Lipid droplets are specialized organelles for the cellular storage and metabolism of neutral lipids, primarily in the form of triacylglycerols and sterol esters. They are of various sizes and have a unique structure consisting of a hydrophobic core, containing the lipids, and a surrounding phospholipid monolayer, or hemimembrane, which contains various integral and peripheral membrane proteins. These proteins include enzymes involved in lipid droplet formation, lipid metabolism and metabolic signalling, but also histones and ribosomes. Lipid droplets enable dynamic storage of lipids used for energy metabolism, biosynthesis of cellular membranes and lipid signalling pathways, but also sequesters toxic lipids and stores membrane proteins.
Lipid droplets are vesicle-like structures that can vary in size and number. Staining of proteins in larger lipid droplets usually give a characteristic ring-like appearance due to their localization to the outer phospholipid layer and absence from the hydrophobic core. However, given the fact that lipid droplets can be difficult to distinguish from different vesicles and other membranous structures, co-localization with hydrophobic marker dyes or marker proteins is usually required to confirm the localization to lipid droplets.
Read more about the proteome of lipid droplets as a substructure of vesicles.