Axon end-bulb swellings and rapid retrograde degeneration after retinal lesions in young animals.

After making a lesion of the retina, anterograde (Wallerian) and retrograde degeneration of optic axons occurred more rapidly in newborn mice than in adults. Axon sprouting occurred only in adult mice, perhaps because retrograde degeneration may have been too rapid and severe for sprouting to occur in newborns. Retinal lesions in mice of any age produced end-bulb swellings initially on both sides of the lesion. In all animals, dense packing of lysosomes and other organelles occurred in end-bulbs on the side of Wallerian degeneration but did not occur in end-bulbs on the retrograde side, where accumulation of smooth endoplasmic reticulum was the most characteristic change. Retrograde end-bulbs appeared much like growth cones, which sprouted in adults, but degenerated in younger animals. Continuing daily enlargement of end-bulb swellings was noted on the Wallerian side of lesions in adults, but not in newborns. Such enlargement is believed to have resulted from retrograde axoplasmic transport and suggests that such transport may be greater in adults than in newborns.