CELL DIVISION, MORPHOLOGY, VIABILITY 765 



MEASUREMENT OF EFFECT 



In determining the relation of a dose of radiation to the biological 

 changes induced, it is essential that allowance be made for two factors 

 that characterize this kind of experiment: (1) latency in the appearance 

 of the effect and (2) the extent to which recovery may have occurred 

 between treatment and observation. Each warrants a more detailed 

 analysis. 



Biological effects do not become observable immediately on the cessa- 

 tion of treatment, even though the ionizations and excitations produced 

 by radiations occur within a fraction of a second after the impingement of 

 the photons. Different biological effects may become detectable in 

 minutes, hours, days, or even years after treatment. In part, this is due 

 to the time required for the series of physical and chemical changes that 

 must intervene between the photon absorption and the morphological 

 change. For example, in alteration of the form of the nucleolus by ultra- 

 violet the maximum observable change occurs at about 35 minutes after 

 treatment (Carlson and McMaster, 1951). In part the latency is due to 

 the series of biological changes that must run their course before the 

 appropriate stage for detecting the change is reached. Changes in rate 

 of mitosis can be determined only at the end of an interval of time during 

 which the rate is measured. Chromosomal aberrations are first detect- 

 able hours or days after treatment and then only at certain mitotic 

 stages and in certain kinds of tissues, while gene mutations may have to 

 pass through two or three generations of individuals, and so may not be 

 detected for months or years after they have been produced. Though 

 the present chapter deals only with what we term immediate effects, 

 these may not be observable until several hours after treatment. 



A striking characteristic of living substance is the capacity to repair 

 deleterious changes produced within it by external agents. The effects 

 dealt with in this chapter, with the exception of cell lethality and pos- 

 sibly the chromosome "stickiness" evident soon after treatment, are 

 subject to repair within the cell. 



The intermediaries by which ionizations within the cell lead to changes 

 in the rate of mitosis are not known. Probably the initial effect may be 

 viewed as either the destruction of something within the cell — perhaps an 

 enzyme or substrate that is necessary for mitotic progression — or the 

 production of a substance that exerts a toxic effect on mitosis. Recovery, 

 then, would result either from the replacement of the destroyed enzyme or 

 substrate or from the removal of the toxic substance. From the time 

 that treatment begins and the first effects are induced, therefore, the 

 recovery processes will begin and the cell will act to restore the original 

 conditions. At any given time between the start of irradiation and the 

 completion of recovery, the amount of radiation effect within the cell will 



