UF, Harvard Researchers Put A Finger On Understanding How Limbs Develop

August 19, 2004

GAINESVILLE, Fla. — Fingers are key to the art of communication, whether it’s a politician flashing a thumbs-up to a cheering crowd or a bride displaying a diamond-bedecked ring finger.

Now scientists at the University of Florida and Harvard University have described how the art of cellular communication — how cells “talk” and what happens when they stop — plays a crucial role in normal limb development and the formation of digits in mice, a discovery that sheds light on the same process in people. The researchers detail their discovery in today’s issue of the journal Cell.

Why the five fingers on a hand form into the sizes and shapes they do, and the fundamental mechanisms that cause some people to be born without fully formed fingers or extra fingers has been a mystery until now. Understanding the development process could someday help doctors correct defects before birth, or help regenerate limbs lost to accident or amputation, researchers say.

“Everybody’s goal is to figure out the normal process well enough so then you can go back and maybe help a human,” said Brian Harfe, a developmental biologist at UF’s College of Medicine and the paper’s lead author. “For example, if a baby is missing a pinkie, and we have learned enough about how this digit is formed in the first place, we might eventually be able to repair the defect by using what we know to induce a normal digit to grow.”

The findings also could shed light on the development of the body’s more-critical organs, he said.

“This is the first time anyone has figured out how the body regulates the size — not just of the limb, but possibly of other organs during development,” he said.

The researchers studied cells in the mouse embryo limb bud that express an active gene called Sonic Hedgehog, which is essential for normal limb development. The gene expresses a protein that acts like a dispatcher, barking chemical orders to other molecules and initiating limb growth. The researchers followed the cells that expressed the gene and found that in many of these cells, the Sonic Hedgehog gene eventually stops sending its message and migrates to another part of the developing limb. These cells then form a “wedge” that directly blocks another important signaling pathway in the limb. When communications break down between key molecules, the signal for limb growth shuts down at the right time and a normal limb results.

Although the discovery was made in mice, scientists say the same pathway is believed to function in human cells.

Harfe, an assistant professor of molecular genetics and microbiology, studied mice bred to harbor a pair of visible genetic markers in Sonic Hedgehog-expressing cells. That enabled him to follow what happened to the cells as a limb developed, even after they stopped expressing the gene.

“Sonic Hedgehog turns off as you start to form the fingers,” Harfe said. “Previously we had no way of following what happens to the cells that were expressing this gene once it turned off. We needed to design a way to follow the fates of these cells once they stopped expressing the Sonic Hedgehog gene. Once we did that, we learned that they formed this wedge and that the cells that formerly expressed Sonic Hedgehog actually form the last two fingers.”

Harfe found that the length of time and the concentration of Sonic Hedgehog that cells were exposed to determined which digit the cells would form.

“There has always been a huge debate in the field as to how you get a pinkie as opposed to an index finger or a thumb,” Harfe said. It is known that Sonic Hedgehog is expressed in a gradient, or in a decreasing concentration over distance, he said. “What we found is that both of the last two digits are formed directly from the cells that formerly expressed Sonic Hedgehog.”

The cells that were exposed to the highest concentrations of Sonic Hedgehog, both because they were closest to it and for the longest periods of time, become the fourth and fifth digits in mice, akin to the ring and pinkie fingers in people. The digits farther away from the source of the gene form the second and third fingers, analogous to the index and middle fingers in people. The cells with no exposure to Sonic Hedgehog form the thumb, or first digit.

Sun Xin, an assistant professor of medical genetics at the University of Wisconsin at Madison, said, “I think Dr. Harfe’s research described in the Cell paper is very important to the limb development field. The research allowed the authors to put forward a new model of how different structures form along the anterior/posterior axis of the limb. It will allow us to rethink the role of many other molecules involved in anterior/posterior patterning.”