Old and unattracted

With a limited number of guidance molecules in the nervous system, the same molecules get used in multiple places. Now Derryck Shewan, Christine Holt, and colleagues (University of Cambridge, Cambridge, UK) have shown that an intrinsic timing mechanism allows netrin to be used as both an attractant and repellant for the same set of growth cones during different periods of their outgrowth.

Holt had already shown that, at the beginning of the pathway traversed by frog retinal axons, netrin leads the axons out of the eye. To study the rest of the pathway, Shewan achieved the finicky feat of culturing the entire pathway. He confirmed that netrin could later act as a repellent that probably helps to prevent overshoot of the axons' final target.

That was a nice result. But the surprise...