Practice question

Differentiation of limb skeleton from multipotent mesenchyme into cartilage requires master chondrogenic transcription factor Sox9. Sox9 expressed in all precartilaginous condensations throughout stylopod, zeugopod, and autopod directly activates Col2a1, Acan, Comp, and cartilage matrix genes while repressing osteogenic Runx2 program until later endochondral ossification. Induced downstream of BMP and FGF signaling from ridge and mesoderm, Sox9 deletion eliminates all limb cartilage yet preserves limb patterning genes, indicating essential execution function in translating positional information into skeletal structures and differentiation. Conserved across chick mouse human models, this mechanism illustrates classic embryological principles integrated with modern molecular genetics.