Hypertrophic differentiation and calcification degeneration

Background: In degenerative intervertebral discs (IVDs) collagen type X expression and calcifications have been demonstrated, resembling advanced osteoarthritis (OA), which is associated with hypertrophic differentiation, characterized by the production of collagen type X, Runt-related transcription factor 2 (Runx2), osteoprotegerin (OPG), alkaline phosphatase (ALP) and calcifications. Objective: The aim of this study was to determine if hypertrophic differentiation occurs during IVD degeneration. Methods: IVDs from all Thompson degeneration grades were prepared for histology, extraction of nucleus pulposus (NP) and annulus fibrosis (AF) tissue (N = 50) and micro-CT (N = 27). The presence of collagen type X, OPG and Runx2 was determined by immunohistochemistry, with OPG levels also determined by Enzyme-linked immunosorbent assay (ELISA). The presence of calcification was determined by micro-CT, von Kossa and Alizarin Red staining. Results: Immunohistochemical staining for collagen type X, OPG, Runx2 appeared more intense in the NP of degenerative compared to healthy IVD samples. OPG levels correlated significantly with degeneration grade (NP: P<0.000; AF: P=0.002) and the number of microscopic calcifications (NP: P=0.002; AF: P=0.008). The extent of calcifications on micro-CT also correlated with degeneration grade (NP: P<0.001, AF: P=0.001) as did von Kossa staining (NP: P=0.015, AF: P=0.016). ALP staining was only incidentally seen in the transition zone of grades IV and V degenerated IVDs. Conclusion: This study for the first time demonstrates that hypertrophic differentiation occurs during IVD degeneration, as shown by an increase in OPG levels, the presence of ALP activity, increased immuno-positivity of Runx2 and collagen type X. (C) 2010 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.