Disturbance of neuronal migration in mouse cerebral cortex by low-dose gamma-radiation

We show that neuronal migration is disturbed by low-dose gamma-radiation of 0.24 Gy in the developing cerebral cortex of mice and suggest that neuronal progenitors in the S phase of mitosis are more sensitive than those in the G1/G0 phase. To pulse-label the fetal cells pregnant Slc:ICR mice were injected with bromodeoxyuridine (BrdU) at 10.00 pm on day 16 of pregnancy or at 9.30 am on day 17. The mice then were exposed to 0.24 Gy gamma- or sham-irradiation at 10.00 am on day 17 of pregnancy. At the time of exposure cells labeled on day 16 were regarded as having completed mitosis, and those labeled on day 17 as being in the S phase. Cell death in the fetal ventricular zone after exposure was negligible. Dams were allowed to give birth and rear their litters. Brain samples obtained from offspring on the 2nd day after birth and at 6 weeks of age were used for the immunohistochemical examination of BrdU-labeled cells. Labeled cells remaining in the ventricular zone were significantly more numerous in irradiated animals than in sham-exposed ones on the 2nd day after birth, in mice treated prenatally with BrdU on day 17 of pregnancy; whereas, mice treated with BrdU on day 16 showed no statistically significant difference. At 6 weeks of age, in both groups most of the labeled cells were present in layers II-III of the SmI cerebral cortex. But, in the irradiated animals the labeled cells in these layers were significantly fewer and those in layers IV-VI more numerous than in the sham-exposed mice in both group treated with BrdU on day 16 or 17.