512 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 24 



must be prepared for effective counting (see Chaps. 10 to 12 on radiation 

 counters, and Chaps. 13, 17, and 18 on the standardization and preparation 

 of samples for measurement). 



The in vivo method is characterized by measurement of a radioisotope in 

 the intact system. This may be accomplished only where the emanations 

 of an isotope are sufficiently penetrating to permit the approximation of a 

 counter to the experimental subject and the detection of particles of nuclear 

 disintegration originating within the latter. Gamma-ray emitters and high- 

 energy beta-particle emitters can be studied by this method. Most measure- 

 ments carried out in this way are largely qualitative, although with a precise 

 understanding of the geometry of the counting arrangement and of the dis- 

 tribution of the isotope within the system, accurate quantitative measure- 

 ments are possible. 



The autoradiographic technique is also largely qualitative. It involves 

 the approximation of a biological sample to a photographic film sensitive to 

 the radiations of the isotope used (see Chap. 14). The resulting pattern on 

 the developed film illustrates the distribution and, to a certain degree, the 

 differential concentration of the isotope on the surface of the sample and, to a 

 greater or lesser extent, depending upon the thickness of the sample and the 

 energy of the radiated particles, the distribution and concentration in the 

 parts of the sample beneath its surface. The most satisfactory autoradio- 

 graphs are those made from very thin samples, e.g., sections of tissues or 

 organs. Techniques have recently been developed for the simultaneous 

 mounting of a tissue section and a sensitive gelatin layer on a microscope 

 slide [113,26]. The film may be developed after an appropriate interval 

 and the section stained; the result is a stained section superimposed on its 

 radioautograph (or vice versa) from which a precise picture of the isotope 

 distribution can be determined. 



24.4. Differential Behavior and Effects of Isotopes. So far there have 

 been few demonstrations that the different stable isotopic species of a given 

 element behave differently in living systems. However, in the case of 

 hydrogen, the differential behavior of deuterium oxide (heavy water) has 

 been demonstrated to have toxic effects on organisms from bacteria to mice 

 [D31]. Evidence also has been accumulated for the differential use of carbon 

 and oxygen isotopes by living systems [C90,O7]. 



Most of the radioactive isotopes in use are of sufficiently high molecular 

 weight so that no differential effect due to the difference between their masses 

 and those of their normally occurring stable analogues is to be expected. 

 However, tritium will most certainly prove an exception to this inasmuch as 

 the difference between its behavior and that of ordinary hydrogen will be 

 greater than that already shown between deuterium and protium. The 

 decay pattern of any given isotope is, of course, unaffected by its participation 



