Consequences of pre-natal radiation exposure for post-natal development

A review of revised observations on Japanese bomb survivors suggests that 10-18 weeks of pregnancy is the period of greatest sensitivity for foetal brain damage leading to severe mental retardation. Severe food deficiencies suggest a cause for the apparently high frequency of severe mental retardation in the unexposed control population and may also have contributed to the dose-dependent increase in those irradiated in utero. The author concludes that there is no confirmed evidence to suggest that the pre-implantation stage of mammalian development is unusually radiosensitive. In the human, the succeeding period of major organogenesis seems to be less sensitive and important than the following 10-18 week period of pregnancy. It is suggested that malformation (teratogenesis) should be distinguished from maldevelopment. Malformations are the result of failure of embryonic organization and ionizing radiation is not an efficient teratogen in this sense. Maldevelopment after exposure to radiation is the consequence of cell depletion of sufficient degree randomly distributed throughout an irradiated tissue. It is concluded that dose thresholds for maldevelopments are to be expected after irradiation both in pre-implantation and post-implantation stages, and that somatic mutation has a possible role as a mechanism without threshold for development damage by pre-natal irradiation, but not likely to be of practical significance. (U.K.)