The number of fingers that grow on a hand is set by the Shh morphogen. Shh is made by cells in a zone at the posterior of the limb bud and sets the anterior–posterior axis by telling neighboring cells to make another morphogen, called Gremlin. Gremlin instructs the tip of the bud to express Fgf, which promotes limb outgrowth and Shh expression.
Escape from this loop in chicks depends on a newly discovered property of Shh-producing cells. Neither they nor their descendents make Gremlin. The cause of this inability is unknown, but high levels of Shh may induce some inherited protein or chromatin alteration that represses Gremlin.
As the cells that once made Shh divide and expand anteriorly out of the Shh zone, a barrier is formed. “Eventually, Gremlin cells are beyond the point where they can reach [Shh],” says Scherz, “so Gremlin is down-regulated.” If this barrier is cut away before the loop terminates, and the two borders stitched together, the limb fills in this space with Shh descendents and grows normally. Different limb sizes might be achieved by changing the size of the barrier cells, the rate of their division, or the distance of Shh transport.
Brian Harfe (University of Florida, Gainesville, FL), Scherz, Tabin, and colleagues also show that both Shh concentration and exposure time control digit development. Using fate mapping experiments in mice, the group shows that the two most posterior digits and part of the middle digit are made of former Shh-expressing cells.
These fingers were thus differentiated from each other despite exposure to the maximum concentration of Shh. But the more posterior the finger, the longer its exposure time. “There has to be some kind of counting mechanism,” says Scherz, “that builds up in cells based on longer exposure,” such as a transcription factor or phosphorylated protein, whose levels control digit morphology. ▪