EFFECTS OF X-IRRADIATION OF MICE EXPOSED IN UTERO DURING DIFFERENT STAGES OF EMBRYOLOGICAL DEVELOPMENT ON DURATION OF MATURE LIFE ' lutXALD J. NASH-AND JOHN \Y. <;o\YKX s In an carlic-r paper (Nash and Go\ven, 1962) we have shown that postnatal growth, as measured by weight changes to 75 days, following x-ray irradiation in ntero, was dependent on both the levels of irradiation and the embryological ages at the time of irradiation. The embryological ages in order of increasing sensitivity were 64, 17.1, 1-H and 101 days. Growth was measured for the periods from mating to birth, and to 12, 26, 40, 60 and 75 days postnatal ages. Body weight response was found to be noticeably dependent upon the age at which observations were recorded. Treatments that produced significantly lowered body weights did not usually show maximum effects until 40 days after birth. Growth changes, expressed as continuing reductions of the body weights with days post-parturition, were most obvious at 75 days of age. The reductions were greatest for the 320 r-treated mice, the lowering of the growth rates being dose-dependent. The data on which these observations were made have one common property ; the mice were alive at the time when the observations were initiated. This results in the population under observation being biased toward greater viability. This limitation is intrinsic at whatever level of development the data on the irradiation effects are taken. To, in part, reduce this difficulty in evaluating the irradiation effects on the animal's subsequent well-being, several other characteristics, such as anatomical changes in young born dead, lifetime fecundity and fertility and various parts of the lifespan. have been utilized for measuring as yet unrecognized physio-logical changes in those exposed to irradiation. This paper deals with those changes, which may be immediate or long delayed, which affect lifespan after the beginning of the reproductive period. Lifespan is a unique instrument in that it integrates the effects of external as well as internal agents influencing life processes. It covers both the short-term and long-term effects as well as the constitutional characteristics of those exposed. It has more variation, both innate and accidental, than other measures so differences between agents believed to affect life become harder to demonstrate. Yet it is in this characteristic most of us are interested. Irradiation of the mammalian embryo exposes rapidly dividing cells during the intense process of organ formation and differentiation. It is a time when 1 Journal Paper of the Iowa Agricultural and Home Economics Experiment Station, Ame--, Iowa. Project Nos. 1180 and 1187. This work has received assistance from Contract AT( 11-1) -107 from the Atomic Energy Commission, and Grants RH328 and Al-06239 National Institutes of Health. -Department of Zoology, Rutgers University, New Brunswick, New Jersey. :; Department of Radiology and Radiation Biology, Colorado State University, Fort Collins Colorado. 425
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