348 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



estimates of bone growth are possible from measurement of the auto- 

 radiogram. Thus, for example, the distal end of the femur of pig ^ on a 

 controlled diet grew 2.1 mm in 10 days. That of animal B, on a low 

 dietary intake, grew 7.9 mm in a 45-day experimental period, whereas 

 the corresponding part of the bone of pig C, on a high dietary intake, 

 grew 10.0 mm in the 45-day period. The growth of other regions of the 

 bone can be readily measured in the same manner. 



This type of information is also of practical importance from the health- 

 physics standpoint in studying the removal of various bone seekers from 

 bone (41). In general, it can be stated that the removal of isotopes such 

 as radiostrontium and radiocalcium primarily depends upon the region in 

 which they were originally deposited and upon the growth of the bone. 

 Referring to the various regions as denoted in Fig. 7-7, numerous studies 

 of bone autoradiograms have led to the following conclusions: (a) Activ- 

 ity deposited directly below the epiphyseal plate will not be removed until 

 by bone growth it becomes located in the trabecular region where resorp- 

 tion is taking place and removal is relatively rapid. (6) Subperiosteal 

 deposition will not be removed until by bone growth the activity becomes 

 located in the endosteal portion of the shaft, (c) Activity deposited in 

 the region of endochondral growth is apparently not significantly removed 

 by the growth or resorption process, (d) Resorption is taking place con- 

 tinually in the trabecular regions, and activity deposited here is relatively 

 rapidly removed. 



Cellular Biosynthesis. Howard and Pelc (42) have described experi- 

 ments that demonstrate the time of synthesis of desoxyribonucleic acid 

 (DNA) in relation to the mitotic cycle. This procedure, in contrast to 

 Geiger counting methods, allowed study of specific cells from among 

 many. The method was based on resolution adequate to distinguish 

 activity in single cells or cell parts, and on removal of all radioactivity 

 from the cell except that which was in the compound of interest. Roots 

 of Viciafaha seedlings were grown in water containing about 0.2 /zc/ml of 

 P^2 and 16 mg/liter of carrier phosphate. After treatment of 2 to 48 hr 

 the roots were fixed in alcohol-acetic acid and put on slides for the prep- 

 aration of autoradiograms by the stripping-film method. Experiment 

 showed that DNA was probably the only P'^--containing compound left 

 by this treatment. Further evidence was the fact that no autoradiogram 

 was obtained when the section was treated with a solution of desoxyri- 

 bonuclease. It was shown that in the meristem of Vicia faha the syn- 

 thesis of DNA occurs during interphase and that about 6 hr elapses 

 between the end of synthesis and the beginning of visible prophase. 

 Similar studies were carried out using S'^^-labeled sodium sulfate at 

 1 /ic/ml. Since the pretreatment of the section was expected to remove 

 all inorganic and acid-soluble sulfur, it was reasoned that the autoradio- 



