The Radioautographic Procedure

Radioautography has been defined as a process in which an image is produced in a photographic emulsion by radiation emanating from a source in an object which is in close contact with the emulsion. This image is represented by developed silver grains seen over a tissue section, and the fit between the position of the radioautograph- 

In the field of lipids the only studies made until relatively quite recently were on frozen tissue sections, which ranged in thickness from 5 to 10 p. These were used in combination with emulsions in the form of stripping film which reduces the contrast of the preparation and makes more accurate localization quite difficult. Better resolution was sought by reducing the thickness of the section and application of liquid emulsion, and some improvement was obtained in the study of cholesterol localization in aorta (Charman and Lipsky, 1967). Another approach was the use of Carbowax, a water miscible wax, which allows the preparation of relatively thin sections (about 4 p). This method of tissue preparation was utilized for the study of phosphatidylinositol metabolism in the pancreas (Hokin and Hueb-ner, 1967) and of the distribution of cholesterol- in arteries and other tissues (Kramsch et al., 1967). The suitability of the Carbowax embedding for the study of lipids has been based on its being miscible with water and on the feasibility of performing lipid stains on sections prepared from Carbowax-embedded tissue. 

4 (left) and 5 (right). Sections of rat heart labeled for 30 seconds by perfusion with oleic acid- H and post perfused for 20 minutes with nonradioactive medium. In both figures the radioautographic reaction is concentrated over a lipid droplet. Significant reduction in the size of silver grains was achieved in Fig. 5 by the use of physical developer. Fig. 4, x 30,000 Fig. 5, x 51,000. (From O. Stein and Stein, 1968, reproduced by permission of the Editor of/. Cell Biol.) 

Similar conclusions were reached by Uzman and Hedley-Whyte (1968) and Hedley-Whyte et al. (1969). These authors compared quantitatively the distribution of grains over preparations prepared with conventional dehydration and embedding, when about 90% of the label had been lost, to those in which retention was improved by partial dehydration, and found no difference in the grain distribution over various structures. 

Thus it seems possible to conclude that for the type of lipids (cholesterol, free and ester, triglycerides and phospholipids) and tissues studied (liver, adipose tissue, aorta, adrenal, intestine, nervous tissue, and macrophages) no significant translocation of the labeled lipid was observed. 

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