Practice question

Morphogens function as long-range positional signals secreted from localized organizing centers, forming gradients that decrease with distance from source. Cells measure local concentration via receptor occupancy and convert it into distinct transcriptional outcomes using threshold-sensitive enhancers, embodying French flag paradigm proposed by Wolpert. For example, varying Sonic hedgehog levels in neural tube activate high-threshold Nkx2.2 versus lower-threshold Olig2, specifying ventral neuronal subtypes. This dose-dependent interpretation enables single molecule to pattern multiple fates spatially, coordinating tissue proportions without requiring direct contact between source and distant responding cells, central to field patterning.