134 



BARBARA E. HOLMES 



The figures are taken from Dr. Looney's paper. He gave 3(K)0 r to the 

 regenerating Uver lobe (a dose which had been shown to reduce 32p 

 uptake into the DNA to about half the normal amount) the rest of the 

 body being sliielded as well as possible. Tritiated thymidine was given 

 immediately after irradiation and the animal killed 3 hr later. Figure 

 1 is a histogram showing grain count distribution. The irradiated cells 



HISTOGRAM OF LOG OF GRAIN COUNT DISTRIBUTION OF 

 NUCLEI (Figure l) WITH SUPERIMPOSED NORMAL CURVE. 



f//^i IRRADIATED 



CONTROL 



0-80 l-OO 120 I 40 I-60 I-80 200 2 20 

 LOG GRAIN COUNTS PER NUCLEUS 



2-40 



Fig. 1. — Histogram of the logaritliin of grain count distiil)utioii of nuc 



curve superimposed. 



with tlie normal 



have a smaller grain count than those from an unirradiated but otlier- 

 wise exactly comparable animal but there is no decrease in the number 

 of cells in synthesis. At this time after hepatectomy no mitosis has yet 

 occurred in either cell population. The reduction in grain counts in the 

 irradiated cells might indicate a lower rate of DNA synthesis. In order 

 to prove that this was the correct interpretation, measurements were 

 made of the content of DNA in individual cells by a light absorption 

 method using Feulgen-stained cells. Figure 2 shows the results. Un- 

 labelled, that is to say non-synthesising, cells indicated the 2n, 4n and 

 Sn amounts of DNA. Most hepatocytes in these animals contained a 4n 

 amount which increased to an 8w amount before cell division. Cells 

 which were synthesising DNA at 17 hr after hepatectoirry become 



