Collagen expression in chicken tibial dyschondroplasia

Collagen expression in growth plate cartilage derived from broiler chickens with tibial dyschondroplasia was studied and compared with samples from unaffected birds. Normal growth plate contains 12% collagen (dry weight) and dyschondroplastic growth plate 19% collagen compared with articular cartilage, which contains 55%. Dyschondroplastic growth plate collagens were more resistant to extraction by pepsin treatment than were those from unaffected growth plate. Normal and dyschondroplastic growth plate cartilages contain similar amounts of type I collagen (5% of the total collagen) but dyschondroplastic growth plate cartilage contains slightly less type II and type XI collagens, and significantly more type X collagen (25% as compared to 11%) than in normal growth plate. The levels of the mature collagen cross-link, hydroxylysyl-pyridinoline, are very low in normal growth plate but are six times higher in dyschondroplastic lesions. Immunolocalisation studies show that there is little change to the normal patterns of collagen organisation in dyschondroplastic growth plate. Investigation of metalloproteinase activity showed there to be a reduction in MMP-2 levels in dyschondroplastic growth plate compared to normal growth plate. In vitro studies on articular, normal growth plate and dyschondroplastic growth plate chondrocytes cultured in alginate or on plastic revealed differences between the cell types. When plated on plastic, articular chondrocytes rapidly assume a fibroblastic morphology. In contrast, normal growth plate chondrocytes retain their polygonal morphology whereas chondrocytes derived from dyschondroplastic cartilage initially exhibit both fibroblastic and polygonal phenotypes but gradually change to totally fibroblastic. These morphological changes are reflected by the collagen synthesis in vitro. Chondrocytes derived from normal articular cartilage synthesised collagen types I, II and X when cultured in alginate but type X synthesis was lost when cultured on plastic. Chondrocytes derived from normal growth plate cartilage synthesised predominantly type X collagen when cultured in either system. Chondrocytes derived from dyschondroplastic growth plate exhibited a similar phenotype to normal growth plate chondrocytes when cultured in alginate beads, but showed signs of dedifferentiation with reduced type X collagen and increased type I collagen when plated on plastic. These results suggest that the chondrocytes in dyschondroplastic growth plate cartilage are at a different stage of maturity than normal resulting in a cartilage that is failing to turn over at a normal rate.