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

The topographic position of growth cones (GCs) shows the course of ingrowin g 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 segment s were studied within cross-sections of the distal, middle, and prechiasmat ic 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 wer e found near the ventral periphery. With increasing age, simultaneous ingro wth occurred within an area that expanded dorsally. In parallel, GCs also o ccurred in dorsal regions and eventually in the dorsal periphery. GCs inter mingled everywhere with more mature axon profiles. However, youngest profil es 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 associa ted 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 ret inal sector as well as axons of neighboring ganglion cells which continue t o leave the mitotic cycle while the front of neurogenesis has spread into t he periphery. (C) 2000 Elsevier Science B.V. All rights reserved.