Fractionated irradiation-induced altered spatio-temporal cell distribution in the rat forebrain

Ionizing radiation as one of the strongest cytogenetic factors can induce significant injury to the adult brain. In the present study, adult male Wistar rats were exposed to whole-body irradiation with fractionated doses of gamma rays (a total dose of 3 Gy). Seven, 14 and 21 days after irradiation the cell types located in the neurogenic anterior subventricular zone (SVZa) were labeled using immunohistochemistry for SVZa-derived young neurons and astrocytes. Cell counting was performed in four anatomical parts along the pathway known as the rostral migratory stream (RMS) represented by the SVZa, vertical arm, elbow and horizontal arm of the RMS. A considerable increase was seen in the number of neuroblasts in the SVZa, vertical arm and elbow on day 7 after irradiation. Until days 14 and 21 there was a marked decline in the density of young neurons, mostly in the horizontal arm of the RMS. In contrast, the number of astrocytes gradually increased in the caudal parts of the RMS until day 14 after irradiation. Strong enhancement was replaced by a steep decline within the RMS up to 21 days after treatment. Our results showed that the radiation response of proliferating cells originating from the SVZa may play a contributory role in the development of more adverse late radiation-induced effects.