Neuronal growth cone collapse triggers lateral extensions along trailing axons

Axonal outgrowth is generally thought to be controlled by direct interactio n of the lead growth cone with guidance cues, and, in trailing axons, by fa sciculation with pioneer fibers. Responses of axons and growth cones were e xamined as cultured retinal ganglion cell (RCC) axons encountered repellent cues. Either contact with cells expressing ephrins or mechanical probing i ncreased the probability of lead growth cone retraction. Lateral extension of filopodia and lamellipodia hundreds of microns behind the lead growth co ne was correlated with its collapse. Transmission electron microscopy showe d that some of the lateral extensions originate from the pioneer axon, wher eas others represent growth cones of defasciculating trailing axons.