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Figure 8


Fig. 8. Regulation of osteoblastic differentiation in cranial dermal bone. (A) Table summarizing the expression pattern data for the developing dentary and frontal bones. Four major cell types could be distinguished based on skeletogenic markers: proliferating cells at the periphery of the osteogenic front, cells expressing only Runx2, preosteoblastic progenitors and surrounding cells expressing Runx2, Coll II, Coll IX, Ptc1, Gli1, PTHrP-R and Bmp4. (1) These cells differentiate into the CLO (chondrocyte-like osteoblast) cells expressing a unique combination of Opn, Coll II and Coll IX (but not Sox9 or aggrecan) as well as Ihh, Ptc1, Gli1, Bmp4 and PTHrP. As these cells develop into mature osteoblasts, they downregulate Coll II and Coll IX expression and, out of the analyzed markers, they express only Opn, BspII and Ihh. (2) Alternatively, some cells could differentiate directly into mature osteoblasts. (B) During craniofacial development, mesencephalic cranial neural crest cells migrate to populate mesenchyme of the future face and skull. Cells of the early cranial skeletogenic condensations depend on BMP2/4/7 activities to form preosteoblastic progenitors, whereas high levels of BMP2 and/or BMP4 alone induced a chondrogenic fate. Differentiation into the chondrocyte-like osteoblasts is regulated by both IHH and PTHrP activities.





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