In a 2000 paper published in Science, Fallon and graduate student Randall Dahn showed that the interdigit tissue was more important than previously thought. It was not just a spacer between developing digits; experimental manipulations showed that it controlled how the neighboring digit would develop. The team proposed that different signal levels in each interdigit resulted in specific digit identities.
"We thought that bone morphogenic protein (BMP) signals from interdigit cells were sent to the digit primordium, a rod of cartilage, in the neighboring digit ray, breaking up the cartilage into phalanges," Fallon says.
Sean Hasso, Fallon's current graduate student, wanted to know precisely which cells in the digital ray give rise to phalanges and which molecular events determine the number, size and shape of each phalanx. Performing microsurgery and cell marking studies on the embryonic chick autopod, Hasso showed that the cells that eventually form phalanges arise from the growth of mesenchyme at the tip of the digital ray.
"This finding is absolutely contrary to what we and other scientists had been thinking-which was that growth and phalanx formation occurred in the cartilaginous rods in the digit primordia," says Fallon. "These observations were the foundation for further studies."
In the next set of experiments, Takayuki Suzuki, a post-doctoral fellow in the Fallon lab, conducted an in-depth examination of several aspects of genetic expression in the PFR. He observed that the up-regulation of a gene called Sox9 indicated that the cells of the PFR commit to becoming cartilage.
The scientists were most interested to see that these cells also up-regulated a BMP receptor. Suzuki devised an assay to quantitate BMP receptor signaling in t
|Contact: Dian Land|
University of Wisconsin-Madison