The vertebrate transcription factor protein Runx2 is regarded as a "master regulator" of bone formation due to the dramatic loss of the osseous skeleton in the mouse homozygous knockout. However, Runx2 mRNA also is expressed in the pre-hypertrophic cartilaginous skeleton of the mouse and chicken, where its developmental function is largely unknown. Several tiers of Runx2 regulation exist in the mouse, any of which may account for its seeming biological inactivity during early stages of skeletogenesis. Unlike mouse and chicken, zebrafish require Runx2 function in early cartilage differentiation. The present study reveals that the earlier functional role of Runx2 in cartilage differentiation is shared between zebrafish and Xenopus. A combination of morpholino oligonucleotide injections and neural crest transplants indicate that Runx2 is involved in differentiation of the cartilaginous hyobranchial skeleton in the frog, Xenopus laevis. Additionally, in situ hybridizations show runx2 mRNA expression in mesenchymal precursors of the cartilaginous skull, which reveals the earliest pre-patterning of these cartilages described to date. The early distribution of runx2 resolves the homology of the larval suprarostral plate, which is one of the oldest controversies of anuran skull development. Together these data reveal a shift in Runx2 protein function during vertebrate evolution towards its exclusive roles in cartilage hypertrophy and bone differentiation within the amniote lineage.
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