Radiosensitivity and differentiation of ganglion cells within fetal mouse retinal explants in vitro

Fetal mouse retinas were explanted at 13-14 days of gestation, and exposed to gamma radiation in vitro. Not all regions of the retina were equally susceptible to radiation-induced necrosis; when exposed to 5000 rads soon after explanation, each explant had a single small radioresistant nubbin of apparently intact tissue, located near the optic nerve-head. This region of radioresistant tissue was larger when the dose of radiation was reduced and when the explants were exposed at later times in vitro, indicating the existence of a gradient of radioresistance across retinal explants which spread outward through at least the first week in vitro, the period examined. Based upon the extensive in situ literature which has correlated the emergence of radioresistance with the differentiation of retinal neurons, we conclude that the in situ central-to-peripheral sequence of cellular differentiation continues in vitro within our retinal explants. Whereas the ganglion cell axonal outgrowth from control retinas grown in isolation on collagen substrates underwent a gradual disintegration over 3 weeks in vitro, the sparse axonal outgrowth from explants exposed to 5000 rads disintegrated abruptly at 5-7 days in vitro. This did not appear to be due to direct damage from radiation, but instead reflected the fact that axons in irradiated cultures arose from central retinal regions only, while many axons in control cultures emerged from later-differentiating peripheral regions. We suggest that disintegration of individual axons in the outgrowth may occur rapidly and in a central-to-peripheral sequence. These findings should be useful in designing assays for trophic factors which may prevent ganglion cell axon disintegration in this in vitro model system.