Practice question

Sonic hedgehog is quintessential secreted morphogen modified with cholesterol and palmitate moieties, diffusing to form ventral-to-dorsal gradient in vertebrate neural tube and posterior-to-anterior gradient in limb bud. Concentration-dependent activation of Gli transcription factor family specifies floor plate at highest levels, motor neurons at intermediate Olig2 threshold and distinct interneuron classes at progressively lower thresholds. Mutations in Shh cause holoprosencephaly and limb malformations, confirming essential patterning role. Unlike cytoskeletal proteins or polymerases, Shh satisfies criteria of long-range signaling, threshold-dependent fate specification and direct influence on differential gene expression, embodying classic morphogen functionality.