Dispersion of growing axons within the optic nerve of the embryonic monkey

To determine whether individual optic fibers grow along constant sets of neighboring fibers, a group of 160 axons and 25 axonal growth cones were traced through a set of 500 serial electron micrographs of an optic nerve taken from a 39-day-old monkey embryo (Macaca mulatta). In single transverse sections, growth cones contact an average of 7.9 fibers, whereas axons contact 5.3 other fibers. The particular set of fibers in contact with one another changed rapidly, and, on average, growth cones and axons lost half of their original neighbors over a distance of only 8-10 micron. Between the first and last sections of the series, 92% of all initial contacts were lost. Individual axons moved freely between fiber fascicles, and the distance separating initial neighbors increased progressively. Most remarkably, the sets of fibers touched by the tips and the shanks of growth cones had no common neighbors in 17 out of 25 cases. These results demonstrate that, in primates, fibers in the optic nerve do not retain a particular set of immediate neighbors during their outgrowth.