Radiation-induced diffuse brain injury in the neonatal rat model--radiation-induced apoptosis of oligodendrocytes.

The mechanism of radiation-induced diffuse brain injury was investigated using a model of delayed myelination in the irradiated neonatal rat brain in which the number of oligodendrocytes decreases without associated necrosis of the cerebral white matter. Immunohistochemical analysis using antibody against the large myelin-associated glycoprotein, a specific marker of oligodendrocytes at an early stage of development, showed that the number of the oligodendrocytes associated with myelination decreased in the irradiated hemisphere 1 day after irradiation and remained low until 5 days after irradiation. In situ terminal deoxynucleotidyl transferase-mediated nick end-labeling assay revealed that apoptosis mainly occurred in the cerebral white matter of the irradiated hemisphere. Three hours after irradiation, apoptotic cells were found in the subcortical white matter and the periventricular white matter. Six hours after irradiation, apoptotic cells were found in the internal capsule, and the numbers of apoptotic cells in the periventricular white matter and subcortical white matter increased. One day after irradiation, the number of apoptotic cells in the periventricular white matter decreased. Three days after irradiation, apoptotic cells were not observed in the cerebral white matter. These results suggest that the oligodendrocytes associated with myelination may be damaged via radiation-induced apoptosis, and depletion of the oligodendrocytes may cause delay of myelination.