GLIAL DOMAINS AND AXONAL REORDERING IN THE CHIASMATIC REGION OF THE DEVELOPING FERRET

This study has examined the developing glial architecture of the optic pathway and has related this to the changing organization of the cons tituent axons. Immunocytochemistry was used to reveal the distribution of glial profiles, and DiI was used to label either radial glial prof iles or optic axons. Electron microscopy was used to determine the dis tribution of glial profiles, axons, growth cones, and wrists at differ ent locations along the pathway. Three different glial boundaries were defined: Two of these are revealed as changes in the distribution of vimentin-immunoreactive profiles occurring in the prechiasmatic optic nerve and at the threshold of the optic tract, respectively, and one b y the presence of glial fibrillary acidic protein (GFAP)-immunoreactiv e profiles at the chiasmatic midline. The latter, midline boundary may be related to the segregation of nasal from temporal optic axons. The boundary at the threshold of the optic tract coincides with the segre gation of dorsal from ventral optic axons that emerges at this locatio n in the pathway. The segregation of old from young optic axons is sho wn to occur only gradually along the pathway. Glial profiles are most frequent in the deeper parts of the tract, coursing parallel to the op tic axons and orthogonal to their usual radial axis. These are suggest ed to arise from later-growing radial glial fibers that are diverted t o grow amongst the older optic axons. Those glial profiles may subsequ ently impede axonal invasion, thus creating the chronotopic reordering by forcing the later-arriving axons to accumulate superficially. (C) 1994 Wiley-Liss, Inc.