Neurites extending on fixed and unfixed corpus callosum or on unfixed corpus callosum treated with polyornithine. A, Fluorescein-labelled neurons cultured for one week on unfixed corpus callosum (edges of the fiber tract are indicated by open arrows). The neurites extending from these neurons were mostly unfasciculated and generally extended in parallel with the longitudinal axis of the fiber tract. B, Neurons cultured for one week on fixed corpus callosum. These neurons were mostly clustered (white asterisks) and many of the neurites extending on the fixed fiber tract were fasciculated (white arrows). From one neuron cluster (white arrowhead), unfasciculated neurites radiated outward in all directions over the fixed fiber tract. C, D, Phase-contrast micrographs of the underlying anatomy of panels A and B, respectively. E, A fluorescein-labelled explant attached on the corpus callosum (cc; edges are indicated by open arrows) in an unfixed tissue section that had been mounted on an untreated culture dish. The neurite outgrowth halo (after 6 days in culture) was oriented in parallel with the underlying fiber tract. F, An explant attached on the corpus callosum in an unfixed tissue section that had been mounted on a polyornithine-treated culture dish. The neurite outgrowth halo (after 3 days in culture) extended radially with no parallel orientation. cc, corpus callosum; ctx, neocortex; cg, cingulum. Scale bars: A, B, C, D, 200 μm; E, F, 100 μm.