Ontogeny of the retina and optic nerve in Xenopus laevis. II. Ontogeny of the optic fiber pattern in the retina.

The ontogeny of optic fiber patterns within the Xenopus laevis retina has been studied in silver-stained serial sections and whole mounts. Ganglion cell axons ("pioneers") appear much earlier than previously recognized, at Nieuwkoop-Faber ('56) Stage 28. Fibers fasciculate into bundles as they arise in the retina and reach the chiasma by NF Stage 32. Because the retina grows in rings at its margins, ganglion cell axons differentiate at the periphery and join the nearest fiber bundles (fascicles) to reach the optical disc. A radial fascicle pattern develops by Stage X (Grant et al., '80) within the optic fiber layer of the retina and acts as a template of pathways that lead all later appearing fibers out of the retina. Since the retina grows asymmetrically along its dorso-ventral axis, the fascicle pattern develops asymmetrically, with dorsal quadrant fascicles forming an arcuate pattern in contrast to shorter, thicker ventral quadrant fascicles. The growth pattern of the retina and the "principle" of fiber following suggest that "positional information" is acquired by ganglion cells according to a system of polar coordinates (MacDonald, '77). A radial distance "r" from the center is determined temporarily by ganglion cell birthday and an angular value "theta" is given by the angular position of the fascicle followed by its axons to the optical disc. Wedges of ganglion cells send their axons into a common fascicle which retains its integrity as it leaves the retina at the optic disc. This pattern of optic fiber development suggests a model for retino-topic ordering of the optic fiber projection in Xenopus, with the fascicle acting as the "pattern-forming unit of connectivity."