What are the forces residing at the presumptive chiasm of embryonic mice that control the directionality (i.e., side specificity) of the optic axons? In an attempt to answer this question, the overall trajectories of individual fascicles of early growing axons and the various environments that they encounter along their pathway have been charted from the eye through the nerve and into the base of the diencephalon. Serial sections and reconstructive computer graphic techniques were used for the analysis.
The early optic axons (embryonic (E) day 13.5) arrive at the chiasm in a stereotyped topographic arrangement. However, the fiber array at the primitive chiasm is not retinotopically organized nor is it maintained with the same level of spatial precision as it is at the disc. Thus, the annular, inverted retinotopic contingent of “pioneering” axons that exists in the primitive nerve becomes reorganized at the chiasm into a crescent-shaped configuration, with fascicles from ventrotemporal and ventronasal retina at either side of the crescent and with fascicles from dorsal retina interposed. Because of their gross locations in the crescent, particular clusters of fibers, each largely originating from different retinal sectors, but “contaminated” with fibers from other regions, come in contact with different types of nonneuronal structures at the chiasm. One, a dense, knotlike glial formation that lies along the margin of the diencephalic-telencephalic junction, directs all adjacent (ventronasal) fibers contralaterally. The other, a discrete pathway of lengthy marginal glial processes, separated by an anastomotic system of large extracellular spaces, guides all nearby fibers from ventrotemporal retina ipsilaterally. The results suggest that fiber topography as well as local environmental factors may play important roles in guiding axons at the chiasm.