Osteochondrosis/dyschondroplasia: a failure of chondrocyte differentiation

Abstract

SummaryOsteochondrosis/dyschondroplasia is a focal disorder of endochondral ossification which occurs in most species and is characterised by retention of epiphyseal or growth plate (physeal) cartilage. Epiphyseal and physeal cartilage in different species is very similar at the molecular and cellular levels. Therefore, the high frequency of lesions, ease of induction and a similar pathology to that found in other species makes avian tibial dyschondroplasia (TD) a useful model for the general study of osteochondrosis. In addition some of the methods successfully applied to the investigation of avian TD may indicate strategies for the investigation of equine OCD/dyschondroplasia.TD is characterised by an avascular mass of cartilage containing partly hypertrophied (transitional) chondrocytes. Changes occurring during the initiation of a lesion can be obscured by subsequent pathology and caution must be exercised in interpreting results of studies of established lesions. In‐situ biochemistry and immunocytochemistry of chondrocytes in TD indicates that the accumulation of cartilage is not caused by increased chondrocyte proliferation or decreased osteoclast/chondroclast activity. Anaerobic glycolysis is not increased in the transitional chondrocytes, showing that oxygen supply to these cells is not limiting. These initial studies confirmed that TD is the result of a failure of chondrocytes to differentiate fully and are supported by an investigation in which genetic selection for TD resulted in accumulations of transitional chondrocytes.Transitional chondrocytes contain high concentrations of TGF‐b and the product of the proto‐oncogene, c‐myc. In TD these cells are deficient in TGF‐β and c‐myc. Alkaline phosphatase and Type X collagen mRNA are increased but a lower concentration of collagen X within the matrix indicates a defect in its secretion or incorporation within the matrix. Type II collagen expression remains unchanged in dyschondroplasia. These data enable the point at which the arrest of chondrocyte differentiation occurs to be identified more precisely.These methodologies are starting to reveal the sequence of events which results in the initiation of dyschondroplasia in domestic poultry. Strategies developed, tested and applied in the investigation of TD can be applied to studies of osteochondrosis/dyschondroplasia in other species including equids.