Fig. 6.

Levels of filopodial and lamellipodial activity remaining on the axon shaft after growth cone reorganization correlate with axon branching. A, Examples from four different neurons, showing different levels of activity along the axon shaft ranging from 0 (no visible activity), 1 (transient filopodial or lamellar activity), 2 (small, lamellar expansions that consolidate into varicosities) to 3 (large, more symmetrical lamellar expansions). Each series of images shows the same region of the axon over time, beginning in each case with the primary growth cone. B, Bar graphs showing percentage of axon regions with activity levels of 0–3 (indicated at bottom right) that either branched or remained unbranched. Note that most of the axon regions with no activity remain unbranched, whereas most axon regions with high levels of activity develop branches. C, Bar graph showing average velocity of the growth cone that gave rise to different levels of axon activity (0–3). Slower rates of growth cone advance correlate with higher levels of axon activity. D, Bar graph showing average levels of activity in unbranched and branched regions of axon shafts. Asterisks indicate p< 0.01 in a two-tailed t test. E, Bar graph showing average number of branches extending from axon regions with different levels of activity. More branches extended from axon regions with higher activity. In C–E,numbers above the bars indicate 70 μm axon regions analyzed. Error bars are SEs in each of the graphs.