Fig. 5.

Localized induction of branching by afferents.A, Schematic illustration of our culture system, where neurons were grown along a border of afferents with one part of their dendritic tree touching the afferent net while the other part was growing on polylysine. B, Micrograph illustrating how axons extending from the explants (right and left lanes) formed sharp borders along a stripe of a nongrowth-permissive substrate (middle lane).C, Micrograph illustrating the presence of the afferent border even after removal of the nonadhesive substrate (right side). D, E, Example of a DiI-labeled neuron with phase contrast growing along a border of afferent axons for 11 d, after the nonadhesive substrate had been peeled off. Only dendrites in contact with the afferents developed a rich branching pattern, as can be seen in the fluorescence picture. Dendrites from the same neuron, but growing on polylysine, remain unbranched.F, Quantification of branching in border neurons. Basal dendrites of all cell types (n = 32) as well as apical dendrites of pyramidal neurons (n = 11) developed branches only on those dendrites in contact with the afferents. This suggests that the afferents promote branching by means of a local effect on the dendrites rather than on the cell globally. Error bars represent SEM. Scale bars: B, 500 μm;C–E, 20 μm.