Modification of Cunningham's method for the detection of proteases in tissue sections

A modification of Cunningham's method to show the protease activity in fresh tissue sections on gelatin film is proposed. The diazotization of the gelatin film in two successive baths of Fast blue B salt sharply increases the colour intensity of the film, improving the contrast between the intact zones and the enzymatic lysis zones of the substrate. The various washings of the stained film have been simplified. After incubation, the histological staining of the sections by solid nuclear red is an improvement over the much more tricky Feulgen reaction, combined with PAS, used by Cunningham.