PATHOLOGIC EFFECTS 31 



women. Radiation administered locally to the testes in doses which would be sublethal if 

 administered to the whole body may cause considerable degrees of irreparable injury in the 

 sperm-producing tissues; and in persons who had only borderline fertility before exposure, 

 permanent infertility or subfertility may result. Limited experience with the Marshall 

 Islanders, the exposed Japanese, and certain accident cases suggest that substantial fractions 

 of the midlethal dose for man (around 400 to 600 r) did not have serious permanent effect 

 on fertility. However, gonadal doses are not known with certainty in these cases and the 

 numbers of such cases studied extensively for this purpose for a long period of time after 

 exposure have been few. 



Central Nervous System: The adult nervous system may be affected by ionizing radia- 

 tions in several ways. In the course of conventional cancer therapy, when parts of the nervous 

 system must be exposed, several thousand roentgens may permanently injure the blood vessels 

 of the brain or spinal cord, leading to ischemic damage. Many thousands of roentgens when 

 delivered rapidly may quickly destroy certain elements in the central nervous system or, in 

 other instances, so derange the function of vital centers as to cause death at once. Doses in 

 the hundreds of roentgens seem to have little measurable effect on adult nervous tis.sues. Re- 

 cent reports that subtle functions of the brain are disturbed by doses of a few roentgens still 

 await confirmation. 



Embryonic Development: Mammalian embryos are readily affected by low doses of 

 ionizing radiations. In laboratory mammals with certain genetic traits, as little as 20 r may 

 alter development. Doses of 100 to 300 r cause a predictable spectrum of malformations de- 

 pending on the stage of development of the embryo when exposed. These malformations can 

 be well understood in terms of classical experimental embryology. High doses are generally 

 lethal to embryos. Little is known about the effects of radiation on man during early develop- 

 ment except that malformation or death follows irradiation of embryos in a dose range com- 

 parable to that known to harm other mammals. Virtually nothing is known about the effects 

 on late fetuses, and scarcely more regarding the effects of exposure of infants and children on 

 subsequent development. 



Studies of possible correlation of the frequency of congenital abnormalities with levels 

 of natural radiation have been suggested. While one survey has been interpreted to show in- 

 crease of congenital malformations in areas relatively high in natural radioactivity, more de- 

 tailed and better controlled studies will be necessary before it can be concluded that these low 

 doses of radiation are capable of producing human anomalies. 



It must also be remembered that there are various other agents causing malformations 

 during development, of which German measles is a well-known example. 



Factors Influencing Sensitivity: Very young or very old animals have increased sensi- 

 tivity to lethal effects, and there is some experimental evidence to indicate that in some species 

 the 50% acute lethal dose (LD.n) may decrea.se progressively with increasing age during 

 aduh life, while in other species a decrease in LD.o may not be observable until later life. 

 Information about the influence of genetic constitution on radiation effects is meager at 

 present, most of the research on the problem having been done on genetically homogeneous 

 or "inbred" mouse strains. In the mouse data it seems that there is a nonspecific component of 

 life shortening that is comparatively independent of genetic makeup, and in addition a specific 

 component, reflected particularly in susceptibility to leukemia, which varies from strain to 

 strain. The contribution of these strain-specific diseases to the total mortality is greater in the 

 mouse than in other species on which information is available, but in spite of this the range 



