Cell recognition by neuronal growth cones in a simple vertebrate embryo.

The mechanism that guides neuronal growth cones to their targets in vertebrate embryos has been difficult to study primarily because of the complexity and large number of neurons found in many vertebrate nervous systems. The spinal cord of a simple vertebrate, the fish embryo, is used to analyze pathfinding mechanisms. The early embryonic spinal cord consists of a relatively small number of identifiable neurons. From the beginning of axonal outgrowth the growth cones of these identified neurons extend along stereotyped and precise pathways in the spinal cord. Laser ablation experiments (i) support the hypothesis that early growth cones that pioneer specific spinal tracts appear to recognize cues on subsets of longitudinally arrayed neuroepithelial cells and (ii) demonstrate that later growth cones that selectively fasciculate in these spinal tracts appear to recognize cues on specific subsets of axons.