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This Week in Science

ERVE growth is directed in the developing vertebrate embryo much as it is in the developing insect: neuronal growth cones (amoeboid motile extensions at the tips of growing axons) appear to receive specific directional cues from cell surfaces during their growth along the spinal column (page 740). "Pioneer" and "follower" growth cones are aided by nonneuronal and neuronal cells, respectively, during pathfinding; when the cuegivers are killed with a laser, growth along the spinal cord is arrested. Kuwada chose a fish system in which to study the early stages of growth and positioning of nerves because the transparent embryos have relatively few yet distinctive neuronal cells. By filling cells with dyes, axon growth into predictable locations could be followed or individual neurons by light and electron microscopy. Although these fish are among the simplest vertebrates, it is likely that their axonal growth mechanisms are similar to those of complex vertebrates during establishment of neuronal networks.