244 



B. Y. GRAYEVSKY, et (ll. 



tissues, other things l^eing equal. On the other hand, the higher the 

 mitotic activity in irradiated tissues the more intensive the process of 

 restoration. However, the main factor hmiting regeneration abiUty 

 after treatment is the quantity of cells with lethal chromosome re- 

 arrangements, since the non-viable offspring of these cells cannot 

 provide for tissue repair despite their intensive multiplication. 



Doses of 700 to 1000 r of X-rays seem to bring about the formation of 

 lethal chromosome rearrangements in an overwhelming majority of 

 cells of different tissues. For example (see Fig. 5) 90 to 95 per cent of 



OJ o 

 en ^ 



<L) o 



100 

 80 

 60 

 40 

 20 



r'' 



/: 



400 800 



1,500 2,000 

 Dose of X-rays (r) 



3.000 



Fig. 5. — Dose -dependence of tlie pei'centage of cells with anaphase bridges and acentric 

 fragments in the eijitheliiiin of mouse cornea. 



the cells of mouse cornea were found to have anajihase bridges and 

 acentric fragments after exposure to 700 to 1000 r. The mitotic index 

 reached a rather high level after a period of complete inhibition of 

 division even though higher doses of radiation were given Fig. 6 

 (Shapiro, 1960). 



3. The repair of radiation injury at the cellular level may be a result 

 of restitution or substitution of damaged structures and functions. 

 The recovery of the ability of a cell to divide may be an example. 

 Relatively low doses of radiation induce a temporary loss of ability to 

 undergo mitosis followed by the return of this process to normal. 

 Only partial i*ecovery of mitotic ability occurs after irradiation with 

 high doses. Without a special observation on a single cell it is difficult 

 to be sure whether this lowering of the mitotic index is caused by the 

 loss of ability to divide in a portion of cells or by a prolongation of 

 interkinesis. 



