Asymmetrical threat averted

摘要

The somites are embryonic elements that give rise to the muscles, skeleton and some skin layers of the trunk. They form in a symmetrical fashion, but to do so they must be shielded from asymmetrical cues. The human body looks deceptively symmetrical from the outside. In con-trast to this external symmetry, our internal organs are in an asymmetrical, yet reproducible, arrangement. The heart is on the left and the lung next to it is smaller than that on the right. Within the abdominal cavity, the intestines form a clockwise loop; the stomach, pancreas and spleen are on the left, whereas the liver and gall bladder are on the right. But our body plan does not start out this way, and some body parts—the somites, for instance — must remain bilaterally symmetrical, A striking series of findings by Kawakami et al. and Vermot and Pourquie, published on pages 165 and 215 of this issue, along with a complementary manuscript in Science, teach us how somite formation proceeds in a bilaterally synchronized fashion (Fig. 1). Furthermore, Kawakami et al. and Tanaka et al., also writing in this issue (page 172), shed new light on how asymmetry, which is necessary for the physiological function of many organs, is first established in the embryo (Box 1, overleaf).