900 
GENETIC AND BIOLOGICAL EFFECTS OF RADIATION 
cells, 1 ml of fetal calf serum and varying 
amounts of NCTC 109 such that all cultures con- 
tained 5 ml of growth medium in a total volume 
of 7 ml. The final cell cultures contained 7.4, 15, 
and 29 /iCi ooSr-^oY/ml. The incubation time, 
colchicine treatment, cell harvest and slide prep- 
aration were the same as described for the 
X-irradiation. 
RESULTS AND DISCUSSION 
Table I summarizes the data now available 
from the studies of in vitro irradiation. As ex- 
pected, the incidence of chromatid aberrations 
in both porcine and human cells was not signifi- 
cantly increased by X-irradiation. This is con- 
sistent with the general observation that cir- 
culating leukocytes from normal healthy 
individuals are in the interphase, Gi, phase of 
the cell cycle. Thus aberrations scored as chro- 
matid are presumed to have occurred spon- 
taneously, not as a result of radiation exposure 
prior to stimulation by PHA. To minimize sub- 
jective influences in interpreting unstained gaps 
and terminal chromatid deletions these two 
categories were combined. Pooling the chro- 
matid aberrations for the various exposures 
yielded spontaneous aberration rates of 0.018 
aberrations/cell for swine and 0.044 aberra- 
tions/cell for humans. The value for human 
chromosomes is somewhat higher than the value 
of 0.014 aberrations/cell obtained by Bender 
and Gooch.2 In a study on swine chromosomes, 
McFee et al.^ observed a much higher chro- 
matid aberration incidence (5% and higher) 
Table I. — Chromosomal Aberrations Occurring after 
In Vitro Irradiation 
No. of Cells CTD* + CD* + Rings + 
Scored CTG AL Dicentrics 
P** H** P H P H P H 
X-Ray 
Control 160 94 3 4 0 0 0 0 
50R _ 168 139 2 5 4 6 2 2 
100 R 171 186 2 10 5 10 4 8 
200 R 165 171 5 7 17 26 6 25 
CI0Sj._90Y 
Control - 133 3 1 0 
7.4 /iCi/ml ... 162 6 5 2 
15 /iCi/inl 169 40 11 1(1)*** 
• CTD, chromatid deletions, CTG, chromatid gaps. CD, chromosome 
deletions, AL, achromatic lesions on both arms. 
** P, porcine. H, human. 
*** The number in parenthesis refers to a quadricentric chromo- 
some. 
but failed to comment on possible causes of such 
a high value. 
To avoid uncertainties of interpretation, chro- 
mosome terminal deletions were combined with 
achromatic lesions, and the frequency of these 
is linearly related to dose. The average one-hit 
aberration coefficients calculated from the data 
are 0.4 X 10-^ cell-iR-i for pig and 0.7 X 10"^ 
cell-^R~^ for human cells. Comparable values 
of 1.1 X 10~^ cell-^R-^ are reported for swine ^ 
and 2.4 X 10-^ cell-iR-i for humans.^ 
Multiple break aberrations are detected by 
the presence of ring configurations or multi- 
centric chromosomes in the metaphase stage. 
Absence of this type aberration in both swine 
and human controls is consistent with their re- 
ported low frequency^ and the small numbers 
of cells observed so far in this study. The two- 
hit aberration coefficient for human cells was 
0.46 X 10-5 cell-iR-2. The apparent linearity 
of the swine data prevent calculation of a single 
value since proportionality with the square of 
the dose is anticipated. Our data give values 
ranging from 0.5 X 10"^ cell-^R-^ to 0.09 X 
10-5 cell-iR-2. Comparable values of 0.52 X 
10-5 cell-iR-2 for human ^ and 0.092 X 10"^ 
cell-^R-2 for swine'' have been obtained by 
others. 
While there are notable inconsistencies within 
the present data which may be resolved as more 
data are accumulated, it appears that, at com- 
parable radiation doses, aberrations occur with 
a lower frequency in swine than in human cells; 
At this time no comparison of swine with hui' 
man chromosome breakage by 90Sr-^°Y betai 
radiation is possible since exposures of human 
leukocytes has not progressed to the point of 
providing results. At the highest dose of 
9oSj._9oy, 29 /xCi/ml, extensive cell death oc- 
curred, yielding an insufficient number of meta- 
phases for study. It is interesting to note that 
chromatid aberrations increase with radiation 
dose because, in this case, the radiation is con- 
tinuous with S and G2 as well as Gi stages ir- 
radiated. The relatively high frequency of 
chromatid, relative to chromosome, aberrations 
is similar to the in vivo results of Brooks and 
McClellan* following injection of large doses 
of oogr-^oy into Chinese hamsters. 
As will be discussed later, dose rate is highly 
