Abnormal axonal growth in the dorsal lateral geniculate nucleus of the cat.

Retino-geniculate axons in the cat were induced to grow abnormally by cutting one optic nerve in kittens. Surviving optic tract axons that had grown into the denervated regions were then filled in the adults with horseradish peroxidase to reveal the terminal arbors of individual axons. Two types of abnormal axonal growth are described--translaminar growth and monocular segment growth. Translaminar growth is the most common and occurs between laminae in the binocular part to the nucleus. Axons giving rise to translaminar growth do not branch as they pass through the denervated regions of the nucleus. Instead, the abnormal branches originate from portions of the terminal arbor within the normal target lamina. These axons look like normal retino-geniculate axons in terms of their branching patterns, cytological features, and patterns of synaptic contacts except that parts of their terminal arbors have expanded to innervate inappropriate laminae. The distribution of translaminar branches overlaps the distribution of a restricted group of surviving large neurons that have not undergone denervation atrophy. Monocular segment growth invades the lateral pole of the nucleus directly from the optic tract. These branches arise from axons passing through or near the denervated region and appear to represent the formation of new terminal arbors. The synaptic swellings arising from these branches have cytological features like the synaptic swellings arising from translaminar branches and they form similar patterns of synaptic contacts. However, monocular segment branches degenerate more rapidly when damaged and they are not associated with surviving large neurons.