9 septembre 2016

De nageoires à mains

Extrait du fascinant article:

To the naked eye, there is not much similarity between a human hand and the fin of, say, a goldfish. A human hand is at the end of an arm. It has bones that develop from cartilage and contain blood vessels. This type of tissue is called endochondral bone.

A goldfish grows just a tiny cluster of endochondral bones at the base of its fin. The rest of the fin is taken up by thin rays, which are made of an entirely different tissue called dermal bone. Dermal bone does not start out as cartilage and does not contain blood vessels.

These differences have long puzzled scientists. The fossil record shows that we share a common aquatic ancestor with ray-finned fish that lived some 430 million years ago. Four-limbed creatures with spines — known as tetrapods — had evolved by 360 million years ago and went on to colonize dry land.

(...) When he is not digging for fossils, Dr. Shubin runs a lab at the University of Chicago, where he and his colleagues compare how tetrapods — mice, for example — and fish develop as embryos.

Their embryos start out looking very similar, consisting of heads and tails and not much in between. Two pairs of buds then develop on their flanks. In fish, the buds grow into fins. In tetrapods, they become limbs.

In recent decades, researchers have uncovered some of the genes that govern this development. In 1996, a team of French researchers studying mice discovered genes that are essential for the development of their legs.

When the scientists shut down two genes, called Hoxa-13 and Hoxd-13, the mice developed normal long bones in their legs. But their wrist and ankle bones failed to appear, and they did not grow any digits.

This discovery suggested that Hoxa-13 and Hoxd-13 genes tell certain cells in the tetrapod limb bud that they will develop into hands and feet.

(...) In 2013, a postdoctoral researcher in Dr. Shubin’s lab, Tetsuya Nakamura, started using Crispr to manipulate fish embryos. He chose zebrafish to study, because their transparent embryos make it easy to track their development.

(...) The new study was important because it revealed that the development of fins and limbs follows some of the same rules, said Matthew P. Harris, a geneticist at Harvard Medical School. In both cases, the Hox genes tell a clump of embryonic cells that they need to end up at the far end of an appendage. “The molecular address is the same,” said Dr. Harris (...)

In zebrafish, the cells that get that molecular address end up making dermal bone for fin rays. In tetrapods like us, the research indicates, the same cells produce endochondral bone in our hands and feet.

The new discovery could help make sense of the intermediate fish with limb-like fins that Dr. Shubin and his colleagues have unearthed. These animals still used the molecular addresses their ancestors used. But when their cells reached their addresses, some of them became endochondral bone instead of fin rays. It may have been a simple matter to shift from one kind of tissue to another.

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