Date Published: January 19, 2012
Publisher: Hindawi Publishing Corporation
Author(s): Ralf-Peter Herber, Justine Fong, Seth A. Lucas, Sunita P. Ho.
Adaptation of a rat dentoalveolar complex was illustrated using various imaging modalities. Micro-X-ray computed tomography for 3D modeling, combined with complementary techniques, including image processing, scanning electron microscopy, fluorochrome labeling, conventional histology (H&E, TRAP), and immunohistochemistry (RANKL, OPN) elucidated the dynamic nature of bone, the periodontal ligament-space, and cementum in the rat periodontium. Tomography and electron microscopy illustrated structural adaptation of calcified tissues at a higher resolution. Ongoing biomineralization was analyzed using fluorochrome labeling, and by evaluating attenuation profiles using virtual sections from 3D tomographies. Osteoclastic distribution as a function of anatomical location was illustrated by combining histology, immunohistochemistry, and tomography. While tomography and SEM provided past resorption-related events, future adaptive changes were deduced by identifying matrix biomolecules using immunohistochemistry. Thus, a dynamic picture of the dentoalveolar complex in rats was illustrated.
The load-bearing bone is a dynamic tissue and continuously adapts to changes in loads . In the periodontium, the cementum of a tooth is attached to the alveolar bone by the periodontal ligament (PDL), and the root is contained within the alveolar bone socket. Cementum and bone are calcified tissues of similar chemical composition, but cementum is far less dynamic . The vascularized and innervated PDL consists of basic constituents that resist and dampen mechanical loads. Different types of collagen and noncollagenous proteins including polyanionic water attracting molecules, the proteoglycans (PGs), all of which accommodate cyclic occlusal loads of varying magnitudes and directions. Unlike other ligaments within the musculoskeletal system, the blood vessels in the PDL are continuous with blood vessels in the endosteal spaces of bone . Although PDL and bone are two dissimilar tissues in physical and chemical properties, the continuity formed by blood vessels enables a flow of nutrients and maintains cellular activity responsible for PDL turnover and bone remodeling and modeling. Development and growth superimposed with functional loads  may cause posterior lengthening of the rat jaw , and can contribute to PDL turnover, bone remodeling, and load-related modeling during the lifespan of a rat. As a result, rat molars are thought to exhibit an inherent distal drift , but this theory continues to be controversial [7, 8]. Regardless, the drift of the molars causes bone resorption located on the distal side of the root and bone formation on the mesial side. In this study, the distal side of the root and the adjacent alveolar bone will be referred to as the distal root bone complex (bone resorption side), and the mesial side of the root and adjacent bone as the mesial root bone complex (bone apposition side). Specific to this study are the various imaging modalities implemented to investigate the physical, chemical, and biochemical changes reflective of distal drift in a rat bone-PDL-cementum complex.
Maxillae from 7-week to 4-month-old male Sprague Dawley rats were used. Rats were obtained using animal tissue transfer according to guidelines of Institutional Animal Care and Use Committee (IACUC), University of California San Francisco (UCSF).
In the 19th century, Wolff discussed adaptation of bone due to mechanical forces . The occlusal force, primarily used in grinding the hard diet fed to rats, is the most prominent force in the periodontium . Within this adaptation lies growth and function-related changes in bone and cementum, which will be illustrated through various imaging modalities.
The functional dentoalveolar complex in rat molars is a highly dynamic system with interactions between functional forces, 3D form, tissues, cells, and biomolecules. The periodontium is an interesting model to study the mechanisms of biomineralization. The inherent distal drift requires ongoing bone formation on the mesial side and resorption on the distal side of the root, to facilitate tooth migration and maintain functional PDL-space. For a better understanding of function-related adaptation, it is necessary to discuss observations at a macroscopic and a microscopic level and correlate them using complementary techniques. With Micro-XCT and post processing of 3D images, we were able to describe the anatomy of the dentoalveolar complex including, approximation of macroscopic occlusion, root geometry, and anisotropy in bone morphology due to the distribution of the microscopic resorption pits on bone and root. With SEM, we increased the resolution and identified structures created by the resorption activity. Attenuation profiles derived from Micro-XCT virtual sections, together with the fluorochrome study, highlighted advancement of the mineralization fronts in the mesial root-bone complex. Fluorochrome labeling pointed out that biomineralization, in relation to repair, can also exist in the distal complex. H&E staining verified structural features from Micro-XCT and SEM studies, and provided a basic understanding of the organic matrix. TRAP allowed for identification of multinucleated cells in the resorption pits of bone and root, found almost exclusively in the distal root-bone complex. Increased RANKL expression as a parallel event to TRAP could be found predominantly close to the distal complex of bone, and to a minor degree at the root surface. We could identify the omnipresence of OPN in the tissue, and related it to its multiple functions in the resorption and remodeling of mineralized tissues. Utilizing a variety of techniques had a synergetic effect to describe and understand the complex dynamic system of the rat periodontium. These results elucidate that load-mediated perturbations and subsequent adaptation of the rat dentoalveolar complex, should acknowledge baseline function based adaptation of bone-PDL-cementum. Local remodeling and modeling related events associated with the physiological distal drift should also be identified before additional experimental variables are imposed.