Ultrastructural immunolocalization of dentin matrix protein 1 on Sharpey's fibers in monkey tooth cementum

Establishment of in vitro three-dimensional cementocyte differentiation scaffolds to study orthodontic root resorption

Orthodontic-induced root resorption is a severe side effect that can lead to tooth root shortening and loss. Compressive force induces tissue stress in the cementum that covers the tooth root, which is associated with activation of bone metabolism and cementum resorption. To investigate the role of cementocytes in mechanotransduction and osteoclast differentiation, the present study established an in vitro three-dimensional (3D) model replicating cellular cementum and observed the effects of static compression on the cellular behavior of the cementocytes. Cell Counting Kit-8 assay, alkaline phosphatase staining and dentin matrix protein 1 quantification were used to evaluate the cementocyte differentiation in the 3D scaffolds. Cellular viability under static compression was evaluated using live/dead staining, and expression of mineral metabolism-related genes were analyzed via reverse transcription-quantitative PCR. The results suggested that the cementocytes maintained their phenotype and increased the expression of osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL) and sclerostin (SOST) in the 3D model compared with cells cultured in two dimensions. Compression force increased cell death and induced osteoclastic differentiation via the upregulation of SOST and RANKL/OPG ratio, and the downregulation of osteocalcin. The effect of compression showed a force magnitude-dependent pattern. The present study established an in vitro model of cellular cementum to study the biology of cementocytes. The results indicated that cementocytes are sensitive to mechanical loading and may serve potential roles in the metabolic regulation of minerals during orthodontic root resorption. These findings provide a novel tool to study biological processes in the field of orthodontics and expand knowledge of the biological function of cementocytes.

Quantitative characterizations of the Sharpey's fibers of rat molars

BACKGROUND AND OBJECTIVE

The Sharpey's fibers of periodontal ligament (PDL) anchor the PDL to alveolar bone and cementum and are essential for the function of PDL. While qualitative analyses of the Sharpey's fibers have been widely explored, a comprehensive quantitative characterization of the Sharpey's fibers is not available. In this work, we selected rat molars as a model and comprehensively characterized the PDL Sharpey's fibers (diameter, density, length, embedding angle, and insertion angle).

MATERIALS AND METHODS

A total of 24 rat mandibular molars, eight maxillary first molars, and their surrounding alveolar bone were harvested, fixed, rendered anorganic and observed under scanning electron microscopy (SEM). The mandibles and maxillae (n = 4) were harvested, processed, sectioned, and stained with Sirius red for histological observation. SEM images were used for quantitative analyses of diameters and densities of the Sharpey's fibers, while Sirius red staining images were used to measure lengths and angles. The Sharpey's fibers were comprehensively characterized in terms of positions (cervical, middle, and apical thirds), PDL fiber groups (alveolar crest, horizontal, oblique, apical, and interradicular groups), sides (cementum and bone sides), and teeth (mandibular first, second, third molars, and maxillary first molar).

RESULTS

Our results showed that the characteristic parameters of the Sharpey's fibers varied in different positions, fiber groups, sides, and teeth. Specifically, the median diameter of the Sharpey's fibers on the bone side was significantly greater than that on the cementum side, while the median density of the Sharpey's fibers on the bone side was significantly lower than that on the cementum side, regardless of the positions and teeth. For the same tooth, the median length of the embedded Sharpey's fibers on the bone side was more than two times greater than that on the cementum side. Among all fiber groups, the alveolar crest group had the maximum length of the Sharpey's fibers on the bone side and the minimal length of the Sharpey's fibers on the cementum side. There is an approximate 5-15° difference between the embedding angle and the insertion angle in each group. The oblique group had the smallest embedding angles on both the bone and cementum sides.

CONCLUSION

This study provides a comprehensive and quantitative characterization of the Sharpey's fibers using rat molars as a model. Overall, these parameters varied according to different vertical positions, fiber groups, teeth, and jawbones. The quantitative information of the Sharpey's fibers presented in this work facilitates our understanding of PDL functions and advances the development of biomimetic materials for periodontal tissue regeneration.

The Cementocyte-An Osteocyte Relative?

Cementum is a mineralized tissue covering the tooth root that functions in tooth attachment and posteruptive adjustment of tooth position. During formation of the apically located cellular cementum, some cementoblasts become embedded in the cementoid matrix and become cementocytes. As apparently terminally differentiated cells embedded in a mineralized extracellular matrix, cementocytes are part of a select group of specialized cells, also including osteocytes, hypertrophic chondrocytes, and odontoblasts. The differentiation and potential function(s) of cementocytes are virtually unknown, and the question may be posed whether the cementocyte is a dynamic actor in cementum in comparable fashion with the osteocyte in the skeleton, responding to changing tooth functions and endocrine signals and actively directing local cementum metabolism. This review summarizes the literature with regard to cementocytes, comparing them to their closest "cousins," the osteocytes, where insights gained from osteocyte studies serve to inform the critical examination of cementocytes. The review identifies important unanswered questions about these cells regarding their origins, differentiation, morphology and lacuno-canalicular system, selective markers, and potential functions.

Ultrastructural immunolocalization of laminin 332 (laminin 5) at dento-gingival interface in Macaca fuscata monkey

Although laminin 332 (laminin 5), an extracellular matrix molecule involved in cell adhesion and migration, has been localized at the interface between the tooth enamel and junctional epithelium, its ultrastructural localization remains to be fully clarified. The purpose of the present study was to investigate the ultrastructural distribution of laminin 332 at the dento-gingival interface in Japanese monkey (Macaca fuscata) using pre- and post-embedding immunoelectron microscopy. Pre-embedding immunoelectron microscopy revealed a broad band of internal basal lamina together with supplementary lamina densa, and both showed immunolabeling for laminin 332. Immunoreaction products for laminin 332 were observed in the rough-surfaced endoplasmic reticulum of the junctional epithelial cells close to the tooth enamel. Post-embedding immunoelectron microscopy revealed an increase in the number of immunogold particles toward the coronal portion, resulting in a large accumulation of particles on the basal lamina, preferentially on the lamina densa. Concomitantly the dental cuticle at the dento-gingival interface was sporadically, but specifically, immunogold-labeled with anti-laminin 332 antibody. These data suggest that junctional epithelium actively produces laminin 332, and that the products accumulate at the dento-gingival interface during cell migration coronally towards the gingival sulcus.