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The present investigations on the long-lived rats previously irradiated with 700 rads of X-rays were made on the bone marrow cells because they constitute a sensitive tissue (Core and Mole 1966) critical to survival of irradiated animals. The animals surviving after a high sublethal irradiation seem to be healthy after recovering through a period of time following irradiation. Nevertheless, development of a number of residual damages and a faster advancement of degenerative changes and neoplasias in irradiated animals leading to a shortening of their lif espan (United Nations Scientific Committee on Effects of Atomic Radiation 1972) prove that these organisms are not quite recovered. Recent experiments made by Covelli and Metalli (1973) have indicated that ageing mice survived after 700 rads of whole-body X-irradiation were deficient in their haemopoietic system. The long-term deficiency according to these authors could not be explained only by a reduced number of pluripotential stem cells in irradiated survivors. As has been suggested by Gong et al. (1969) "residual injuries" lagging behind the sublethal irradiations may be rather the consequence of permanently altered functions of the survived stem cells. Nowadays it is known, that chromosome aberrations, the most obvious residual damages produced by irradiation, represent a great deal of genetic injuries and that primary lesions may undergo further changes from their original radiation damages. This has led to extensive research on the induction of chromosome aberrations in mammalian somatic tissues. Numerous reports have confirmed that various chromosome anomalies may persist for many years in somatic cells of humans X-rayed for diagnostlc reasons (Bloom and Tjio 1964), in individuals which were therapeutically (Court Brown et al. 1965) or accidentally (Goh 1975) exposed to radiation and also in persons subjected to occupational exposure (Ishihara and Kumatori 1969). The longlasting effect of radiation have been also shown upon animal chromosomes in Chinese hamster (McKay and Shaw 1974), swine (Mailhes and McFee 1974), monkey (Kosichenko 1973) and recently in rats (Kohno and Ishihara 1976). Some of the results have indicated that aberrations observed in circulating leucocytes were identical to those found in bone marrow cells. Moreover, the existence of a selection process of persisting abnormal cells in vivo and further evolution of these cells to clones were shown.

Late effects of X-irradiation in rats. 1. Lasting chromosome abnormalities in bone marrow cells.