Practice question

Hindlimb-specific development relies on Pitx1 transcription factor selectively expressed in hindlimb lateral plate mesoderm and limb bud mesenchyme from early stages. Pitx1 directly activates Tbx4, modulates hindlimb-specific cis-regulatory elements, drives expression of hindlimb outgrowth and skeletal genes, and represses forelimb program. Tbx5 drives forelimb, Sox9 governs universal chondrogenesis, Wnt7a dorsalizes both limbs. Pitx1 loss in mouse or altered expression in sticklebacks converts hindlimb morphology toward forelimb-like structure with patellar and flexor changes, demonstrating selector role in leg identity and muscle patterning.