EFFECTS OF X-IRRADIATION UPON POSTNATAL GROWTH IN THE MOUSE 1 DONALD J. NASH -AND JOHN W. GOWEN Department of Genetics, Iowa State University, Ames, Relatively little attention has been directed towards the long-term effects of embryonic or fetal irradiation on the subsequent vigor and well being which the irradiated may attain in later life. However, since an individual's development, in a sense, unfolds from conception until death, it was felt desirable to investigate systematically effects of in utero irradiation upon postnatal development. The present study has emphasized effects on development after parturition in mice, as measured by such responses as growth from birth to maturity, lifetime fecundity. and total lifespan, of which the growth effects will be reported in this paper. The mammalian embryo is in a unique stage of development because of the great number of cells that are actively undergoing differentiation. Radiant energy absorbed during a period of development may act as an agent directing the organ-ism's development into new paths. The redirection may stimulate either nuclear changes which have permanent continuity in later cell generations or cytoplasmic influences which may possibly be more transient. Of the various agents which may make these changes, ionizing radiations are especially useful since their pene-trant actions have a general distribution throughout the entire organism. Patterns of sensitivity which are characteristic of the embryo may be revealed, therefore, by the selective response of the exposed structures. Numerous congenital malformations have been reported as a result of embryonic radiation. Most of the earlier works are difficult to interpret since careful control of the embryo's age at irradiation was not made and the physical factors of radiation often were not standardized. These changes involved many organs of the de-veloping individual. They indicate well defined critical periods during which the cells are susceptible to redirection of development. They further indicate that the susceptibilities change as development progresses, making some elements more re-sistant and causing other elements to enter more sensitive periods. In general for any one type sensitivity the periods appear to be quite restricted in the length of time over which they are active. There is furthermore a dose-dependence. Meas-urement of these effects has largely been confined to the observations made on the young prior to parturition or but a few days thereafter. Long-term effects have scarcely been considered. An additional major shortcoming of many of the earlier experiments is the fact that the animals used were either animals of unknown heterogeneous origin 1 Journal Paper J4151 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Projects 1180 and 1187. This work has received assistance from Contract AT (11-1) -107 from the Atomic Energy Commission. 2 Present address : Department of Botany and Plant Pathology, The Pennsylvania State University, University Park, Penna. 115
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