Light and electron microscopic study of the initial attachment of principal fibers to the alveolar bone surface in rat molars

Microstructural heterogeneity of the collagenous network in the loaded and unloaded periodontal ligament and its biomechanical implications

The periodontal ligament (PDL) is a highly heterogeneous fibrous connective tissue and plays a critical role in distributing occlusal forces and regulating tissue remodeling. Its mechanical properties are largely determined by the extracellular matrix, comprising a collagenous fiber network interacting with the capillary system as well as interstitial fluid containing proteoglycans. While the phase-contrast micro-CT technique has portrayed the 3D microscopic heterogeneity of PDL, the topological parameters of its network, which is crucial to understanding the multiscale constitutive behavior of this tissue, has not been characterized quantitatively. This study aimed to provide new understanding of such microscopic heterogeneity of the PDL with quantifications at both tissue and collagen network levels in a spatial manner, by combining phase-contrast micro-CT imaging and a purpose-built image processing algorithm for fiber analysis. Both variations within a PDL and among the PDL with different shapes, i.e. round-shaped and kidney-shaped PDLs, are described in terms of tissue thickness, fiber distribution, local fiber densities, and fiber orientation (namely azimuthal and elevation angles). Furthermore, the tissue and collagen fiber network responses to mechanical loading were evaluated in a similar manner. A 3D helical alignment pattern was observed in the fiber network, which appears to regulate and adapt a screw-like tooth motion under occlusion. The microstructural heterogeneity quantified here allows development of sample-specific constitutive models to characterize the PDL's functional and pathological loading responses, thereby providing a new multiscale framework for advancing our knowledge of this complex limited mobility soft-hard tissue interface.

Proteoglycans in the periodontium: A review with emphasis on specific distributions, functions, and potential applications

Proteoglycans (PGs) are largely glycosylated proteins, consisting of a linkage sugar, core proteins, and glycosaminoglycans (GAGs). To date, more than 40 kinds of PGs have been identified, and they can be classified as intracellular, cell surface, pericellular, and extracellular PGs according to cellular locations. To illustrate, extracellular PGs are known for regulating the homeostasis of the extracellular matrix; cell-surface PGs play a role in mediating cell adhesion and binding various growth factors. In the field of periodontology, PGs are implicated in cellular proliferation, migration, adhesion, contractility, and anoikis, thereby exerting a profound influence on periodontal tissue development, wound repair, the immune response, biomechanics, and pathological process. Additionally, the expression patterns of some PGs are dynamic and cell-specific. Therefore, determining the roles and spatial-temporal expression patterns of PGs in the periodontium could shed light on treatments for wound healing, tissue regeneration, periodontitis, and gingival overgrowth. In this review, close attention is paid to the distributions, functions, and potential applications of periodontal PGs. Related genetically modified animal experiments and involved signal transduction cascades are summarized for improved understanding of periodontal PGs. To date, however, there is a large amount of speculation on this topic that requires rigorous experiments for validation.

Hyaluronan-CD44 interactions mediate contractility and migration in periodontal ligament cells

The role of hyaluronan (HA) in periodontal healing has been speculated via its interaction with the CD44 receptor. While HA-CD44 interactions have previously been implicated in numerous cell types; effect and mechanism of exogenous HA on periodontal ligament (PDL) cells is less clear. Herein, we examine the effect of exogenous HA on contractility and migration in human and murine PDL cells using arrays of microposts and time-lapse microscopy. Our findings observed HA-treated human PDL cells as more contractile and less migratory than untreated cells. Moreover, the effect of HA on contractility and focal adhesion area was abrogated when PDL cells were treated with Y27632, an inhibitor of rho-dependent kinase, but not when these cells were treated with ML-7, an inhibitor of myosin light chain kinase. Our results provide insight into the mechanobiology of PDL cells, which may contribute towards the development of therapeutic strategies for periodontal healing and tissue regeneration.

Direct microCT imaging of non-mineralized connective tissues at high resolution

The 3D imaging of soft tissues in their native state is challenging, especially when high resolution is required. An X-ray-based microCT is, to date, the best choice for high resolution 3D imaging of soft tissues. However, since X-ray attenuation of soft tissues is very low, contrasting enhancement using different staining materials is needed. The staining procedure, which also usually involves tissue fixation, causes unwanted and to some extent unknown tissue alterations. Here, we demonstrate that a method that enables 3D imaging of soft tissues without fixing and staining using an X-ray-based bench-top microCT can be applied to a variety of different tissues. With the sample mounted in a custom-made loading device inside a humidity chamber, we obtained soft tissue contrast and generated 3D images of fresh, soft tissues with a resolution of 1 micron voxel size. We identified three critical conditions which make it possible to image soft tissues: humidified environment, mechanical stabilization of the sample and phase enhancement. We demonstrate the capability of the technique using different specimens: an intervertebral disc, the non-mineralized growth plate, stingray tessellated radials (calcified cartilage) and the collagenous network of the periodontal ligament. Since the scanned specimen is fresh an interesting advantage of this technique is the ability to scan a specimen under load and track the changes of the different structures. This method offers a unique opportunity for obtaining valuable insights into 3D structure-function relationships of soft tissues.

An immunohistochemical study of the attachment mechanisms in different kinds of adhesive interfaces in teeth and alveolar bone of the rat

BACKGROUND AND OBJECTIVE

This study was designed to examine the histological and immunohistochemical nature of different kinds of adhesive interfaces in the rat molar region under identical experimental conditions and to discuss the structural and functional similarities between these adhesive interfaces.

MATERIAL AND METHODS

Four kinds of adhesive interfaces - an initial attachment layer for principal fibers on the developing alveolar bone surface, a reattachment layer for principal fibers on resorbed alveolar bone surface, cement lines on the alveolar bone surface unrelated to the principal fibers, and the cemento-dentinal junction - were examined in 25-d-old male Wistar rats. Routine histological staining, immunohistochemical staining for bone sialoprotein and osteopontin, and digestion tests with trypsin were conducted.

RESULTS

The adhesive interfaces showed very similar histological and immunohistochemical features: they were intensely hematoxylin-stainable, deficient in collagen fibrils, and rich in bone sialoprotein and osteopontin. After trypsin treatment the four adhesive interfaces had lost immunoreactivity to bone sialoprotein and osteopontin, and the two adjacent tissue parts held together finally separated at the adhesive interfaces.

CONCLUSION

The above findings suggest that (i) the different types of adhesive interfaces in the rat molar region have a common structure in that they are filled with highly accumulated bone sialoprotein and osteopontin and deficient in collagen fibrils; (ii) accumulated bone sialoprotein and osteopontin are closely associated with the adhesion at the interfaces; and (iii) the adhesive interfaces have a similar developmental process.

Immunohistochemical characterization of noncollagenous matrix molecules on the alveolar bone surface at the initial principal fiber attachment in rat molars

This study was designed to immunodetect proteoglycans (PGs) and the noncollagenous glycoproteins, bone sialoprotein (BSP) and osteopontin (OPN) on developing alveolar bone surface in rat molars by the indirect immunoperoxidase method, and to discuss the roles of these molecules at the initial principal fiber (PF) attachment. To characterize PGs, antibodies against five species of glycosaminoglycans (GAGs), chondroitin-4-sulfate (C4S), chondroitin-6-sulfate (C6S), unsulfated chondroitin (C0S), dermatan sulfate (DS), and keratan sulfate (KS) were used. Maxillary alveolar bone facing the distal root of the second molar was examined in 20- and 25-day-old male Wistar rats. Routine histological staining was also used. A hematoxylin-stained, fibril-poor layer always appeared on the alveolar bone surface just prior to the initial PF organization. This layer was strongly immunoreactive for C4S, C0S, OPN, and BSP, and weakly for C6S, but not for DS and KS. Then the initial PFs were attached to this layer. When new bone containing Sharpey's fibers covered this layer, it remained as a hematoxylin-stained, fibril-poor layer between Sharpey's fiber-containing and -lacking bone. The layer was consistently immunoreactive for OPN and BSP but had no immunoreactivity for GAGs. The results suggest that the accumulation of C4S-, C0S-, and C6S-carrying PGs, and of BSP and OPN is a primary event at the initial PF attachment, and is involved in the adhesion of PFs and mineralization of the initial attachment layer. The BSP and OPN act to maintain the interface integrity between Sharpey's fiber-containing and Sharpey's fiber-lacking alveolar bone after the PF attachment is established.

Effects of growth/differentiation factor-5 on human periodontal ligament cells

OBJECTIVES

Growth/differentiation factor-5 (GDF-5), a member of the transforming growth factor-beta superfamily, shows a close structural relationship to bone morphogenetic proteins and plays crucial roles in skeletal, tendon, and ligament morphogenesis. The mRNA encoding GDF-5 is also expressed during odontogenesis, especially in dental follicle tissue. While this suggests that GDF-5 participates in the formation of alveolar bone and the periodontal ligament, cementum, and dental root, the physiologic role of GDF-5 in these tissues in adulthood remains unclear. We therefore investigated GDF-5 effects upon cultures of human periodontal ligament (HPDL) cells.

MATERIAL AND METHODS

HPDL cells were obtained from healthy periodontal ligaments of individuals. Tetrazolium reduction assay was carried out for cell proliferation assay. Alkaline phosphatase (ALP) activity was estimated by measuring light absorbance at 405 nm. Reverse transcription-polymerase chain reaction (RT-PCR) and northern analysis were performed for gene expression in cultured HPDL cells. Sulfated glycosaminoglycan (sGAG) synthesis was evaluated by histochemical staining and a quantitative dye-binding method.

RESULTS

Expression of GDF-5 and its receptor was demonstrated in HPDL cells by RT-PCR. ALP activity in HPDL cells was significantly decreased by addition of rhGDF-5 at 10-1000 ng/ml (p < 0.05). Although northern analysis showed little change in gene expression for collagen alpha2(I) in rhGDF-5-stimulated HPDL cells, rhGDF-5 dose-dependently enhanced cell proliferation. This proliferative effect persisted for 16 d. Alcian blue staining and dye-binding assays indicated that sGAG synthesis was enhanced by rhGDF-5.

CONCLUSION

rhGDF-5 may provide an environment fostering periodontal healing or regeneration by affecting extracellular matrix metabolism.

Hard tissue formation on dentin surfaces applied with recombinant human bone morphogenetic protein-2 in the connective tissue of the palate

The purpose of this study was to evaluate whether hard tissue might be formed on dentin surfaces applied with recombinant human bone morphogenetic protein-2 (rhBMP-2) in palatal connective tissue. Fifty-eight dentin blocks were prepared from rat roots, demineralized with 24% EDTA (pH 7.0), applied with 0, 50 and 100 microgram/ml rhBMP-2, and labeled as groups 0, 50 and 100. The dentin blocks were then transplanted into palatal connective tissue of rats, and specimens were prepared at two and four weeks after surgery for histologic and histomorphometric examinations. The results showed that the percentage of newly formed hard tissue in relation to the total dentin block surface length in groups 0, 50 and 100 was 0.0%, 2.8% and 4.4% at two weeks, and 0.0%, 1.6% and 12.8% at four weeks, respectively. New hard tissue formation in groups 50 and 100 was significantly promoted as compared to group 0 (p < 0.01). These findings thus indicate that rhBMP-2 application to dentin enhanced new hard tissue formation on dentin surfaces in the connective tissue of the palate.

A light microscopic study of the attachment mechanism in different kinds of adhesive lines in rat molars

This study was designed to observe drifting molars of 70-day-old rats by light microscopy, and to elucidate whether there are similar attachment mechanisms at different kinds of adhesive lines in periodontal mineralized tissue of the rat molar region. Three kinds of adhesive lines--cement lines on resorbed alveolar bone, cement lines on resorbed roots, and cemento-dentinal junctions were examined. The two kinds of cement lines showed similar histological and histochemical features, they were proteoglycan-rich and fiber-poor. They appeared to form on the resorbed tissue before principal fiber reattachment. After covering by new bone or by reparative cementum, the cement lines retained the original features. The cemento-dentinal junction showed features very similar to those of the cement lines. Previous studies have suggested that the cemento-dentinal junctions bind the cementum and dentine by adhesion of proteoglycans. Structural similarities suggest that cement lines provide similar links between new bone and resorbed bone and between resorbed root and reparative cementum. In conclusion, this study suggests that there is one attachment mechanism for the different kinds of collagen based hard tissue in the rat molar region.

The initial attachment of cemental fibrils to the root dentin surface in acellular and cellular cementogenesis in rat molars

To elucidate the initial attachment mechanism of cemental fibrils to the root dentin surface in acellular and cellular cementogenesis, developing rat molars were observed by light microscopy and scanning electron microscopy combined with NaOH maceration. The NaOH maceration was used to observe details of the positional association of cemental and dentinal fibrils during cementogenesis. An initial hematoxylin stained, cementum layer began to form on the root dentin surface with the first dentin mineralization in both acellular and cellular cementogenesis. The initial attachment of cemental fibrils to the dentin surface also began at this point. At the initial attachment the intermingling of cemental and dentinal fibrils occurred only in places. With advanced cementogenesis the initial cementum layer became the fibril-poor cemento-dentinal junction. This suggests that cemental fibrils attach on the initial cementum layer, and not directly on dentinal fibrils, so that the layer results in the fibril-poor cemento-dentinal junction. The present study suggests that an intervening adhesive is necessary for the cemento-dentinal attachment at any stage of cementogenesis in rat molars.