ABSTRACT
Tibial dyschondroplasia (TD) is a form of aberrant endochondral ossification in chickens, in that a plug of avascular cartilage (TD lesion) is formed within the growth plate. Histologically, the lesion is filled with apparently transitional chondrocytes that have been unable to differentiate to hypertrophic chondrocytes. We have examined the spatial expression of mRNAs for type X collagen, Indian hedgehog (Ihh) and Parathyroid Hormone-related protein (PTHrP) in the TD growth plate by in situ hybridization in order to ascertain at which stage chondrocyte differentiation is arrested in TD. In the normal growth plate, type X collagen mRNA was expressed by both prehypertrophic and hypertrophic chondrocytes. Indian Hedgehog mRNA was detected in a band of prehypertrophic chondrocytes and PTHrP expression was localized to a narrow band of prehypertrophic chondrocytes and in osteoblasts within the diaphysis. In TD sections, collagen X expression was seen within differentiating cells, within a small number of lesion cells, and within hypertrophic chondrocytes on the diaphyseal side of the lesion. Ihh expression was also seen within the differentiating cells and throughout the lesion. These data indicate that chondrocyte differentiation is arrested at the transitional stage just prior to hypertrophy. Contrary to the previously reported PTHrP expression patterns in TD chicks by immunohistochemistry, PTHrP mRNA was not detected in the TD lesion. This observation probably reflects the cessation of PTHrP gene expression by chondrocytes in the more severe TD lesions. The results from the present study also imply that the arrest of cell differentiation in TD is independent of PTHrP and that endochondral ossification in the post-hatch avian growth plate may involve additional regulatory pathways.
