1108 RADIATION BIOLOGY 



after injection of this isotope. It is noteworthy that erythropoiesis does 

 occur (except after Zr 93 -Cb 93 ) in the red pulp, which is the site of lodgment 

 of many isotopes. With practically all radioactive isotopes, the mega- 

 karyocytes, after a preliminary decrease, increase tremendously in num- 

 ber after weeks and months and often show great nuclear abnormalities 

 (Murray, 1948a). 



BONE 



Bone is a specialized connective tissue consisting of bone cells (osteo- 

 cytes) embedded in a matrix of collagenous fibers and amorphous cement- 

 ing substance which is heavily infiltrated with small crystals of hydroxy- 

 apatite, calcium phosphate, carbonate, and small amounts of citrate. 

 On the surface of a developing spicule of bone are cells called osteo- 

 blasts, which are believed to make the bone, and some of them become 

 bone cells. In areas where bone is being destroyed, there are large multi- 

 nucleated cells called osteoclasts. Their presumed role is the destruc- 

 tion of bone. 



In normal animals growth of the bone in length depends on (1) con- 

 tinued multiplication of the cartilage cells in the epiphyseal plate and, 

 near the bone marrow, calcification of the intercellular substance between 

 them; and (2) continued invasion of the calcified cartilage matrix by 

 vascular connective tissue from the bone marrow and deposition of bone 

 in the space thus made available, often on remnants of cartilage which 

 are not destroyed. With cessation of bone growth, multiplication of the 

 cartilage cells and invasion by blood vessels stop, and bone is deposited 

 on the diaphyseal side of the cartilage plate. This plate disappears in 

 man on reaching adulthood, and continuity develops between the marrow 

 of metaphysis and epiphysis. The epiphyseal cartilage participates in 

 the growth of bones for a long time in rats, but in mice the period of 

 growth lasts for only two or three months. 



Bones grow in width by deposition of bone directly on the outer surface 

 of the bony cylinder (periosteal growth). In addition, with the recon- 

 struction of a bone which takes place as it grows, new bone may be laid 

 down inside the bone collar (endosteal growth). This is especially true 

 toward the growing end of a bone. 



Although many believe mature bone is relatively resistant, there is 

 little doubt that it can be severely damaged by irradiation. Thus a 

 peculiar eburnation and devitalization of bone (Friedman, 1942) and 

 massive necrosis after dosages which did not damage the skin (Regaud, 

 1922) have been described. 



There is general agreement that the tissues responsible for growth of 

 bone can be severely damaged by irradiation. Stunting of the growth of 

 the long bones has been observed by many investigators. The rat, 

 because of its rapid and prolonged period of growth, is a sensitive indi- 

 cator of these radiation effects. After 600 r of X rays (LD 50 30-day 



