Pioneer axons lose directed growth after selective killing of guidepost cells.

The first nerve cells to appear in the limb buds of embryonic grasshoppers are a pair which lie at the distal tip and project axons along the length of the limb to the central nervous system (CNS). The stereotyped route navigated by these 'pioneer' axons is followed by other neurones and eventually becomes that of a major adult nerve trunk. The guidance cues which delineate this route are unknown, but it has been suggested that guidance is provided by a set of nonadjacent 'guidepost' cells along which the pioneers grow (Fig. 1). We have now tested this suggestion by selectively destroying identified guidepost cells and observing pioneer axon trajectories in their absence. Our results support the guidepost cell hypothesis.