Endogenous enzymes cause structural and chemical artifacts in methacrylate- and celloidin-embedded sections of unfixed freeze-dried tissues.

Bovine exocrine pancreas and fish (Rivulus ocellatus marmoratus) liver containing pancreatic acini were cryofixed, freeze-dried, and embedded in methacrylate or double-embedded in celloidin and paraffin. In chemically unfixed sections incubated in aqueous solutions, dissolution of zymogen granules was coincident with loss of tissue structure and antigenicity. Type II-S soybean protease inhibitor at 150 mg/liter during section flotation and in aqueous reagents used for immunohistochemistry prevented these artifacts and allowed the use of more dilute antibody solutions. Loss of glycogen from fish hepatocytes was most rapid in areas adjacent to pancreatic acini. Rapid loss of glycogen was attributed to amylase and was prevented by using poly-L-lysine instead of 3-aminopropyltriethoxysilane slide adhesive and by using alcoholic solutions during PAS staining. Inhibition of endogenous enzymes is an important consideration in the development of histological protocols with freeze-dried tissue sections.