@misc{oai:repo.qst.go.jp:00060656,
 author = {Hayata, Isamu and Yoshida, Mitsuaki and Kanda, Reiko and Bing, Wang and Minamihisamatsu, Masako and Akiyma, Miho and Vares, Guillaume and 早田 勇 and 吉田 光明 and 神田 玲子 and 王 冰 and 南久松 眞子 and 穐山 美穂 and Ｇｕｉｌｌａｕｍｅ Ｖａｒｅｓ},
 month = {Aug},
 note = {In this presentation I will first introduce the advantage of using human lymphocytes for the study of biological effects of low dose (rate) radiation, and the results of cytogenetical studies in high background radiation area (HBRA) we performed in the previous nuclear cross-over study. Then I will talk about our plan for the new nuclear cross-over study where we will investigate the biological effects of low dose of dose rate radiation in mouse.
 When human peripheral lymphocytes are exposed to radiation or chemicals in vivo, their resultant chromosome aberrations are different. The former induces chromosome type aberrations involving both chromatids of chromosomes and the latter induces chromatid type aberrations involving only one of the two chromatids. At the metaphase stage in the first cell division cycle after exposure to those clastogens, these two kinds of aberrations show morphologically different features. But they become indistinguishable in the succeeding cell divisions, because the chromatid type rearrangement is copied on both chromatids to become chromosome type. If a cell has a dicentric (chromosome type aberration) accompanied by a fragment to be lost during cell division, the cell is considered in the first cell division cycle. Since most chemical mutagens induce chromatid type aberrations a dicentric accompanied by a fragment is an excellent marker of radiation exposures. 
 Dicentrics and translocations are induced in the human peripheral lymphocytes in about equal frequency by radiation(1). The former is unstable to be eliminated through cell division, while the latter is stable to be accumulated in the human body. A radiation induced translocation is not distinguished from a chemically induced translocation, and therefore translocations are the indicator of total effect of all kinds of clastogenic factors. Induction yield of a dicentric per cSv with X or gamma rays is reported to be 0.3 per 1000 cells in the human lymphocytes (2). If the average dose of exposure per year in human body is assumed to be 0.24 cSv (3), 4.68 dicentrics and 4.68 translocations per 1000 cells would be induced by radiation in the body in 65 years. 
 In the high background radiation area (HBRA) in the Guang-dong province of China where the level of natural radiation is three to five times higher than that in the control area (CA) (4), increase of the frequencies of dicentrics was found in the peripheral lymphocytes of the residents. But the increase of translocations in HBRA was within the range of individual variation in CA (5-7). The frequency of translocations was much higher than the expected value induced by the accumulated dose of radiation both in HBRA and CA. Smoking contributed to the induction of the chromosome aberrations more than the elevated natural radiation in those areas (8).
 The targets of our research in the new nuclear cross-over study are as follows
A)The minimum radiation dose that can be detected by chromosome analysis in mouse.
B)Development of the methods to detect chromosome aberrations to be induced by radiation in comparison to those by all kinds of clastogens in mouse.
C)Adaptive response in the irradiated mouse fetus to be examined by chromosome analysis. 
 In mouse, turnover or cell division cycle of lymphocytes is shorter than that in human. Lymphocyte culture is not easy, and induction rates of dicentrics and translocations by radiation have not yet been extensively studied in mouse. The shape of mouse chromosomes is all acrocentrics, while the majority of human chromosomes are meta-or submetacentrics. It was reported that the morphology of chromosome influences the yield of dicentrics (1). Therefore, the ratio of dicentrics in comparison to translocations to be induced by radiation in mouse would not be the same as that of human. The life span of mouse is about 3 years, while that of human is about 80 years. Therefore, the background frequency of chromosome aberrations in the adult due to natural radiation would be much smaller in mouse than in human.
 Thus mouse chromosome aberrations could be more sensitive biological indicator of man-made radiation exposures than human chromosome aberrations. We investigate chromosomes of the mice which are irradiated chronically at low dose or dose rate with or without a chemical mutagen in the new nuclear cross-over study., International Workshop on Biological Responses to Low Dose Radiation},
 title = {Cytogenetical Studies on Biological Effects of Low Dose (Rate) Radiation},
 year = {2004}
}