Thalamic ablations and neocortical development: alterations in thalamic and callosal connectivity.

Corticofugal pathways (callosal, intracortical, and subcortical) have initial axon outgrowth to many areas where no adult connections will persist. Corticofugal projections also demonstrate considerable reorganization after early damage. At the level of gross projections from specific thalamic nuclei to cortical cytoarchitectonic areas, early thalamocortical projections appear to show greater specificity for their targets than do corticofugal projections, and their potential for reorganization after early damage is not known. In this article, we explore the nature of the reorganization shown by the thalamocortical system after early thalamic lesions, and contrast it with reorganization of the origin of contralateral visual callosal projections in the same animals. Hamster pups were given electrolytic lesions in the posterior thalamus on the day of birth, damaging principally either the ventrobasal (somatosensory) or the dorsal lateral geniculate (visual) nucleus. After 30 d of age, HRP was implanted in either the somatosensory or the visual cortex, matching the area of implant with the intended thalamic lesion. The thalamus was reconstructed to determine the remaining nuclei, and the distribution of retrogradely labeled cells was plotted. For animals with HRP implants in visual cortex, the location of callosally projecting cells from the contralateral cortex was charted. These animals were compared to a group of normal adult animals with HRP implants approximately matched for size and location. In seven of eight adult animals with neonatal thalamic lesions, the remaining thalamus did not reorganize to innervate the thalamically denervated cortex. In contrast, the callosal projections from the contralateral visual cortex showed a wider tangential origin in the experimental animals compared to the controls. This expanded callosal projection included cells from temporal cortex, a projection not seen in normal animals. Thus, thalamocortical and callosal projection systems differ in both the magnitude and the nature of their reorganization after early damage.