Practice question

Source-sink models explain how stable gradients form by balancing localized production, diffusion and removal. Source represents localized synthesis, e.g., zone of polarizing activity secreting Shh or dorsal ectoderm secreting BMP. Sink is region where morphogen is cleared by receptor-mediated endocytosis and lysosomal degradation, proteolytic cleavage by Tolloid, binding to decoy receptors or heparan sulfate sequestration and intracellular degradation. This clearance sharpens gradient, prevents saturation and maintains steady-state shape and length scale. Without sink, ligand would accumulate uniformly, abolishing thresholds. Source-sink distance sets slope, amplitude and positional threshold locations critical for reproducible French flag patterning during limb and neural tube development.