Fig. 8. Three classes of mechanisms modify the Shh extracellular gradient. An intermediate snapshot (30 hours after secretion) of the concentration versus distance profile is shown for cells that have mechanisms of the wild-type chick embryo: a signal accumulation regime, a signal dispersal regime, or a shunting mechanism. Mechanisms that promote the signal accumulation regime hinder Shh transport, thereby causing high accumulation of Shh near the source. By contrast, mechanisms that induce signal dispersal promote Shh transport along the axis, thereby creating a shallow Shh gradient over the entire tissue. Shunting mechanisms degrade Shh over the entire tissue, thereby decreasing extracellular Shh. V3 specification occurs above the 2.5 nM Shh signaling threshold and can be tuned to a particular distance from the source depending on which mechanisms are active.