AUTORADIOGRAPHY 



355 



100 M" using an ocular micrometer calibrated to 4 fx'. Grain (counts were 

 made of the emulsion over the muscularis, the lumen of the gut, and the 

 nuclear and supranuclear zones of selected epitheHal cells. Table 7-3 

 presents typical data. It is clear that the grain density over both the 

 nuclear and supranuclear regions was higher than over the lumen or 

 muscle; the counts over the latter were little, if any, higher than the 

 emulsion background. Comparison of the autoradiograms and specially 

 treated sections indicated that at least some of the activity in the supra- 

 nuclear zone was probably incorporated in the Golgi apparatus. 



Table 7-3. Graixn Counts over 100-/1^ Areas ok IIimthklial Cells from 



Rat Give.n P^^ 



[From Perry Max Johnston, The Localization of P^- in the Supranuclear Zone of the 

 Columnar Absorbing Cell of the White Rat, ./. MorphoL, 95: 77-93 (1954).] 



INTERPRETATION 



In recapitulation, a listing follows of possible sources of error in auto- 

 radiograms, all of which should be kept in mind when one translates the 

 black specks of the photographic image into meaningful information: 



1. Removal or relocation of the radioactive atoms by biological or 

 physical processes during the time between sampling and formation of the 

 photographic image 



2. Extraneous sources of image production, such as chemically active 

 substances in the specimen; pressure on the emulsion; radioactivity in the 

 films, chemicals, or glass used in processing; and stray light or ionizing 

 radiation 



3. Fading of the latent image or desensitization of the emulsion by the 

 specimen 



4. Nonuniform development 



5. Scratches in the film, deposition of debris 



6. Effects of staining solution on the emulsion 



7. Movement of sample on film during exposure 



An example of artifacts commonly encountered is presented in Fig. 

 7-20, which represents a stripping-film autoradiogram of cartilage of a 

 9-day chick embryo that received Ca'' at the start of hatching. The 



