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Figure 3. In situ hybridizations of runx2, sox9, collagen2a1, and cbfb. All images are lateral views, anterior to the right. Runx2 expression begins in the presumptive suprarostral plate (SP). It expands to the anlage of the cranial trabeculae (CT) and a faint portion of the hyoid arch crest (HA) by stage 35/36. Runx2 staining includes the future ceratobranchial (CB) and ceratohyal (CH) cartilages by stage 39, and is additionally found in the otic capsule (OC) by stage 41. The expression of runx2 in the ceratohyal decreases during stage 41 as this element begins to chondrify. The distribution of sox9 is more extensive than that of runx2 by stage 33/34, with strong expression in the mandibular (MA), hyoid (HA), and branchial (BA) arch neural crest streams. Its expression in the crest becomes restricted by stage 35/36 with a clear distribution in the precursors of the future palatoquadrate and continued expression in the hyoid and branchial arches. By stage 39, both runx2 and sox9 are co-expressed in the future cartilaginous elements. Sox9 expression in the ceratohyal is also diminished by stage 41 with the onset of chondrogenesis. Col2a1 is distributed through the notochord and otic capsule along with a subset of the branchial arches (BA) and presumptive suprarostral plate by stage 33/34. Col2a1 is expressed widely throughout the head during the stages examined with intensifying staining in the presumptive cranial cartilages by stages 39 and 41. The runx co-factor cbfb is expressed throughout the head during development, with stronger expression in the presumptive ceratohyal and palatoquadrate by stage 39.
