Practice question

Sonic hedgehog family member secreted from posterior mesenchyme establishes anterior-posterior pattern in developing and regenerating limbs. During salamander and chick limb regeneration, SHH expressed in posterior blastema activates downstream Gli transcription factors, with gradient high posteriorly specifying digit five to four to three identities and low anterior leading to thumb. Ectopic SHH beads anteriorly duplicate digits mirror-symmetrically, while SHH blockade produces digit loss and shortened limbs. Beyond digit specification SHH promotes cell cycle progression and survival of blastema cells, regulates blood vessel formation secondarily, but canonical role is promoting digit formation and posterior pattern rather than suppressing growth.