Morphological and biochemical characterization of possible intracellular precursors of milk lipid globules.

Within milk secreting mammary epithelial cells, immediate precursors of milk lipid globules existed as relatively large (diameter greater than 1 micron) cytoplasmic lipid droplets. The periphery of these lipid droplets was characterized by an electron-dense, granular material which lacked unit membrane character. Cytoplasmic lipid droplets which retained surface coat material were isolated from lactating cow and rat mammary glands. Surface coat material on isolated droplets was composed of polar lipids and proteins. In enzyme activities, this surface material was distinct from plasma membrane and endomembranes. In polar lipid composition, surface material on cytoplasmic lipid droplets was similar to milk lipid globule membrane or was intermediate between endoplasmic reticulum and milk lipid globule membranes. A previously unrecognized group of small structures (diameter less than 0.5 micron), which resembled cytoplasmic lipid droplets in matrix and surface coat appearance, was observed and characterized. These structures were isolated and found to contain large amounts of triacylglycerols, which closely resembled triacyl/glycerols of cytoplasmic lipid droplets in fatty acid composition. Surface coat materials of these small structures and of cytoplasmic lipid droplets were similar in enzymatic, polypeptide and polar lipid composition. Morphological evidence that these small structures may fuse with cytoplasmic lipid droplets was obtained. These small structures, for which we propose- the name micro lipid droplets, may provide triacylglycerols to support growth of cytoplasmic lipid droplets.