Chemical gradients tell cells where they are, and simple rules tell them what to become
An embryo has no blueprint and no foreman. Cells read local chemical concentrations, apply threshold rules, and differentiate accordingly. The body plan emerges from local chemical conversations.
The signaling chemicals are morphogens. They are produced at specific locations, diffuse outward, and degrade along the way, leaving a concentration gradient. Each cell reads its local value and responds.
Lewis Wolpert's 1969 French Flag model: a row of cells reads a morphogen gradient high on the left and low on the right, and uses two thresholds to choose among three fates — blue above T1, white between, red below.
No cell-by-cell address system is required. One signal and two decision boundaries are enough.
Figure 1. Wolpert's French Flag model. A morphogen gradient (black curve) decreases from left to right. Two thresholds (dashed lines) divide cells into three fates: blue above T1, white between T1 and T2, red below T2. Drag the sliders to reshape the gradient and move the thresholds. You can also drag the threshold lines directly on the chart.
A source pumps morphogen into the tissue. Molecules diffuse outward and are degraded along the way. The balance between diffusion and degradation sets the steady-state shape.
The reach scales as $\sqrt{D/k}$. Fast diffusion and slow degradation give a long gentle gradient; slow diffusion and fast degradation give a short steep one.
Click anywhere on the tissue to inject a burst of morphogen — watch how it spreads and decays
Figure 2. Morphogen diffusing from a source on the left edge while being degraded everywhere. The heatmap shows concentration across the 2D tissue field. The strip below shows cell fates (blue/white/red). The profile chart plots the average concentration at each column. The dashed curve is the theoretical steady-state: C(x) = e-x/L. Click the heatmap to inject a transient secondary source.
A single gradient sets one axis; real embryos need two. Add a second morphogen with a different source, and every cell reads two concentrations as coordinates in a 2D fate space.
Split that fate space into quadrants and the intersection of the gradients carves the tissue into stripes, patches, or nested regions depending on source placement and reach.
Drag the two source markers (circles) to reposition the morphogen sources
Figure 3. Two morphogen sources create a 2D fate map. Each cell reads both concentrations and picks one of four fates based on which quadrant of the two-morphogen space it falls in. Toggle views to see each morphogen's individual concentration field. Drag the source markers to rearrange the pattern.
The Bicoid protein in fruit fly embryos forms exactly this kind of gradient. Maternal Bicoid mRNA is deposited at the anterior end of the egg; after fertilization, protein diffuses posteriorly into an exponential gradient. High Bicoid means head, low Bicoid means abdomen. Mutations that disrupt Bicoid produce flies with two tails.