Gene expression during cartilage differentiation in turkey tibial dyschondroplasia, evaluated by in situ hybridization

Gene expression and alkaline phosphatase (AP) activity, associated with chondrocyte differentiation, were evaluated in the epiphyses of normal and tibial dyschondroplasia (TD)-afflicted turkeys. In the normal turkey growth plate (GP), osteopontin (OPN) and type X collagen genes were expressed by the hypertrophic cells in both GP and secondary ossification center, parallel to manifestation of AP activity. Collagen type II gene expression was restricted to the nonhypertrophic chondrocytes at the upper part of the GP. OPN or collagen type X genes were not expressed within the TD lesion. However, these genes were expressed in areas proximal and distal to the lesion, suggesting that after reaching partial differentiation, chondrocytes within the developing TD lesion de-differentiate into cells that resemble chondrocytes in the prehypertrophic zone. This suggestion is supported by the observation that the cells in the lesion expressed the collagen type II gene. In some cases, the TD lesion was invaded by fibroblastlike cells that did not exhibit any AP activity or expressed the OPN gene. No lesions were observed in the secondary ossification centers.