Practice question

Posterior mesenchyme Zone of Polarizing Activity produces Sonic hedgehog which acts as long-range morphogen specifying anterior-posterior axis through concentration gradient. High levels activate Gli activator form high posteriorly, repress Gli3 repressor formation anteriorly, establishing digit identity gradient. High levels promote posterior ulnar digits expressing Hand2, intermediate specifies central digits, absence specifies anterior thumb. SHH-coated beads grafted anteriorly replicate ZPA duplication experiments producing mirror-image polydactyly. Moreover SHH induces downstream Fgf4 in AER and BMP antagonists, supporting survival and expansion of patterning domain and maintaining outgrowth coordination.