CHICKEN TIBIAL DYSCHONDROPLASIA - A LIMB MUTANT WITH 2 GROWTH PLATES AND POSSIBLE DEFECTS OF COLLAGEN CROSS-LINKING

In the cartilaginous epiphyseal growth plate, extracellular matrix mol ecules such as collagens are believed to play important roles during b oth normal and abnormal development. One defect of the epiphyseal plat e occurs in chickens with a condition termed tibial dyschondroplasia ( TD). This abnormality occurs in certain strains of juvenile chickens a nd other rapidly developing animals. It is characterized by the presen ce of a mass of avascular, uncalcified cartilage which is retained in the proximal metaphysis of the tibiotarsus. To elucidate the developme ntal events which may be involved in this lesion, we have performed bo th immunohistochemistry and in situ hybridizations for collagen types II and X, known components of the extracellular matrix of the growth p late. By immunohistochemical analyses, the TD lesion contains both of these collagen types; therefore, the presence of these molecules per s e is not sufficient for calcification of vascularization to occur. Sin ce type X collagen is expressed exclusively in hypertrophic cartilage, the chondrocytes in the lesion must have undergone hypertrophy before their developmental arrest. The matrix of the lesion also reacted wit h a monoclonal antibody which is directed against an epitope in the NH 2-terminal telopeptide of the alpha1(II) chain. Our previous data sugg est that this epitope is rendered unavailable in type II collagen whic h has undergone crosslink formation; its availability in the lesion su ggests that crosslinking may be abnormal. Lastly, analyses by in situ hybridization failed to detect mRNA for either type II or type X colla gen within the lesion, but chondrocytes distal to the lesion do contai n mRNAs for these collagens in a spatial pattern suggesting the presen ce of a second growth plate.