Extracellular matrix changes in early osteochondrotic defects in foals: a key role for collagen?

Osteochondrosis (OC) is the most important developmental orthopaedic disease in the horse. Despite some decades of research, much of the pathogenesis of the disorder remains obscure. Increasing knowledge of articular cartilage development in juvenile animals led to the presumption that the role of collagen in OC might be more important than previously thought. To study collagen characteristics of both cartilage and subchondral bone in young (5 and 11 months of age) horses, samples were taken of subchondral bone and articular cartilage from a group of 43 Dutch Warmblood foals and yearlings that suffered from varying degrees of OC. Based on a histological classification, lesions were graded as early, middle and end stage. Collagen content and some posttranslational modifications (lysyl hydroxylation, hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP) cross-links) were determined, as was proteoglycan content. Data were compensated for site effects and analysed for differences due to the stage of the lesion. In early lesions total collagen was significantly decreased in both cartilage and subchondral bone of 5- and 11-month-old foals. Also in cartilage, HP cross-linking was reduced in the early lesions of 5- and 11-month-old foals, while LP cross-linking was decreased in subchondral bone of the end-stage lesions of both 5- and 11-month-old foals. Hydroxylysine content was unaffected. Collagen content remained reduced in cartilage from middle- and end-stage lesions, but returned to normal in subchondral bone. In cartilage there was a decrease in proteoglycan content in the end-stage lesions of both age groups. Thus, alterations of the collagen component, but not of the proteoglycan component, of the extracellular matrix might play a role in early OC. More severe lesions show a more general picture of an unspecific repair reaction. Biomarkers of collagen metabolism can be expected to be good candidates for early detection of OC.