Figure 3 Lipid droplet formation and expansion Figure 3 | Lipid droplet formation and expansion. a | Lipid droplets (LDs) consist of a neutral lipid core surrounded by a phospholipid monolayer. Proteins access the LD surface by relocalizing from the ER bilayer (class I) or from the cytosol (class II). b | LD formation begins with neutral lipid synthesis. The lipids accumulate in the ER bilayer to form a lens. c | Eventually, the bilayer deforms and causes the droplet to bud forming an initial LD (iLD). d | COPI can bud nano-LDs from the iLD, resulting in increased surface tension and reconnection of the iLD with the ER. This contact allows class I proteins to access the droplet, including GPAT4 and DGAT2. These enzymes are involved in triglyceride synthesis and result in LD growth, forming an expanding LD. Gluchowski, N. L. et al. (2017) Lipid droplets and liver disease: from basic biology to clinical implications Nat. Rev. Gastroenterol. Hepatol. doi:10.1038/nrgastro.2017.32