1112 RADIATION BIOLOGY 



is quite different. For example, five and seven months after the injec- 

 tion of plutonium or radium (0.03 juc/g), atypical bone often replaces the 

 marrow in the metaphysis and even much of the shaft (M. A. Bloom and 

 W. Bloom, 1949). 



In a growing bone, isotope-containing bone will be in part resorbed with 

 the remodeling and in part incorporated in new bone. The isotopes 

 enter adult bone by incompletely understood processes of exchange with 

 atoms in the bone-salt crystal complexes. Moreover, in the case of 

 yttrium and plutonium, at least some of these elements become firmly 

 adherent to the organic part of the intercellular substance of bone (Copp, 

 Axelrod, and Hamilton, 1946; Copp, Greenberg, and Hamilton, 1946). 



CARTILAGE 



This tissue is distributed in the respiratory system, joints, and grow- 

 ing bones. In the last, a definite portion of the epiphyseal cartilage is in 

 continuous growth, and a thin zone within it is just as continuously 

 undergoing provisional calcification and shortly thereafter dissolution and 

 replacement by newly formed bone. As might be expected, cartilage in 

 these several positions and different states of activity shows great varia- 

 tions in susceptibility to damage by irradiation. In growing animals it is 

 Difficult to assess accurately just how much of the change resulting from 

 irradiation is due to failure of replacement of cartilage by ingrowth of 

 bone. In nongrowing animals, on the other hand, it is probable that all 

 the changes in cartilage are direct effects of irradiation. The response of 

 cartilage may vary from rather marked resistance in adult animals to a 

 fairly high degree of sensitivity during the period of rapid growth with 

 resulting retardation of skeletal growth. On the basis of investigations 

 on chicks (Perthes, 1903), on young rats (Segale, 1920), and on young 

 rabbits (Bisgard and Hunt, 1936; Brooks and Hillstrom, 1933), it appears 

 that localized exposure of an epiphysis of tibia or femur to 1500 r or less 

 of X rays leads to an arrest of elongation of the bone owing to damage to 

 the epiphyseal cartilage (Hinkel, 1942). Degenerative changes in certain 

 cartilage cells, particularly in the zone of proliferation, occur as early as 

 2 days after irradiation, and at 6 days many of these cells are hypertrophic 

 and vacuolated, with pyknotic nuclei. The columns of cells and the 

 intercellular bridges lose their regular arrangement, the epiphysis becomes 

 thinner, and, after two weeks, the disturbance in the growth mechanism is 

 manifest in both cartilage and new bone of the metaphysis. By six 

 months the epiphyseal cartilage has disappeared ; this is much earlier than 

 it disappeared on the nonirradiated side. There are indications that 

 growing children are much more sensitive to radiation than these animals 

 (Gates, 1943). With total-body as well as with localized irradiation, 

 abnormally swollen cartilage cells have been noted as the first sign of 



