Morphogen

A morphogen is a substance that tells cells where they are in a developing embryo. It is released from a localized source, spreads through the surrounding tissue and is removed or broken down along the way, so its concentration falls with distance. Cells read the local level: above one threshold they switch on one set of genes, at a lower level another, and below that they keep a default fate. Alan Turing coined the word in 1952, and Lewis Wolpert later pictured the idea as a French flag, with three stripes of cell types set by two concentration thresholds.

Classic morphogens include Bicoid, a transcription factor that forms a gradient from the front end of the fruit fly embryo, and Sonic hedgehog, which patterns the vertebrate limb and neural tube. In the neural tube, Sonic hedgehog from the floor plate and BMP from the roof plate form opposing gradients that divide it into distinct zones of nerve cell precursors.

Real gradients are more dynamic than the simple model suggests. Cells respond to how long they are exposed as well as how much, and gradients shift as the tissue grows.