1110 RADIATION BIOLOGY 



level), the continuity of the epiphyseal cartilage of femur and tibia with 

 the spongiosa is frequently interrupted, resulting in a temporary cessation 

 of bone growth in length (Heller, 1948a). Preceded by excessive hyper- 

 trophy of cartilage cells in the zone of ossification at 3 days, the normal 

 interdigitation of cartilage and metaphyseal spongy bone is gradually 

 lost. By 9 days the two are completely separated. In the lamellae of 

 the spongiosa there are some dead bone cells, but none in the dense 

 cortical bone. Osteoblasts disappear after irradiation but gradually 

 reappear after some weeks. Only a slight increase in the number of 

 osteoclasts accompanies the resorption of bone. In some instances this 

 severance of cartilage and spongy bone may persist for a month, but in 

 others a new primary spongiosa is laid down against the cartilage plate 

 as early as 21 days or possibly sooner and growth in length is resumed. 

 The old, disconnected spongiosa is gradually resorbed. Recovery of the 

 growth process is complete at 70 days, although the bones are perma- 

 nently somewhat stunted. 



A single dose of 400 r of X rays merely reduces the number of osteo- 

 blasts at 3 and 7 days while osteoclasts are more numerous in parts of the 

 metaphysis. 



In young chicks (three weeks of age) similar changes occur. Doses of 

 800 or 1000 r total-body X irradiation cause severance of cartilage from 

 spongy bone, effectively stopping growth of the bone. After 1000 r there 

 is a steady progression of damage from 1 to 16 hours, with the disappear- 

 ance of osteoblasts, increase of osteoclasts around the spongy bone, and a 

 decrease in the erosion of cartilage. From 7 hours to 4 days occasional 

 dead osteocytes and a number of empty lacunae are present in the spongy 

 bone. By 4 days separation of cartilage from spongy bone is extensive, 

 some of the latter having been resorbed, and the bone stops growing. 

 After 800 r, the destructive process is the same as after 1000 r, only not 

 quite so rapid. However, by 5 days severance is extensive, osteoblasts 

 are absent, and bone growth stops. After 400 r there is a progressive 

 reduction of osteoblasts around the metaphyseal spicules of bone from 2 

 to 18 hours, but at this dosage no severance or interruption of bone growth 

 occurs. Recovery is rapid, with osteoblastic and osteoclastic activity 

 resumed by 30 hours. No effect on chick bones is noted after 100 r. 



In comparison with the effects of external irradiation on bone at LD 50 

 levels, even small fractions of the LD 50 of the bone-seeking isotopes cause 

 disproportionately greater damage, owing to their continued activity at 

 the points of deposition (Heller, 1948a). Among these isotopes there is a 

 remarkable similarity of reaction, especially between radium and plu- 

 tonium, and the /3 emitters Sr 89 and Y 91 . However, the weak (3 particles 

 of C 14 , which deposit heavily in bone after injection of NaHC 14 3 , do 

 not interfere with the growth of bone, even when 2 juc/g is injected (W. 

 Bloom, 1949). 



