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Shift in Spatial Pattern of Type X Collagen Expression in Embryonic Sterna Undergoing Endochondral Ossification Demonstrated by in Situ Hybridization and Immunohistochemistry

In embryonic chick sterna undergoing endochondral ossification, type X collagen is expressed only in the cephalic presumptive calcification zone and is absent in the caudal permanent cartilagenous zone.'-3 The cephalic zone contains hypertrophic chondrocytes, whereas the caudal zone is populated only by small chondrocytes. To address the question of which chondrocytes in the cephalic zone initially express type X collagen message, actively produce type X collagen, and eventually lose that synthetic activity through stages of calcification, we examined consecutive sections from embryonic chick sterna from stages 42 to 46 by means of in situ hybridization4e5 to type X collagen mRNA using tritium-labeled EcoRI-PvuII fragments (979 bp and 1123 bp) of pYN92E1 as the probe and by peroxidase-antiperoxidase immunostaining for type X collagen using a monoclonal antibody (AC9): Our results indicate the following: ( 1) At stage 42, hypertrophic chondrocytes, which appeared to form from previously existing small chondrocytes in the central