Why do we have five fingers? A gene called sonic hedgehog

Scientist Megan Davey at TEDxPortobello

Scientist Megan Davey at TEDxPortobello

“I can look into your hand and see something of your embryonic past,” says Megan Davey at TEDxPortobello. Davey studies developmental biology at The Roslin Institute, where she uses genomics to seek out new knowledge into the development processes of invertebrates like you and me.

Davey is particularly interesting in understanding the development of the human hand, for which she uses birds as a genetic model, from chickens to ducks to emus.

A developing chicken foot, one of Davey's research interests (Photo: Megan Davey)

A developing chicken foot, one of Davey’s research interests — the dark blue represents the SOX9 gene (Photo: Megan Davey)

Davey has targeted her studies toward the sonic hedgehog (SHH) gene, which is key in the formation of the chicken’s foot and wing. She uses an example of a chicken embryo to explain how this works.

“[The chicken embryo] is showing expression of the gene sonic hedgehog,” Davey says. “Everything that is dark blue is where sonic hedgehog is turned on. On the side of the body there are two little lumps — those are the developing wing and foot; and on one side there is a dark area of cells and those are the cells that are turning on sonic hedgehog.

A developing chicken foot embryo (Photo: Megan

A developing chicken embryo (Photo: Megan Davey)

The idea is that the cells that are very close to those blue cells and the cells which are actually making the sonic hedgehog, they hear that sonic hedgehog signal very loud and they’re told to turn into little fingers. The ones on the other side of the limb — they’re far away the signal and they can’t hear it so well and so they turn into thumbs. So if you don’t have any sonic hedgehog, you just develop thumbs. If you turn on sonic hedgehog for too long or in the wrong place, you end up with a lot of extra fingers and — maybe — no thumbs at all.

So having five fingers is really down to sonic hedgehog being turned on in those precise cells for a precise amount of time at a precise level.”

Davey explains what she’s learned from seeing what happens when sonic hedgehog goes wrong — from cancer to a mutation called TALPID3, which Davey has investigated in both chickens and humans.

To learn more, watch Davey’s whole talk below:

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