A pioneering growth cone in the embryonic zebrafish brain.

During development of the nervous system, growth cones navigate very precisely to their appropriate, often distant, targets. In insects, the task of establishing the earliest pathways is accomplished by a small number of neurons, termed pioneers. These neurons have axons that lay down an early scaffold, which provides a substrate for many later-developing axons. Here we show that a similar type of cell exists in the embryonic vertebrate brain. Using light- and electron-microscopic techniques we have examined the formation of one of the earliest tracts in the zebrafish brain. We find that it is pioneered at a precise time by the growth cone of a single neuron present in a predictable location. These observations show a fundamental similarity in the establishment of axonal pathways in the central nervous systems of both invertebrates and vertebrates.