Fig. 8.

The in vitro results suggest the following model for interstitial branching by cortical neuronsin vivo. Schematics represent different stages in axon branching. A, Growth cone of an efferent cortical axon is advancing along a pathway toward its target, indicated by the circle. B, In response to target-derived signals, the primary growth cone pauses in the vicinity of the target for extended time periods and enlarges. C, After the primary growth cone has resumed forward advance, remnants of the reorganized growth cone appear as filopodial or lamellar activity along the axon shaft.D, A branch from these active membrane regions extends toward the target after various time delays. Typically, branches extend interstitially some distance behind the primary growth cone. However, when the primary growth cone and the branch extend simultaneously, branching appears to occur by bifurcation (data not shown).