INTRODUCTION TO PART I 



The past 10 years and especially the last few have seen a great increase 

 in interest in what ionizing radiation may do to the nervous system, both 

 in respect to the malforming effects induced during early development and 

 the functional and structural changes occurring in later stages. Formerly, 

 it was said — and sometimes still is — that the embryo brain is very radio- 

 sensitive and the adult brain will stand almost anything. Unqualified, these 

 phrases are almost meaningless today. We try to specify what dose of radia- 

 tion produces a given effect, because one type of cell in the adult or 

 embryonic nervous system may respond quite differently from another cell. 

 It makes a difference whether one gives 200 r of conventional 250 kv x-rays 

 to a fetal or neonatal rat in divided doses or in a single dose, or whether 

 one gives a single dose of 700 r. E\en the difference in effects between 

 200 r and 300 r on the fetal and neonatal cortex, cerebellum, and retina 

 can sometimes be remarkable. 



The term radiosensitivity now is used more carefully, because it has 

 meant quite different things to different workers. We can no longer say, 

 "this stage of embryonic life is the most radiosensitive" or "that enzyme 

 system is the most radiosensitive" without qualification. To a geneticist, 

 radiosensitivity means that a chromosome can be changed easily; to a 

 pathologist, it has often meant that a tumor cell is easily killed; and to an 

 endocrinologist, it may suggest that a cells hormone production can be 

 easily stopped. Many laboratories, including our own, are interested in dis- 

 co\ering subtle radiation changes in neurons. For example, we are attempt- 

 ing to demonstrate changes in nucleic acids in adult rat cortical neurons 

 by ultraviolet microscopy following exposure to 200 or 400 r of conven- 

 tional 250-kv x-rays. I note this because it reflects the growing attitude that 

 we ought to be looking for obscure and perhaps totally unexpected changes 

 in the nervous system and other tissues following radiation. Certainly, 

 among the most attractive areas for research are those which relate to the 

 roles that DNA and RNA have in both the development and, later, the 

 function of neurons. 



A diverse array of approaches to problems of developmental radio- 

 biology is represented in these papers on the effects of ionizing on the 

 developing nervous system, including effects of successive small doses of 

 radiation on neuron differentiation, chemical and histochemical changes 

 in irradiated nerve cells of all ages, studies on nucleic acids of ner\e cells 



