COLLAGEN EXPRESSION IN CHICKEN TIBIAL DYSCHONDROPLASIA

Collagen expression in growth plate cartilage derived from broiler chi ckens with tibial dyschondroplasia was studied and compared with sampl es from unaffected birds, Normal growth plate contains 12% collagen (d ry weight) and dyschondroplastic growth plate 19% collagen compared wi th articular cartilage, which contains 55%, Dyschondroplastic growth p late collagens were more resistant to extraction by pepsin treatment t han were those from unaffected growth plate. Normal and dyschondroplas tic growth plate cartilages contain similar amounts of type I collagen (5% of the total collagen) but dyschondroplastic growth plate cartila ge contains slightly less type II and type XI collagens, and significa ntly more type X collagen (25% as compared to 11%) than in normal grow th plate. The levels of the mature collagen cross-link, hydroxylysyl-p yridinoline, are very low in normal growth plate but are six times hig her in dyschondroplastic lesions. Immunolocalisation studies show that there is little change to the normal patterns of collagen organisatio n in dyschondroplastic growth plate. Investigation of metalloproteinas e activity showed there to be a reduction in MMP-2 levels in dyschondr oplastic growth plate compared to normal growth plate, In vitro studie s 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 chondrocy tes derived from dyschondroplastic cartilage initially exhibit both fi broblastic and polygonal phenotypes but gradually change to totally fi broblastic. These morphological changes are reflected by the collagen synthesis in vitro. Chondrocytes derived from normal articular cartila ge synthesised collagen types I, II and X when cultured in alginate bu t type X synthesis was lost when cultured on plastic, Chondrocytes der ived 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 th e chondrocytes in dyschondroplastic growth plate cartilage are at a di fferent stage of maturity than normal resulting in a cartilage that is failing to turn over at a normal rate.