical alignment along the reserve crown. Single oxytalan fibres deviated from their general course in order to attach to the cementum. In these cemental entheses the oxytalan fibres ran parallel to collagen fibre bundles. The interpretation of such morphological features emphasized the oxytalan fibres' capacity to improve the stability of periodontal blood vessel walls during masticatory movements. Level c, especially in regions next to the persisting epithelial root sheath, is the site of oxytalan fibre generation. This is a prerequisite for the facilitation of periodontal regeneration and reorganization during dental growth and eruption.

Tooth-root form and function in platyrrhine seed-eaters

Research into the functional and adaptive basis of tooth crown form has provided a useful framework for the inference of diet in extinct primates. However, our understanding of variation in tooth-root form is limited. Studies within the clinical literature emphasize the influence of tooth-root surface area on stress resistance, but it is not known if root form has diversified during primate evolution in relation to dietary specialization. This hypothesis was tested by quantifying maxillary canine and postcanine tooth-root surface areas in four platyrrhine species that differ in the material properties of their diet: Cebus apella, Cebus albifrons, Chiropotes satanas, and Pithecia pithecia. Pairwise comparisons between closely related taxa support predictions based on dietary differences. Taxa that regularly consume resistant seeds (Cebus apella and Chiropotes satanas) exhibit significantly larger relative surface area values for those teeth used in seed processing than closely related taxa that consume resistant foods less often (Cebus albifrons and Pithecia pithecia). Additionally, relative molar-root surface area appears to be greater in Pithecia than in Chiropotes, as predicted from the more folivorous diet of Pithecia. Tooth-root surface area was also found to vary along the tooth row and should therefore have a significant influence on antero-posterior bite-force gradients. The results of this study suggest a close relationship between tooth-root form and patterns of occlusal loading. Further elucidation of this relationship could improve our inferences of diet in extinct taxa, and augment research into the mechanics and evolution of feeding.

Effects of mechanical tension on matrix degradation by human periodontal ligament cells cultured in collagen gels

BACKGROUND

Periodontal ligament (PDL) cells are thought to play a crucial role in the remodelling of periodontal tissues during orthodontic tooth movement.

OBJECTIVE

The objective of this study was to analyse the effects of mechanical tension on matrix degradation by PDL cells cultured in collagen gels.

METHODS

The gels were prepared free-floating or attached to the culture wells and cultured for up to 22 d. In free-floating gels very little mechanical tension is generated within the matrix, whereas in attached gels tension is highly increased.

RESULTS

At d 8, free-floating gels had contracted to 2% of their original wet weight. Attached gels had contracted to only 40%, but by d 15 all gels had spontaneously detached from the wells and had contracted rapidly. The collagen content of free-floating gels had decreased to 30% of the initial value at d 22. Collagenase activity was detected in the culture media of the free-floating gels and the presence of matrix metalloproteinases (MMPs) 2 and 9 was shown by zymography. In addition, histological sections showed matrix degradation around the cells. This shows that ligament cells in free-floating gels are actively resorbing the collagen matrix. The collagen content of attached gels did not change during the first 8 d but, after detachment, it rapidly decreased to 2%. Therefore, mechanical tension seems to prevent degradation of the matrix. In contrast, relaxation of the tension enhances the resorptive activity.

CONCLUSIONS

The sensitivity of PDL cells to mechanical tension may be essential for the remodelling of periodontal tissues and their adaptation to physiological and orthodontic forces.

Regulatory mechanisms of periodontal regeneration

The periodontal ligament, located between the cementum and the alveolar bone, has a width ranging from 0.15 to 0.38 mm. Regeneration and homeostasis of the periodontal ligament are highly significant functions in relation to periodontal therapy, tooth transplantation or replantation, and orthodontic tooth movement. The purpose of this review is to discuss the regulatory mechanisms of regenerative and homeostatic functions in the periodontal ligament based on currently published studies and also on our own experimental data. We consider the capability of the ligament tissue to promote or to suppress calcification in connection with bone and cementum formation and the maintenance of the periodontal ligament space. Also discussed are the involvement of the periodontal ligament tissue in the regenerative ability, cell proliferation, growth and differentiation factors, extracellular matrix proteins, homeostatic phenomena, function of Malassez epithelial rests, tooth movement, or occlusal loading. Regulatory mechanisms for regeneration and homeostasis of the periodontal ligament are hypothetically proposed.

Immunohistochemical localization of insulin-like growth factor-II and its binding protein-6 in human epithelial cells of Malassez

So-called epithelial rests of Malassez are derived from the Hertwig's root sheath and are located in the periodontal ligament, with still unknown functions. Different pathological conditions may lead to proliferation of these otherwise non-proliferative cell clusters. The insulin-like growth factor (IGF) system is an important growth factor system controlling proliferation and differentiation. In our study on Malassez cells from extracted human deciduous teeth, we investigated their structure by means of light and electron microscopy. Although they appeared as cellular clusters with a uniform epithelial phenotype, immunohistochemical analyses of components of the IGF system revealed an unique pattern: weak immunoreactivity could be seen for IGF-II while among all IGF binding proteins (IGFBPs) only IGFBP-6 and weakly IGFBP-4 were detectable in epithelial cells of Malassez. Since IGFBP-6 has a very high affinity for IGF-II and can inhibit its functions, we discuss that, in the normal periodontal ligament, autocrine IGFBP-6 may function as an antiproliferative molecule suppressing mitogenic effects of IGFs on Malassez cells.

Dendritic fibroblasts in three-dimensional collagen matrices

Cell motility determines form and function of multicellular organisms. Most studies on fibroblast motility have been carried out using cells on the surfaces of culture dishes. In situ, however, the environment for fibroblasts is the three-dimensional extracellular matrix. In the current research, we studied the morphology and motility of human fibroblasts embedded in floating collagen matrices at a cell density below that required for global matrix remodeling (i.e., contraction). Under these conditions, cells were observed to project and retract a dendritic network of extensions. These extensions contained microtubule cores with actin concentrated at the tips resembling growth cones. Platelet-derived growth factor promoted formation of the network; lysophosphatidic acid stimulated its retraction in a Rho and Rho kinase-dependent manner. The dendritic network also supported metabolic coupling between cells. We suggest that the dendritic network provides a mechanism by which fibroblasts explore and become interconnected to each other in three-dimensional space.

A cell line with characteristics of the periodontal ligament fibroblasts is negatively regulated for mineralization and Runx2/Cbfa1/Osf2 activity, part of which can be overcome by bone morphogenetic protein-2

The periodontal ligament (PDL) is a connective tissue located between the cementum of teeth and the alveolar bone of the mandibula. It plays an integral role in the maintenance and regeneration of periodontal tissue. The cells responsible for maintaining this tissue are thought to be fibroblasts, which can be either multipotent or composed of heterogenous cell populations. However, as no established cell lines from the PDL are available, it is difficult to assess what type of cell promotes all of these functions. As a first step to circumvent this problem, we have cloned and characterized cell lines from the PDL from mice harboring a temperature-sensitive SV 40 large T-antigen gene. RT-PCR and in situ hybridization studies demonstrated that a cell line, designated PDL-L2, mimics the gene expression of the PDL in vivo: it expresses genes such as alkaline phosphatase, type I collagen, periostin, runt-related transcription factor-2 (Runx2) and EGF receptor, but does not express genes such as bone sialoprotein and osteocalcin. Unlike osteoblastic cells and a mixed cell population from the PDL, PDL-L2 cells do not produce mineralized nodules in the mineralization medium. When PDL-L2 cells were incubated in the presence of recombinant human bone morphogenetic protein-2 alkaline phosphatase activity increased and mineralized nodules were eventually produced, although the extent of mineralization is much less than that in osteoblastic MC3T3-E1 cells. Furthermore, PDL-L2 cells appeared to have a regulatory mechanism by which the function of Runx2 is normally suppressed.

Slow local movements of collagen fibers by fibroblasts drive the rapid global self-organization of collagen gels

Aclassic model for tissue morphogenesis is the formation of ligament-like straps between explants of fibroblasts placed in collagen gels. The patterns arise from mechanical forces exerted by cells on their substrates (Harris et al., 1981). However, where do such straps come from, and how are slow local movements of cells transduced into dramatic long-distance redistributions of collagen? We embedded primary mouse skin and human periodontal ligament fibroblasts in collagen gels and measured the time course of patterning by using a novel computer algorithm to calculate anisotropy, and by tracking glass beads dispersed in the gel. As fibroblasts began to spread into their immediate environments, a coordinated rearrangement of collagen commenced throughout the gel, producing a strap on a time scale of minutes. Killing of cells afterwards resulted in a partial relaxation of the matrix strain. Surprisingly, relatively small movements of collagen molecules on the tensile axis between two pulling explants induced a much larger concomitant compression of the gel perpendicular to the axis, organizing and aligning fibers into a strap. We propose that this amplification is due to the geometry of the collagen matrix, and that analogous amplified movements may drive morphological changes in other biological meshes, both outside and inside the cell.

On the origin of intrinsic matrix of acellular extrinsic fiber cementum: studies on growing cementum pearls of normal and bisphosphonate-affected guinea pig molars

Cementum pearls (CPs) belong to a type of acellular extrinsic fiber cementum (AEFC) that form on the maturing enamel of guinea pig molars. This study aimed to elucidate the forming process of intrinsic matrix of AEFC using the CPs of normal and bisphosphonate-affected guinea pig molars as experimental models. A group of guinea pigs were subjected to continuous administration of 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) for 2 wk to inhibit mineralization of growing CPs. Fenestration of the enamel organ and migration of periodontal cells on to the exposed surface of maturing enamel appeared to be unaffected by HEBP, whereas de novo formation as well as growth of pre-existing CPs did not proceed under the same conditions. Immunoreactions for osteopontin were located exclusively on the mineralized matrix of preformed CPs, implying the absence of additional deposition or accumulation of putative intrinsic cementum matrix on the affected CPs, where the propagation of mineral phase had been arrested. In both normal and HEBP-treated groups, distinct enzymatic reactions for alkaline phosphatase appeared on the cells of the periodontal ligament associated closely with the sites of CP formation, and along the mineralization front of CPs. These observations suggest that the mineralization process per se plays a central role in the deposition of AEFC matrix and that alkaline phosphatase of periodontal cells penetrating through the enamel organ to the maturing enamel surface plays a key role in the mineralization process of CPs.

Expression profile of active genes in human periodontal ligament and isolation of PLAP-1, a novel SLRP family gene

Periodontal ligament (PDL) is one of the most important tissues in maintaining the homeostasis of tooth and tooth-supporting tissue, periodontium. In this study, we investigated the expression profile of active genes in the human PDL obtained by collecting sequences with a 3'-directed cDNA library, which faithfully represents the composition of the mRNA population. We succeeded in obtaining a total of 1752 cDNA sequences by sequencing randomly selected clones and identified a total of 1318 different species as gene signatures (GS) by their sequence identity, 344 of which were known genes in the GenBank, and 974 of which were new genes. The resulting expression profile showed that collagen type I and type III were the most abundant genes and that osteogenesis-related proteins, such as SPARC/osteonectin and osteoblast specific factor 2, were highly expressed. By comparing the expression profile of PDL with 44 profiles similarly obtained with unrelated human cell/tissue, nine novel genes, which are probably expressed specifically in PDL, were discovered. Among them, we cloned a full-length cDNA of GS5096, which is frequently expressed in freshly-isolated periodontal tissue. We found that it encodes a novel protein, which is a new member of the class I small leucine-rich repeat proteoglycan family, and designated it PLAP-1 (periodontal ligament associated protein-1). PLAP-1 mRNA expression was confirmed in in vitro-maintained PDL cells and was enhanced during the course of the cytodifferentiation of the PDL cells into mineralized tissue-forming cells such as osteoblasts and cementoblasts. These findings suggest the involvement of PLAP-1 in the mineralized matrix formation in PDL tissues.

The putative collagen binding peptide hastens periodontal ligament cell attachment to bone replacement graft materials

BACKGROUND

Bone replacement graft (BRG) materials are often used to treat periodontal defects, to promote cellular invasion, and to encourage bone regrowth. Periodontal ligament fibroblasts (PDLF) incorporate these materials and form the basis of the renewed connection between the existing and newly formed alveolar bone and the tooth surface. A peptide (P-15) that mimics the putative cell-binding domain of collagen has been reported to promote dermal fibroblast attachment and proliferation.

METHODS

PDLF were quantitatively examined for their ability to adhere to a variety of BRG materials fluorometrically. In addition, scanning electron microscopy was used to examine the changes in morphology exhibited by these cells as they attached and spread on several BRG materials. Finally, BRG materials containing the P-15 peptide were quantitatively examined for their ability to promote PDLF attachment and proliferation.

RESULTS

Freeze-dried allograft bone supports greater PDLF attachment than does several xenograft and alloplastic anorganic bone replacement materials. An anorganic BRG material containing the P-15 peptide promoted more rapid cell attachment and spreading than a similar anorganic BRG material lacking this peptide. Finally, none of the BRG materials examined promoted PDLF proliferation.

CONCLUSIONS

Our data indicate that the addition of the P-15 peptide increases the rapidity of PDLF attachment to xenogeneic bone replacement materials. This increase in the rate of attachment may have clinical significance in the context of the dynamic regulation of cell attachment during periodontal regeneration. However, this peptide does not promote an increase in stable cell attachment or proliferation in vitro.

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.

Epithelial rests of Malassez express immunoreactivity of TrkA and its distribution is regulated by sensory nerve innervation

The periodontal ligament is the connective tissue that fills the space between the tooth and its bony socket. It is abundantly innervated by the sensory and sympathetic nerves. We first investigated the immunoreactivity of TrkA, which is a high-affinity receptor of nerve growth factor (NGF), in the periodontal ligament of rats. Immunoreactivity was observed at the epithelial cells in the cervical and furcation regions of the molars. These epithelial cells, which gather together to form clusters or networks, are known as the epithelial rests of Malassez. Immunoreactivity was not observed in other non-neuronal cells, such as osteoblasts, fibroblasts, odontoblasts, cementoblasts, endothelial cells, and/or osteoclasts. On the basis of these findings, we investigated the possible involvement of sensory nerve innervation in the immunoreactivity of the epithelial cells. Denervation of the inferior alveolar nerve resulted in a marked decrease in the distribution area and size of the clusters of immunoreactive cells compared with those of sham-operated rats. These findings suggest that sensory nerve innervation may have a regulatory role in maintenance of the epithelial rests of Malassez expressing TrkA in the periodontal ligament.

Apoptosis in the early developing periodontium of rat molars

Development of the periodontium involves a series of complex steps that result in the formation of root dentine, cementum, bone and fibres of the ligament. These precisely controlled and timed events require the participation of the enamel organ derived epithelial cells of Hertwig's (HRS) and ectomesenchymal cells of the dental follicle. These events involve rapid turnover of the tissues and cells, including disappearance of epithelial cells of HRS. Thus, it seemed likely to us that programmed cell death (apoptosis) may play a role in the development of the periodontium. Fragments of first molars, obtained from 14- and 29-day-old rats, were fixed in glutaraldehyde-formaldehyde and processed for light and electron microscopy. For the TUNEL method for detection of apoptosis, specimens were fixed in 4% formaldehyde and embedded in paraffin. Results confirmed that epithelial cells of HRS maintain a close relationship with the forming dentine root, and that they may become trapped in the dentino-cemental junction. Some of the epithelial cells exhibited ultrastructural features which are consistent with the interpretation that they were undergoing programmed cell death, i.e. apoptosis. Periodontal fibroblast-like cells showed typical images of apoptosis and engulfed apoptotic bodies. TUNEL positive structures were present in all corresponding regions. It seems therefore that apoptosis of epithelial cells of HRS and fibroblast-like cells of the periodontal ligament constitutes an integral part of the developmental process of the tissues of the periodontium.

Enamel matrix-derived protein stimulates attachment of periodontal ligament fibroblasts and enhances alkaline phosphatase activity and transforming growth factor beta1 release of periodontal ligament and gingival fibroblasts

BACKGROUND

Although it is claimed that enamel matrix-derived proteins (EMP) can be used to promote new attachment formation around periodontally involved teeth, the underlying biological mechanism is not understood. It was the aim of the present study to investigate the effects of EMP on the behavior of human periodontal ligament (HPLF) and gingival fibroblasts (HGF) in vitro, with special focus on their attachment properties, the expression of alkaline phosphatase (ALP) activity, the release of transforming growth factor (TGF)beta1, and their proliferative rate.

METHODS

Fibroblast populations were obtained from 10 individuals with a healthy periodontium and cultured in chemically defined medium on culture plates coated with EMP, purified collagen type I, or their respective vehicles. Experiments were performed in the absence of serum for periods up to 48 hours.

RESULTS

It was shown that HGF barely attached and spread on EMP-coated substrata, whereas HPLF attached and spread within 24 hours. However, when cultured on purified collagen type I, both cell types showed rapid attachment and spreading. Furthermore, the expression of ALP activity was significantly enhanced under the influence of EMP, especially in HPLF. HPLF and HGF both released significantly higher levels of TGFbeta1 in the presence of EMP. EMP did not influence 3H-thymidine incorporation by HPLF and HGF.

CONCLUSIONS

Our results indicate that HPLF and HGF respond differently to EMP. A more rapid attachment of HPLF to this substratum might contribute, during the initial stages of periodontal healing, to selective outgrowth and colonization of exposed root surfaces in vivo.