Practice question

Morphogen is defined as secreted signaling molecule that forms concentration gradient from localized source and elicits distinct cellular outcomes at different thresholds, thereby providing positional information across field. Source secretes ligand such as Activin, Sonic hedgehog or Bicoid that diffuses away, creating decreasing concentration with distance. Cells measure ambient dose via receptor occupancy, translating into graded intracellular pathway activation like phosphorylated Smad levels, leading to differential gene expression programs. Long-range action over many cell diameters distinguishes morphogens from short-range paracrine factors, allowing single signal to organize entire tissue proportionally and coordinate patterning according to French flag paradigm.