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.