Migrating trunk cells (green) avoid Slit- expressing cells (red).

When your gut tells you to run, it is wise to listen. Embryonic neural precursor cells do just that, according to De Bellard et al. (page 269), who identify the basis of an avoidance mechanism for migrating neural crest cells.

Groups of embryonic neural precursors take separate paths on their way to forming the peripheral nervous system. Vagal cells, which emerge from the neck region, migrate long distances to enter the gut. Cells in the trunk originate closer to the gut yet never enter this region. The new results show that the gut is off limits because trunk cells sense Slit proteins, chemorepellents involved in axon guidance in flies and vertebrates.

In chick embryos, Slit expression marked the entrance to the gut. The group injected Slit- expressing cells into embryos and found that trunk cells stopped in their tracks to avoid migrating through regions containing the chemorepellent. Vagal cells, in contrast, were unaffected by Slit proteins, as trunk but not vagal cells express the Slit receptors Robo1 and Robo2.

Repulsion required contact between trunk cells and cells expressing surface-bound Slit. Slit administered in vitro is reused uniformily, so a simple repulsion effect is not obvious. But this form of Slit does cause trunk cells to migrate faster than vagal cells. In vivo, these two effects may combine to turn trunk cells away from the gut and speed their migration in the other direction. ▪