340 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



4. Under safelight conditions the emulsion, of appropriate size, is 

 mechanically peeled from the glass plate and floated on water at 20°C, 

 with the side that was formerly in contact with the glass now facing 

 upward; that is, the emulsion side is face downward. After about 2 to 

 3 min, during which time the emulsion has swelled, the slide carrying the 

 tissue section is slipped under the emulsion, so that the emulsion covers 

 the section and drapes around the slide to make a tight fit. The prepara- 

 tion is dried and set aside for exposure. It has been emphasized that the 

 stripping should be done carefully in a humid atmosphere or underwater 

 to avoid static charges, which fog the film (8, 33). Lotz and Johnston 

 (34) have discussed the advantages in using a strip of emulsion long 

 enough to wrap completely around the breadth of the slide, enabling the 

 ends to overlap at least 3^^ in. 



5. After exposure the preparation is photographically processed and 

 can be stained if desired. 



Other Methods. Boyd and Wilhams (33) and MacDonald et al. (35) 

 have described various techniques for the stripping-film method. It is 

 pointed out that commercial stripping films are available with a 5- to 10-^ 

 emulsion supported by a 5- to 10-/i base, and that the stripping base can 

 be used either to protect the emulsion while staining or to protect the 

 tissue while developing. 



APPLICATIONS 



It is not feasible to review the numerous individual applications that 

 have been made with autoradiograms in biological studies. Rather, an 

 attempt will be made to indicate and illustrate the main fields of useful- 

 ness, with the expectation that the investigator will consider the principles 

 involved in terms of his specific problems. The applications are classified 

 as below in order to emphasize functional aspects. It is recognized that 

 the terminology is arbitrary and that any given study often provides 

 information in several of the categories. 



Chemical. Consideration is given to a few chemical applications that 

 are particularly important m biological studies. As discussed in Chap. 2, 

 many radioisotopes under certain conditions tend to form aggregates or 

 so-called radiocolloids. The existence of these aggregates in a solution 

 to be administered to a biological system may have a profound effect on 

 the results obtained. The presence of radiocolloids can be readily dem- 

 onstrated by simple autoradiographs, as discussed by Yagoda (4). 

 Similarly it is possible to study the appearance and disappearance of such 

 aggregates in the blood. Figure 7-5, taken from Gallimore (36), shows 

 the autoradiographic appearance of rat l)lood at 2 to 10 min and 2 to 

 24 hr, respectively, after intravenous injection of polonium chloride. 

 This demonstrates that the polonium immediately after injection existed 



