Practice question

Definitive morphogen properties include ability to act at distance via extracellular diffusion, to form concentration gradient from localized source, to evoke distinct cellular responses at different thresholds, to pattern fields without requiring additional relay signals and to act directly activating target genes. Acting only through direct cell-cell contact contradicts long-range diffusive nature essential for forming broad gradient spanning many cell diameters up to millimeter scale. Concentration-dependent gene expression, functioning as transcription or paracrine factor and establishing body axes are genuine attributes reflecting gradient interpretation. Contact-only action characterizes juxtacrine Notch signaling, not morphogens like Bicoid, Shh or BMP which spread extracellularly.