Cell Death in Avian Tibial Dyschondroplasia

Tibial dyschondroplasia (TD) is a local defect of growth plates in fast-growing poultry where the transitional zone cartilage fails to resorb and persists as an avascular plug that prevents endochondral bone formation. We compared the differences in the cartilages from normal and TD-affected growth plates using the reduction of MTS to assess cartilage viability. Chondrocyte apoptosis was determined using biochemical measurement of DNA fragmentation, and in situ labeling of nuclei with fluorescein-dUTP using terminal deoxynucleotide transferase (TdT)-mediated nick end labeling (TUNEL) of isolated chondrocytes and growth plate sections. The TD-affected cartilage showed a significantly lower level of MTS reduction and a decrease in trichloroacetic acid (TCA)-precipitable DNA content. The TD cartilages had a higher percentage of fragmented DNA, which was also evident with agarose gel electrophoresis. A significantly higher number of chondrocytes isolated from TD-affected cartilages had condensed morphology, shrunken nuclei with little cytoplasm, and were TUNEL positive as identified by the incorporation of fluorescein-dUTP into the nuclei. In vivo results similarly showed a significant population of chondrocytes in transition zones undergoing condensation and apoptosis as determined by in situ TUNEL staining of growth plate sections. Normal growth plates, under similar conditions, showed no significant apoptosis of chondrocytes from hypertrophic and chondrolyzing zones. The condensation and apoptotic cell death may be responsible for the reduction of growth plate viability as well as the reduction in DNA content and increased DNA fragmentation. While the cause of the pathogenesis of TD is unknown, it appears that the aberrant death of chondrocytes in hypertrophic regions of growth plates may be responsible for the accumulation of cartilage and the arrest of endochondral bone formation.