W. A. THOMAS, R. A. FLORENTIN, S. C. NAM, J. M. REINER AND K. T. LEE 
867 
If one were dealing with a delimited cell pop- 
ulation, such tables could be used to predict ab- 
j solute numbers of labeled cells that would be 
present in each grain class after a selected num- 
ber of divisions. In in vivo situations, however, 
one is seldom dealing with a delimited popula- 
tion. It is meaningless to ask which batch of 
cells in the aorta of experimental animal B cor- 
responds to the batch counted in baseline ani- 
mal A; the most one can say is that both 
batches were selected, as nearly as possible, 
from corresponding regions of the two organs. 
To compare predictions such as can be made 
from generation tables and observations made 
at some time after divisions have taken place, it 
is necessary to convert both predictions and ob- 
servations to percentages as shown in this table. 
Also, when cells in the original population di- 
vide, some of them will have their grain numbers 
diluted by division so that they fall below the 
counting threshold. When we express our theo- 
retical predictions of the grain count distribu- 
tion after division, we correct the total number 
of cells for the predicted number that "disap- 
pear" below the threshold ; and the percentages 
are obtained by dividing by this corrected total. 
In an actual experiment, the predicted per- 
centage with 4 grains after one division may be 
compared with the observed percentage with 4 
grains. If the values are similar, we could say 
that the results with the 4-grain category are 
consistent with the hypothesis that one division 
has taken place in the observed population. If 
the other grain classes also are consistent with 
predictions for one division, we would consider 
that we had strong evidence for the occurrence 
of just one division in the labeled population. If, 
on the other hand, values predicted for one divi- 
sion and values observed are not similar, one 
would try other whole or fractional numbers of 
divisions until the best fit is achieved. 
The results of the 30-day experiment are 
shown in Table VI. Analyses of the data were 
made in all grain categories. The results were 
similar for all categories, so that what is pre- 
sented in this table is a simplified analysis of 
two grain classes, 14-23 and > 24. The ob- 
served values of the stock swine were similar to 
those of the cholesterol swine, indicating no dif- 
ference in generation time between the two 
groups up to 30 days after the administration 
of ^H-thymidine. It is also apparent that, for 
both groups, the best fit of the observed values 
among the predicted values was for one divi- 
sion. The results of the 2-day, 7-day, and 15-day 
experiments were also consistent with the 
occurrence of only one division. This is the divi- 
sion that took place shortly after the pulse-la- 
beling period. From the labeled-mitoses experi- 
ment, we know that the initial division of the 
labeled cells was completed around 37 hours 
after the administration of ^H-thymidine. 
Therefore, the generation time for both stock 
and cholesterol diet-fed swine is longer than 28 
days. 
Since most of the post-pulse labeled cells in 
the swine carotid artery of the previous experi- 
ment had less than 9 grains, we examined the 
4-8 grain category of both stock and cholesterol 
groups. The results are presented in Tables VII 
and VIII. The values for the cholesterol group 
are significantly higher than those of the stock 
group in most periods of observation. This indi- 
cates that a greater number of cells is synthe- 
sizing DNA and being labeled in the post-pulse 
period among the cholesterol-fed swine than 
among the stock-fed swine. This suggests that 
more SMC enter S (the DNA synthesis part of 
Table VI. — Brain count data of all grain categories from the 30-day experiment* 
% predicted 
after % % predicted 
Stock (6) Cholesterol (6) divide once after all 
Grain at 30 days at 30 days only and divide once 
c^as3 (% observed) (% observed) % do not divide only 
14-23 53.9»» 56.4** 40.0 55.3 
(av. of 49.0, 53.4, (av. of 42.6, 61.5, 
35.6, 49.5, 59.0, 73.0, 65.6, 60.7, 
77.1) 44.8) 
^24 46.1 43.6 60.0 44.7 
* Comparison of predicted percentage of cells in 14- to 23-grain class among those left with > 14 grains, after one half of cells have divided 
once and after all cells have divided once with observed percentage in two groups. 
** Difference between stock and cholesterol not significant by Mann-Whitney U test. 
