Organic Substances 119 



Method 



Fix tissues in 95-100 per cent alcohol When mounting the 

 sections on sHdes, float them on water for only a few seconds. 



Carry slides through xylene and alcohols. From last alco- 

 hol, transfer them directly into methenamine-silver solution 

 (see p. 60) buffered to pH zi= 9 and prewarmed to 37° C. 

 Keep them in the incubator for about 30 minutes or until 

 urate shows up in a black shade. Rinse slides, remove unre- 

 acted silver with a dilute solution of Na thiosulfate. Gold 

 toning is optional. Counterstain as desired. 



Under low and medium powers the crystalhne structure of 

 the deposits appears to be preserved to perfection; under 

 high power the slender needles are resolved into rows of fine 

 black granules. 



The only source of error is the presence of calcifications; 

 they will be disturbing only if present in large masses. Other- 

 wise, silver phosphate and carbonate are relatively easily 

 soluble in methenamine and will be washed out of the tissue 

 during incubation. In the case of massive deposits, some sil- 

 ver phosphate may remain undissolved. However, since it is 

 not reduced to metallic silver ( provided that the slide is not 

 exposed to strong light ) , the rinse in thiosulfate will remove 

 it. In case of doubt, treat section for 1 or 2 minutes with a 

 0.2-0.5 per cent solution of nitric or hydrochloric acid in ab- 

 solute alcohol before transferring it to the silver solution ( the 

 acid must be washed off first with absolute alcohol). This 

 treatment will completely eliminate calcifications. Urate de- 

 posits will remain undissolved, but their crystal structure 

 wiU be somewhat distorted. On the other hand, m'ate depos- 

 its are removed readily by a dilute solution of lithium car- 

 bonate, which will leave calcifications intact. Confusion with 

 melanin and premelanin, which are also argentaffin and will 

 blacken under the same conditions, is not likely. 



