928 RADIATION BIOLOGY 



For a given cell, the nucleus is a more sensitive indicator of damage than 

 the cytoplasm, and the cell in mitosis is usually more susceptible to 

 injury than the cell at rest. But even in mitosis there are differences, 

 for cells in prophase and metaphase are the most sensitive to radiation 

 injury (Sparrow, 1951). It is known that the absorption of radiation 

 depends upon the atomic constitution of the absorbing medium. While 

 nuclear sensitivity may perhaps be attributed to the differential absorp- 

 tion of radiation by the nucleus and cytoplasm, tissues with nearly the 

 same cross section can vary greatly in their susceptibility. It is evident, 

 therefore, that other intrinsic factors are involved. Although it has 

 been demonstrated by fractional irradiation that ions produced in the 

 cell nucleus are more effective biologically than those produced elsewhere 

 (Zirkle, 1932; Henshaw, 1938), nuclear injury in certain cells may be a 

 result of toxic factors originating in the cytoplasm (Duryee, 1949). A 

 normal cell injected with cytoplasm from an irradiated cell has been 

 shown to exhibit typical radiation effects. Thus, the special sensitivity 

 of the nucleus to radiation may, under certain conditions, be more 

 apparent than real, at least in terms of the initiating mechanism. 



The sensitivity of the cell during division may reside in some facet 

 of its instability, since mitosis is characterized by a number of physico- 

 chemical changes, in, for example, chromosomal mass and surface, 

 viscosity, permeability, conductivity, and energy requirements. Poly- 

 merization and depolymerization of nucleic acids, as well as changes in 

 the relative amounts of nuclear and cytoplasmic nucleic acids, are thought 

 to occur during the mitotic cycle, and such changes may affect sensitivity. 

 Recent work with pepsin-albumin films reveals that form, even on the 

 molecular scale, can greatly influence the radiation response (Mazia and 

 Blumenthal, 1948). The physical and chemical factors that contribute 

 to nuclear sensitivity are numerous and only partially understood. They 

 are discussed in some detail in the excellent review by Sparrow (1951). 



Recovery phenomena have been evoked to explain the difference in 

 response of slowly and rapidly dividing cells. It is believed that the 

 slowly dividing cell has a greater chance to recover since the death of a 

 cell frequently occurs at mitosis from structural alterations incurred 

 sometime before (Lasnitzki, 1943a). However, with a and neutron 

 irradiation, or with high dosages of X rays, degenerated cells appear in 

 appreciable numbers before resumption of the mitotic process (Lasnitzki, 

 1943b; Spear and Tansley, 1944; Tansley et al., 1948). Although there 

 is ample evidence that radiation interferes with the synthesis of desoxy- 

 ribonucleic acid (DNA), which could explain the postirradiation mitotic 

 inhibition, the disturbance in rapidly growing and in adult tissues does 

 not differ greatly (Mitchell, 1942; Hevesy, 1945, 1949; Holmes, 1949; 

 Kelly and Jones, 1950). Unfortunately, there are few, if any, data 

 relating the recovery of nucleic acid formation to the mitotic activity of 



