Fiber order of the normal and regenerated optic tract of the frog (Rana pipiens)

In the normal frog, axons from the peripheral retina arising at the temporal pole course superficially in the middle stream of the diencephalic optic tract. Axons from the nasal pole course in two streams running in the opposite margins of the tract, dorsonasal axons ventrally, ventronasal axons dorsally. Axons from the dorsal and ventral poles of the retina occupy the intervals between the aforementioned middle and marginal streams. Axons from more central regions of the retina tend to occupy deeper levels of the optic tract. The regenerated optic tract does not regain its normal organization, e.g., axons of peripheral nasal origin are spread out widely over the entire width of the tract. However, axons from the temporal pole of the retina do return approximately to their original location in the middle stream. The concentration of temporal axons in the middle stream of the optic tract after regeneration may now be understood in terms of the expression pattern of the ephrin-A class of receptor tyrosine kinase ligands in the cellular matrix of the optic tract. The ephrin-As, which have a repellent effect on growing temporal retinal axons, are concentrated in and along the margins of the diencephalic optic tract and essentially absent from its middle stream. It is proposed here that peripheral temporal axons may be forced into this middle region by their avoidance of the higher levels of ephrin-A expression in the tract margins. In contrast, the growth pattern of regenerating peripheral nasal axons would not be affected by the ephrin-A gradient in the optic tract.