Possible recruitment of osteoblastic precursor cells from hypertrophic chondrocytes during initial osteogenesis in cartilaginous limbs of young rats.

The appearance of the bone phenotype during rat embryogenesis was studied by in situ hybridization using a cDNA clone to osteopontin. Radiolabeled sense and antisense RNA probes were prepared from the osteopontin cDNA by in vitro transcription. The probes were used to hybridize paraffin sections of the cartilaginous diaphysis from embryonic rats at day 17 of gestation. The hybridization pattern was analyzed by autoradiography. Hybridization with the antisense probe gave patterns of silver grain labeling, indicating the presence of osteopontin mRNA among the hypertrophic chondrocytes. No silver grains could be detected in the corresponding region following hybridization of consecutive sections with the sense probe, showing the specificity of the technique being used. Whether these results indicate that the osteopontin gene is transiently expressed by hypertrophic chondrocytes or that osteopontin is an early marker for osteoblastic precursor cells will have to be explored further.