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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.
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