The course of later generated axons in the developing optic nerve of the chick embryo. A morphometric electron...

The topographic position of growth cones (GCs) shows the course of ingrowing axons within the optic nerve and allows to draw conclusions with respect to the fiber order in this pathway. Therefore, the topographic distribution and frequency of GCs as well as the proximal and distal axon shaft segments were studied within cross-sections of the distal, middle, and prechiasmatic part of the nerve of 3-8-day-old embryos using electron microscopy. The ingrowth of GCs was not confined to a particular region. Initially, GCs were found near the ventral periphery. With increasing age, simultaneous ingrowth occurred within an area that expanded dorsally. In parallel, GCs also occurred in dorsal regions and eventually in the dorsal periphery. GCs intermingled everywhere with more mature axon profiles. However, youngest profiles predominated ventrally, oldest dorsally. Hence, maturity increased from ventral to dorsal. This indicated that the time of arrival of axons and the topographic position in the cross-section correlated significantly. It is concluded that axons are chronotopically organized, but in a probabilistic sense. The predominant ingrowth of axons in the ventral part may be associated largely with the first wave of neurogenesis of retinal ganglion cells. The ingrowth in dorsal regions of the cross section may be related to later generated axons that enter the nerve following older axons of the same retinal sector as well as axons of neighboring ganglion cells which continue to leave the mitotic cycle while the front of neurogenesis has spread into the periphery.