90 Microscopic Histochemistry 



APPENDIX 

 Ascorbic Acid 



Ascorbic acid has the unique property of reducing an acidi- 

 fied solution of silver nitrate almost instantly. This unusual 

 reducing power can be utilized for its histochemical identi- 

 fication. The first histochemical experiments were made by 

 Szent-Gyorgyi^*® even before the chemical nature of ascorbic 

 acid was known. 



While the specificity of the reaction (if performed cor- 

 rectly) is reasonably certain, the correctness of localization 

 is much less so. Ascorbic acid is a highly diffusible substance 

 which cannot be expected to stay at its original sites unless 

 the tissue is frozen-dried. Its displacement cannot be avoided 

 even by fixation in formaldehyde vapor^^"^ or by perfusing 

 the tissue with reagent,^^^ although localization (in the gross 

 histological, but not cytological, sense) may be improved by 

 these maneuvers. The experiments of Barnett and Fisher^*^ 

 clearly show the importance of physical factors, such as 

 lipid-water interfaces, on the localization of the reaction. 



According to Huszak,^^^ Giroud and Leblond,^^^ and Bar- 

 nett and Boume,^^^ a negative reaction does not rule out the 

 presence of ascorbic acid because some tissues seem to con- 

 tain a substance which prevents the reduction of silver ni- 

 trate. The last-named authors believe that this inhibitor is 

 glutathion. This substance actually slows down the reaction 

 considerably in test-tube experiments. 



Method^^^ 



The reagent is an aqueous or alcoholic 1-5 per cent solu- 

 tion of silver nitrate acidified with some acetic acid; concen- 



146. Szent-Gyorgyi, A.: Biochem. J., 22:1387, 1928. 



147. Bourne, G.: Australian J. Exper. Biol., 11:261, 1933. 



148. Giroud, A., and Leblond, C. P.: Arch, d'anat. micr., 30:105, 1934. 



149. Bamett, S. A., and Fisher, R. B.: J. Exper. Biol., 20:14, 1944. 



150. Huszak, S.: Ztschr. f. physiol. Chem., 222:229, 1933. 



151. Giroud, A., and Leblond, C. P.: Nature, 138:247, 1936. 



152. Bamett, S. A., and Bourne, G.: J. Anat., 75:251, 1941. 



153. Giroud, A., and Leblond, C. P.: Bull, d'histol. appliq. a la physiol., 

 11:375,1934. 



