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#source-sink model

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Morphogen Induction-l

What does the 'sink' refer to in the source-sink model of morphogen distribution?

Source-sink model describes formation of stable gradient via localized production and distributed removal that together shape distribution. Source is synthesizing center like floor plate producing Shh or zone of polarizing activity in limb bud. Sink encompasses mechanisms clearing morphogen: receptor-mediated endocytosis and lysosomal degradation, binding to heparan sulfate decoys, cleavage by extracellular proteases like Tolloid. Sink prevents accumulation, maintains steepness and sets length scale lambda equals sqrt(D/k). Without degradation, gradient collapses to uniform distribution losing positional information needed for threshold-based patterning in limb bud and neural tube.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Sink, Degradation and Gradient Stability.

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Induction Morphogen -ll

What does the 'sink' refer to in the source-sink model of morphogen distribution?

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.

Ref: Wolpert et al., Principles of Development, 6th ed., Chapter 5: Source-Sink Model of Gradient Formation.

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