Practice question

Classical morphogen gradients rely on long-range secreted proteins that form extracellular concentration fields and pattern tissues across many cell diameters. Hedgehog family members like Sonic hedgehog ventralize neural tube, Wnt proteins posteriorize embryo and maintain stem niches, BMPs dorsalize neural tube and pattern dorsoventral axis consistently. All operate via graded distribution and direct transcriptional threshold responses interpreted by Gli or Smad proteins. JAK-STAT mainly mediates cytokine signaling for proliferation, immunity and maintenance of Drosophila germline stem cells over short range, not forming classic long-range morphogen gradients that specify multiple distinct fates in concentration-dependent manner.