Practice question

Establishment of graded positional fields depends on secreted ligands capable of long-range diffusion and threshold-dependent target activation via high-affinity receptors. Sonic hedgehog exemplifies this: palmitoylated and cholesterol-modified protein released from notochord, floor plate and zone of polarizing activity spreads via Dispatched, heparan sulfate proteoglycans and cytonemes, forming ventral-dorsal and posterior-anterior gradients over 300 micrometers. It directly patterns neural tube, limb digits and gut via Gli code. Myosin motor, DNA polymerase replicative enzyme and cyclins cell-cycle regulators lack extracellular diffusive gradient property and cannot provide spatial coordinate system for fate specification across tissue fields.