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Segmentation Gets Pieced Together

Nature likes segmentation, the process of making repetitive embryonic units that serve as the building blocks of all insect bodies and those of many higher animals as well. In the 27 July issue of Cell, two papers shed new light on how this process works.

Segmentation can be seen in the shells of crustaceans, the body rings of the earthworm, and the backbones of vertebrates. But developmental biologists have only just begun to understand what drives segment formation, particularly in higher organisms. In previous work, developmental biologist Olivier Pourquié of the University of the Mediterranean in Marseille, France, showed that in vertebrates many of the genes involved in forming somites, the units that develop into the vertebrae and muscles of the torso, repeatedly cycle on and off. He suggested that a "segmentation clock" controls the timing of somite formation.

This time Pourquié and his team focused on fibroblast growth factor 8, or FGF8. To test its role in somite formation, the team first treated chick embryos with a drug that inhibits FGF8 binding to receptors. This produced bigger somites. When they upped FGF8 concentration, the somites were smaller. These results suggest to Pourquié that too much FGF8 prevents presomitic cells from differentiating and joining somites. Because the segmentation clock turns off after a certain period, that somite will be smaller.

Another piece of the segmentation puzzle has been put in place by Denis Duboule and colleagues of the University of Geneva in Switzerland. Using mice, they closely monitored the timing and level of expression of Hox genes, which help give each somite its identity--in other words, a sense of its location along the chain of somites. The researchers found that the same signaling that starts the formation of a somite triggers the expression of the Hox genes. This information of where along the chain of somites it is located is critical for further development of the organism.

Yoshiko Takahashi, a developmental biologist at Nara Institute of Science and Technology in Japan, calls the link of Hox genes to the segmentation clock "astonishing work." The results from the two groups, she says, "are having a great impact on understanding a very important phenomenon."