Evidence for an orderly arrangement of optic axons within the optic nerves of the major nonmammalian vertebrate classes.

The pathways of selected optic axons were traced in representative urodele, anuran, teleost, reptile, and avian species by filling the fibers with HRP or by tracing, at the light and electron microscopic (EM) level, the degeneration caused by focal retinal or optic nerve lesions. In all species it was shown that fibers retain retinotopic neighborhood relationships throughout their transit of the optic nerve. Additionally, in anurans, it was found that a subset of large diameter, myelinated fibers take up a random arrangement in the nerve. It is argued that retinotopic fiber organisation is a reflection of contact guidance of axons during fiber outgrowth in the embryo and that this organisation could account for the arrival of fibers in orderly arrays at central nuclei during normal embryonic development.