Development of corticospinal tract fibers and their plasticity II. Neonatal unilateral cortical damage and subsequent development of the corticospinal tract in mice

In this study, the right cerebral cortices of mice on postnatal day 0 (P0) were cryocoagulated with dry ice. Subsequent development of the corticospinal tract (CST) was studied morphologically and quantitatively, and was compared with that in age-matched controls. When the pyramidal tract was traced anterogradely by injecting HRP into the sensorimotor area of the left cerebral cortex of adult operated mice, the right CST originating from the healthy left hemisphere showed remarkable hypertrophy. The number of axons in the CST at the C4–C6 level became maximum on P14 in the control mice and rapidly decreased thereafter. In the operated mice, the axonal number in the right CST also was maximal on P14 and then rapidly decreased. However, the decrease in axonal number after P21 was less in the operated mice than in the controls. Moreover, the number of axons showed a slight increase after P56. These results indicate that the physiological elimination of the parent axons and their collaterals is much lower in the operated mice than in the controls, and that the increase in axon collaterals from parent axons in the hypertrophic right CST persists a long time in the operated mice.