332 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



SPECIFIC TECHNIQUES 



There are four commonly used procedures which differ primarily in the 

 method of contact between tissue section and emulsion. These methods 

 have been reviewed by Heller (7), Fitzgerald et al. (10), Leblond and 

 Gross (6), and Bourne (8). A brief statement of advantages, disadvan- 

 tages, and representative procedural details is presented for each. The 

 original literature should be consulted for application to specific problems, 

 since individual workers develop modifications and refinements that lead 

 to improvement in results. Also it is not feasible to include the numerous 

 details, many of which can be appreciated only as a result of actual expe- 

 rience. Illustrative autoradiograms prepared by the various methods 

 will be found in the section on Applications. 



Simple Apposition. Principle. The specimen to be studied is placed 

 in contact with the photographic emulsion and kept in contact by pres- 

 sure, and at the end of the exposure period the specimen is removed, and 

 the film developed. 



Advantages. The method is rapid and simple, and pretreatment of the 

 sample is minimal, so that radioisotope losses are avoided. Since poor 

 resolution is inevitable, it is possible to make advantageous use of a sen- 

 sitive film to decrease the exposure time and/or the amount of radio- 

 activity required in the sample. The autoradiogram can be used for 

 densitometric measurement, since there will be no interference from the 

 specimen. The sections can be stained after preparation of the auto- 

 radiogram, and there is no interference from the emulsion. 



Disadvantages. Poor contact is responsible for loss of resolution, and 

 cellular localization is usually not possible. It is difficult to superimpose 

 accurately the object and autoradiogram. 



Recommended Usage. This method is satisfactory for gross autoradio- 

 grams, especially for samples in which the radioisotope localizations are 

 widely separated. It has been particularly useful in studies with plant 

 material, bones, frozen tissue, and paper chromatograms. 



GROSS SOFT TISSUES. Soft tissues may be frozen, sectioned with a 

 chilled blade or fine electric saw, and then applied to the photographic 

 emulsion in a frozen state (5). 



BONE. It is not feasible to use a decalcification procedure for studies 

 of most elements that deposit in the crystal or organic matrix of bone. 

 Bone and teeth sections may be cut with various types of saws or may be 

 ground to a smooth surface (21a to 24). Gross et al. (5) have described 

 a procedure in which the sample is glued to a microtome mount and then 

 polished with a motor-driven fine emery stone cooled by a stream of 

 water. A simple miter box and Zona saw have been used satisfactorily 



