Fig. 2.

Branches develop from cortical axons over an extended time period. A, Representative images selected from a series of 1320 time-lapse images acquired at 3 min intervals over a period of 3 d. This neuron developed minor processes first (5 hr), followed by elongation of the axon (20–35 hr) and extension of interstitial branches (35–53 hr). The axon is labeled with anasterisk. One axon branch (at right) was already present when imaging began, but most of the branches extended as a cluster from the region indicated by the arrows. Times shown indicate hours after plating. B, Time course of elongation of minor processes (squares), the axon (circles), and axon branches (triangles), excluding the initial branch of the neuron shown in A. For minor processes and axon branches, total neurite length, not that of a single neurite, is graphed. Note that before extension of branches elongation of the axon has halted. No data were collected during the gaps, but branches did elongate during these periods. C, Frequency histogram summarizing the time course of branching on 42 axons. Bars show numbers of new branches (n = 58) >30 μm that extended during each of the time periods indicated. Most of the branches extended on the second and third day in culture. Only branches with full recorded histories, from the beginning of branch initiation, were included.