Background Bone destruction in the joints of patients with rheumatoid arthritis (RA) is the result of a combined bone resorption by osteoclasts and bone formation by osteoblasts. However, most knowledge about this process comes from in vitro studies, whereas only few studies have addressed the importance of the osteoclasts and osteoblasts using histological methods, and none has used 3D stereological estimators. Therefore, stereology may provide important knowledge about bone formation and resorption in RA.
Objectives To study the in vivo effect of bone resorption and bone formation in the paws of SKG mice using 3D stereological estimators.
Methods Twenty one 9–12-weeks-old female SKG mice were randomised to either i.p. injection with mannan (11 mice) or PBS (10 mice). Arthritis was scored twice weekly by an observer blinded for group distribution. The fluorescent label tetracycline was injected 8 days before termination of the study at the end of week 6.
Right hind paws were fixed in alcohol and embedded undecalcified in methylmethacrylate. Tissue sections of 7 μm were cut exhaustively according to the principles of vertical sectioning. Systematic sampling was used to obtain approximately 12 sections from 10 levels. Using newCAST stereological software, intercepts between a line grid and the tissue of interest were counted by an observer blinded for group distribution. Osteoclast-covered bone surfaces (Oc.S) and eroded surfaces (ES) were estimated on TRAP-stained sections and mineralising surfaces (MS) were estimated on unstained sections. All parameters were assessed in the tarsus on the periosteal and endosteal surfaces, and the presence of adjacent inflammatory tissue was evaluated for each intersection. The relevant reference bone surface (BS) was estimated for all parameters, presenting the results as relative values (MS/BS, ES/BS, and Oc.S/BS).
Results At the end of week 1 and until termination, there was a significant difference in arthritis score (p<0.01).
MS/BS, ES/BS, and Oc.S/BS were elevated in arthritic paws compared to normal paws both at the endosteal surface and the periosteal surface (p<0.001). On endosteal as well as periosteal surfaces MS/BS were elevated on surfaces without inflammation in arthritic mice compared to normal mice (p<0.001).
In arthritic mice the ES/BS and Oc.S/BS on endosteal as well as periosteal surfaces were larger on surfaces adjacent to inflammation compared with the surfaces without inflammation (p<0.01). However, the difference between MS/BS at surfaces with and without inflammation on either periosteal or endosteal surfaces did not reach the level of statistical significance.
Conclusions The study demonstrates that arthritis caused bone formation to occur on more bone surfaces, irrespectively of the adjacent tissue being inflamed. However, bone degradation was present almost exclusively at surfaces with adjacent inflammation. Therefore, arthritic bone degradation is likely to be explained by an imbalance of erosion and formation of bone rather than a general down-regulation of bone formation. The present study is the first to apply 3D stereological estimators to quantify bone formation and degradation in a model of RA.
Disclosure of Interest None Declared
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