The use of trypsin to improve the localization of immunoglobulins in semi-thin frozen sections of the mammary gland.

Frozen sections, 0.5 micron thick, of the lactating mouse mammary gland have been used to localize immunoglobulins A and G and serum albumin throughout the connective tissue stroma, in the lumina of blood vessels, in milk stored in the alveoli and in the lateral spaces between adjacent epithelial cells. In addition, the immunoglobulins were localized to their specific plasma cells in the connective tissue stroma. Serum albumin was further identified within the mammary epithelial cells as small spots of fluorescence scattered throughout the cytoplasm. The immunoglobulins were not localized within these cells in untreated sections, but in sections treated with trypsin and Soybean trypsin inhibitor, it was possible to identify a similar distribution to that for serum albumin. The spots of fluorescence representing the intracellular localization of the immunoglobulins and serum albumin were frequently found in association with the periphery of intracellular lipid droplets.