Bundle formation of principal fibers in rat molars

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.

Histology of human cementum: Its structure, function, and development

Cementum was first demonstrated by microscopy, about 180 years ago. Since then the biology of cementum has been investigated by the most advanced techniques and equipment at that time in various fields of dental sciences. A great deal of data on cementum histology have been accumulated. These data have been obtained from not only human, but also non-human animals, in particular, rodents such as the mouse and rat. Although many dental histologists have reviewed histology of human cementum, some descriptions are questionable, probably due to incorrect comparison of human and rodent cementum. This review was designed to introduce current histology of human cementum, i.e. its structure, function, and development and to re-examine the most questionable and controversial conclusions made in previous reports.

Eruptive and functional changes in periodontal ligament fibroblast orientation in CD44 wild-type vs. knockout mice

BACKGROUND AND OBJECTIVE

Periodontal ligament (PDL) fibroblasts establish principal fibers of the ligament during tooth eruption, and maintain these fibers during occlusion. PDL development and occlusal adaptation includes changes in the orientation of PDL fibroblasts; however, the mechanism for these changes in orientation is unclear. The objective of this study was to compare PDL fibroblast orientation in different stages corresponding with first molar eruption and occlusion in CD44 wild-type (WT) and knockout (KO) mice.

MATERIAL AND METHODS

CD44 WT and KO mice were raised to six postnatal stages corresponding with first molar (M1 ) eruption (postnatal day 8, 11, 14 and 18) and occlusion (postnatal day 26 and 41). Coronal sections of the first mandibular molar (M1 ) were prepared and the orientation of fibroblasts in the cervical root region was measured. Angle measurements were compared across developmental stages and between strains using Watson-Williams F-test (oriana software) and ANCOVA.

RESULTS

PDL fibroblast orientation increased significantly in CD44 WT (9-87°) and KO mice (14-93°; p ≤ 0.05) between intraosseous eruption (day 11), mucosal penetration (day 14) and preocclusal eruption (day 18); however, the PDL fibroblast orientation did not change significantly with the onset of occlusion (day 26) or continued function (day 41). Within each strain, the variance in fibroblast orientation during preocclusal eruption (day 18) was significantly higher than the variance of all other time points (p < 0.0005). CD44 WT and KO mice showed a similar pattern of PDL development and eruption with a significant difference in CD44 WT vs. KO fibroblast orientations only during early function (day 26, 92° vs 116°; p = 0.05).

CONCLUSIONS

The development of PDL fibroblast orientation is highly similar between CD44 WT and KO mice. Between early (day 11) and late (day 18) eruptive stages PDL fibroblast orientation increases, corresponding with the upward movement of M1 . The PDL fibroblast orientation established in preocclusal eruption (day 18) is maintained during early (day 26) and late (day 41) stages of occlusal function, suggesting that PDL cells adapt to mechanical loads in the oral cavity before M1 occlusion.

Histological and histomorphometrical alterations of the periodontal ligament in gerbils submitted to teeth extraction

This study verified the effect of unilateral teeth extraction on the periodontal ligament in gerbils (Meriones unguiculatus). Ten adult male gerbils weighing about 50 g had induced occlusal alterations by upper left molar extractions while the other ten animals, only submitted to surgical stress, were considered as controls. The periodontal ligament was characterized by qualitative and quantitative analysis, histological description and histomorphometric quantification. Significant alterations were observed on the left side of the experimental group (P < 0.05), the hypofunctional region, when it was compared with the contralateral side and the corresponding region of the control group. Two months after occlusal alterations induced by unilateral teeth extraction, atrophic histological alterations and a decrease in the periodontal space on the ipsilateral side characterized the periodontal ligament. In this study it was possible to conclude that the gerbil can be used in experimental models attempting to correlate the periodontium's biological response to various mechanical stresses, as the periodontal ligament was shown to be highly sensitive to occlusal alterations.

Collagen fibril form and function

The majority of collagen in the extracellular matrix is found in a fibrillar form, with long slender filaments each displaying a characteristic approximately 67?nm D-repeat. Here they provide the stiff resilient part of many tissues, where the inherent strength of the collagen triple helix is translated through a number of hierarchical levels to endow that tissue with its specific mechanical properties. A number of collagen types have important structural roles, either comprising the core of the fibril or decorating the fibril surface to give enhanced functionality. The architecture of subfibrillar and suprafibrillar structures (such as microfibrils), lateral crystalline and liquid crystal ordering, interfibrillar interactions, and fibril bundles is described. The fibril surface is recognized as an area that contains a number of intimate interactions between different collagen types and other molecular species, especially the proteoglycans. The interplay between molecular forms at the fibril surface is discussed in terms of their contribution to the regulation of fibril diameter and their role in interfibrillar interactions.

Histochemical studies of glycosaminoglycans in developing periodontal ligaments of ICR mice

Although the periodontal ligament (PL) contains an abundance of glycosaminoglycans (GAGs), there are only a few histochemical studies describing GAGs in the developing PL. In the present study, the relationship between the formation of principal fibers and the molecular species of GAGs in the developing PL was examined by light microscopic histochemistry. Jcl:ICR mice were killed on day 0 to day 28 after birth. Paraffin-embedded tissue sections were routinely made and stained with hematoxylin-eosin (H&E), Azan, or the sensitized high iron diamine (S-HID) procedure combined with enzyme digestions. Before tooth eruption, thin threads of collagen fibers in the PL assembled and constructed principal fibers, which projected from both the side of the alveolar bone and the root of the tooth. After tooth eruption, the principal fibers from both sides were tightly entangled. In the developing PL, the molecular species of GAGs was mainly dermatan sulfate. Moreover, the relative amount of dermatan sulfate increased together with the maturation of the principal fibers, while the principal fibers adjacent to the alveolar bone and cementum contained chondroitin sulfate. These results suggest that dermatan sulfate contributes to collagen fiber assembly in the PL and that chondroitin sulfate relates to PL adhesion to the alveolar bone and to the cementum of the root.

Extracellular matrix in tooth cementum and mantle dentin: localization of osteopontin and other noncollagenous proteins, plasma proteins, and glycoconjugates by electron microscopy

BACKGROUND

Noncollagenous proteins (NCPs) are considered to have multiple functions related to the formation, turnover, and repair of the collagen-based mineralized tissues. Collectively, they comprise a class of generally acidic, mineral-binding proteins showing extensive posttranslational modifications, including glycosylation, phosphorylation, and sulfation. METHODS. We have used colloidal-gold immunocytochemistry and lectin-gold cytochemistry, together with transmission electron microscopy, to examine the organic matrix composition of tooth cementum and the subjacent mantle dentin in rodent molar teeth. Molars were processed for immunocytochemistry using antibodies against osteopontin (OPN), osteocalcin (OC), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), albumin (ALB), and alpha 2HS-glycoprotein (alpha 2HS-GP), or for glycoconjugate cytochemistry using lectin-gold complexes.

RESULTS

Ultrastructurally, at the advancing root edge in developing molars, OPN and BSP initially were associated with small calcification foci in the mantle dentin. With progressing mineralization, OC and alpha 2HS-GP appeared diffusely distributed throughout the calcified mantle dentin, and diminished as a gradient toward the circumpulpal dentin. Immediately following disruption of Hertwig's epithelial root sheath, cementum deposition commenced at the root surface occasionally with the appearance of a cement line rich in OPN. Cementum matrix proper contained abundant OPN, BSP, OC, and alpha 2HS-GP, but no or little BAG-75 or ALB. Protein immunolabeling, as well as lectin labeling for beta-D-galactose and N-acetyl-neuraminic acid and/or N-glycolyl-neuraminic acid, both being prominent sugars of certain NCPs, was primarily concentrated between, and at the surface of, collagen fibrils in acellular extrinsic fiber cementum. OPN, BSP, OC, and alpha 2HS-GP were also prominent components of cellular cementum and of Sharpey's fibers. In cellular cementum, laminae limitantes sometimes present delimiting cementocyte lacunae and cell process-containing canaliculi were also rich in OPN. Along the root surface, occasional cementoblasts exhibited intracellular labeling for OPN over the Golgi apparatus and secretory granules.

CONCLUSIONS

We have identified OPN, BSP, OC, and alpha 2HS-GP as being prominent organic constituents of both mantle dentin and acellular and cellular cementum, and, have elucidated the details of their distribution at the ultrastructural level. The temporal appearance and spatial distribution of these organic moieties in the teeth root are similar to those seen during bone formation and are consistent with proposals that certain NCPs may be involved in regulating calcification and/or participating in cell-matrix and matrix-matrix/mineral adhesion events.

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.