Imaging lipid droplets by electron microscopy.

The lipid droplet (LD) is different from other cellular organelles in that most of its volume is made of lipid esters and its surface is lined by a phospholipid monolayer. This uniquely lipid-dominant structure poses a problem for electron microscopy (EM) because the aldehydes commonly used as a fixative do not react with most lipids. To circumvent this difficulty and utilize the high resolving power of EM, many methods have been developed. In this chapter, we discuss methods that have been used and/or are potentially useful to study LDs. The methods include conventional EM to observe the LD core, cryoelectron microscopy to observe the LD surface, freeze-substitution, immunoelectron microscopy (pre-embedding, post-embedding, and cryosectioning methods), and freeze-fracture. Each method has strong and weak points and therefore some caution is necessary in interpreting the obtained results. In combination with methods of other disciplines, the electron microscopic techniques should contribute significantly to solving the remaining questions on LDs.