Development of a crossed corticotectal pathway following cerebral hemispherectomy in cats: a quantitative study of the projecting neurons.

A hypothetical mechanism for the partial sparing of visual function in the contralateral visual field following cerebral hemispherectomy early in life is the formation of a new corticotectal pathway arising from the remaining primary visual cortex (areas 17 and 18) that projects to the contralateral superior colliculus. To test this hypothesis, the left superior colliculus of intact adult and neonatal (5-15 days old) cats and of adult cats with a left cerebral hemispherectomy sustained neonatally (7-9 days old) or in adulthood, was injected with WGA-HRP and the brains were processed for combined TMB/DAB histochemistry. The primary visual cortex was examined, labelled neurons were counted and the cross sectional area of their somata was measured. The left primary visual cortex of intact adult animals exhibited a mean of 959.68 labelled cells +/- 406.5 (S.E.), with a mean soma size of 366.7 microns2 +/- 131.2. For the neonatal intact cats, there was a mean of 75.31 +/- 21.08 cells within the left primary visual cortex which exhibited a mean soma size of 249.56 microns2 +/- 68.18. The peak cell size distribution for both intact groups was similar at 300 microns2. Virtually no labelled neurons were detected in the right primary visual cortex of intact animals (neonatal or adult). For neonatal-hemispherectomized cats, the remaining right primary visual cortex exhibited a mean cell count of 351.09 +/- 126.3 cells, with a mean soma size of 436.1 microns2 +/- 131.5, and a peak cell size distribution of 400 microns2. Finally, for adult-hemispherectomized animals, the contralateral primary visual cortex exhibited 68.27 +/- 20.13 neurons having a mean soma size of 486.6 microns2 +/- 143.2 with a peak cell size distribution of 500 microns2. These results indicate that reorganization of the corticotectal pathway occurs in both adult- and neonatal-hemispherectomized cats but is more pronounced in neonatal-lesioned animals. In addition, the cells of origin of this reorganized pathway tended to be larger, perhaps in response to a greater axonal arborization.