Ipsilaterally projecting retinal ganglion cells in Xenopus laevis: an HRP study

The routes of ipsilaterally projecting retinal ganglion cell axons in the visual pathway of young postmetamorphic Xenopus laevis were studied by anterograde and retrograde transport of horseradish peroxidase (HRP). In the retina, most cells heavily labeled from injections in ipsilateral thalamus are large multipolar ganglion cells. They are found exclusively in the posterior half of the retina, and their axons occupy a central position in the optic nerve head. Immediately behind the eye, axons of ipsilaterally projecting axons leave the core of the nerve and regroup around the circumference of the nerve. The nerve increases in diameter in the region where the fibers reorganize, and pigmented processes are seen in this region of the nerve. At the point where the optic nerve enters the brain case through the optic foramen, the fibers undergo a second reorganization which results in a laminar arrangement of ipsilaterally projecting axons at the ventral margin of the intracranial portion of the nerve. As soon as the nerve touches the brain, uncrossed axons begin to turn toward the ipsilateral side rather than proceeding further towards the midline of the chiasm. These uncrossed axons keep their internal topographical order at least at the beginning of the marginal tract. All ipsilaterally projecting axons run at the rostral edge of the marginal tract at the lateral wall of the brain until they reach their terminal fields in the thalamic visual nuclei.