Abstract
The zymogen granules of the chief cells of the peptic glands are colored dark blue when the sulfuric acid Nile blue technic for the demonstration of lipofuscin is applied. Weaker blue colors appear in smooth muscle, soft keratin and certain cytoplasms. After methylation the same technic colors these structures in varying shades of pink to red.
Decomposition of the methyl esters by KOH restores the blue staining. Extraction with acetone directly after staining with sulfuric acid Nile blue prevents both the blue and the pink to red colorations. The strong Nile blue staining of peptic gland zymogen granules may correlate with the large excess of free anionic residues over cationic in pepsin, and the considerable excess of aspartic and glutamic acids over arginine, histidine and lysines in the same protein.
Use of alcoholic potassium hydroxide affords a convenient histochemical procedure for conversion of esters to fatty acids, provided the acids are not soluble in the alkali.