Early axon patterns of the spinal cord: experiments with a computer

In the vertebrate central nervous system, most axons appear in one of two elemental patterns--sheets or bundles. Many developmental mechanisms are involved in the formation of the elemental axon patterns, and these mechanisms often act simultaneously. The major axon-patterning mechanisms include differential adhesivity, internal growth constraints on axons, and initial orientation of axonal outgrowth. To evaluate the effects of these mechanisms on the formation of axon patterns, a computer was used to model axonal growth. Experiments with the computer model suggest that axon sheets are produced by the cooperative action of more than one mechanism. Furthermore, in the appropriate combination, these mechanisms produce orderly axon sheets even on patternless substrates. On the other hand, to transform the sheets into axon bundles, the substrate must be patterned.