Inductive mechanism of bystander effects by radiation and its biological significance

The bystander effects through which cells near to radiation-exposed cells are also affected, are reviewed mainly around author's recent investigations. There are two major streams for studies on the effects. One is represented in such experiments as microbeam exposure to targeted cells in the presence of definitive unexposed cells using high linear energy transfer (LET) radiations like α-ray from Pu or He ion beam. Those studies have given evidence that the effects are surely induced by ROS and NO radicals and by signals through the gap junction between exposed and co-existing unexposed cells. The other is in such experiments as identifying the bystander effect factors using low LET ionizing radiations like γ-ray. The studies have been indicative of the presence of long-life bystander effect factor(s). The author has recently shown that, using He ion beam, only chromatid breaks and acentric fragments, thus the slight damages, are observable in the bystander cell chromatin. More clear evaluations of biological effects occurring in directly exposed and unexposed cells, and of interaction between those cells are concluded important. (S.I.)