LONG-TERM SURVIVAL OF CENTRALLY PROJECTING AXONS IN THE OPTIC-NERVE OF THE FROG FOLLOWING DESTRUCTION OF THE RETINA

A significant number of unmyelinated axons and their synaptic endings in the frog, R. pipiens, retained a normal morphology long after separation from their cell bodies. At the end of various survival periods following unilateral removal of the retina, horseradish peroxidase (HRP) was administered to the optic nerve stump by a fiber-filling method. In frogs maintained at 20.degree. C, unmyelinated optic nerve axons conducted HRP from the site of application in the orbit to layers A, C and E of the contralateral optic tectum, even though their retinas were removed up to 69 days earlier. Such fiber-filling was absent beyond 19 days in other frogs surviving at 35.degree. C. No labeled fibers were continuous with any intracerebral neurons. The HRP was always localized intraaxonally, and the marked axons and terminals were ultrastructurally normal. Counts of surviving axons from EM of the optic nerves showed that, at 20.degree. C, > 1/2 of the normal complement of unmyelinated axons disappeared in the first 10 days. All the myelinated axons degenerated during the first 6 wk survival. Approximately 55,000 normal-appearing unmyelinated axons (12% of the unmyelinated fiber population) persisted in the optic nerve at 10 wk following removal of the retina. The survival rate was lower at 35.degree. C. In other frogs, 1 eye was injected with 3H-leucine to initiate axonal transport into the retinal ganglion cell axons. That eye was removed 48 h later. Autoradiographic analysis of brain sections of frogs surviving an additional 31-61 days at 20.degree. C showed strong labeling of the optic tract and layers, A, C and E of the contralateral optic tectum. The absence of displaced ganglion cells that might exist within the optic nerve was verified by other observations. The potential shown by frog optic axons for long-term survival in the absence of the cell-body may express a general property of vertebrate (and invertebrate) axons, rather than a special property of the frog optic nerve.