Electron Microscopy of Milk Secretion in the Mammary Gland of the C3H/Crgl Mouse. II. Identification of Fat and Protein Particles in Milk and in Tissue 2

Air-dried mouse-milk films, frozen-dried mouse milk, frozen-dried mouse lactating mammary tissues, and osmium tetroxide-fixed lactating mammary tissues of the mouse were examined by electron microscopy. Air-dried and frozen-dried preparations treated with a solution of absolute methanol and ethyl ether (1:1) solubilized the fat droplets but failed to remove the protein particles. Tryptic digestion of formalin-fixed, methanol ether-treated grids of the same material removed the protein particles. The results established the proteinaceous and fatty characters of these 2 classes of milk particulates observed in the osmium tetroxide-fixed lactating mammary-gland tissue by electron microscopy.