Changes in fiber order in the optic nerve and tract of rat embryos

Abstract

In order to define the extent to which retinotopic order in the optic pathways may contribute to fiber segregation at the chiasm or to the formation of central maps, the arrangement of fibers in the optic nerve and tract of rat embryos, on embryonic days 16.5 and 18.5, has been studied by placing a small granule of DiI in one of the four quadrants of the retina and tracing the filled fibers through transverse sections of the retinofugal pathway with confocal microscopy. There is a distinct quadrant-specific order in the optic stalk immediately behind the eye, with fibers from the ventral nasal, dorsal nasal, dorsal temporal, and ventral temporal retina arranged sequentially across the rostrocaudal axis of the cross section of the stalk. However, this distinct order is not maintained very far. There is a gradual increase in the degree of overlap between fibers from the different quadrants as the fibers pass towards the chiasm. The dorsal groups of fibers intermingle extensively along almost the entire length of the stalk, but the fibers from ventral sectors remain separate until they reach the prechiasmatic region, where the ventral temporal and the ventral nasal fibers spread throughout the rostrocaudal extent of the stalk and the chiasm. The initial quadrant-specific order is completely lost at the chiasm. However, beyond the optic chiasm, the fibers are reorganized into another distinct order. In the optic tract, there is a segregation of dorsal from ventral fibers, but the nasal and temporal groups remain intermingled. The results of this study indicate that the earliest fibers in the developing optic tract are arranged according to topographical rules that differ from those obtaining behind the eye. Since all topographical order is lost between these two levels, there must be an active sorting mechanism in the region where the chiasm joins the tract. Possibly this mechanism is related to the development of the dorsoventral axis of the topographic maps in the central visual targets.

Evidence for an Instructive Role of Retinal Activity in Retinotopic Map Refinement in the Superior Colliculus of the Mouse

“…This suggests that mechanisms for the formation of NT and DV retinotopy are separable, with mapping of the NT axis particularly dependent on activity. This is consistent with findings in many species (Springer and Mednick, 1983;Rager et al, 1988;Reese and Baker, 1993;Chan and Guillery, 1994), including the mouse , that axons from the DV axis of the retina are already mapped in the optic tract before they enter their target, but axons from the NT axis are completely mixed. Interestingly, a similar asymmetry was observed in the LGN of ␤2 mutants, in which the primary effect of the ␤2 mutation on retinotopy was a blurring of fine scale topography along Figure 7.…”

Expression of chondroitin sulfate proteoglycans in the chiasm of mouse embryos

“…These changes in fiber order segregate retinal axons according to their positions along the nasal-temporal and dorsal-ventral axes of the retina and also reposition the fibers according to their ages in different parts of the chiasm (Godement et al, 1990;Guillery, 1991;Chan and Guillery, 1994;Colello and Coleman, 1997;Colello and Guillery, 1998;. One control for such axon patterning is the interaction of retinal axons with guidance molecules at the chiasm.…”

Slit Proteins Regulate Distinct Aspects of Retinal Ganglion Cell Axon Guidance within Dorsal and Ventral Retina

“…Furthermore, several cues that are uniformly expressed throughout the retina are required specifically for guidance within either dorsal or ventral regions (Ott et al, 1998;Birgbauer et al, 2000). After exiting the eye, axons from dorsal and ventral retina are initially segregated but become intermingled at the chiasm before reorganizing and becoming segregated again within the optic tract (Chan and Guillery, 1994;Chan and Chung, 1999). Whether Slits are involved in regulating this aspect of RGC axon guidance has not been established (Thompson et al, 2006).…”