Cellular interactions that guide sensory and motor neurites identified in an embryo slice preparation.

We used cultured cross sections ("slices") of avian embryos to identify interactions that guide neurites during their encounters with seven tissues that impose a stereotyped gross anatomical nerve pattern. We show that cultured slices retain tissue morphology, molecular distribution patterns, and guidance cues. They also allow us to directly visualize responses of labeled sensory and motor neurons deposited on the slice's surface. This assay has high predictive power. Contact-mediated avoidance or stimulation and long-range attraction or repulsion are each distinguishable because each predicts different neurite lengths and trajectories. The analysis shows that all but one of these mechanisms contributes to guidance. Three tissues similarly stimulated neurite elongation, suggesting common responses to a contact-mediated stimulation. Four tissues similarly elicited avoidance on contact, suggesting a common contact-mediated inhibition. Neurite orientations implicate a previously unsuspected long-distance attraction to one tissue, dorsal anterior sclerotome. Long-range repulsion plays no detectable role. Each tissue elicits the same response in two different neural populations, sensory and motor neurons. These results suggest that a small set of repeated mechanisms mediates responses to tissues that axons contact serially during pathfinding.