This week's image of the week shows an example for the growing number of organelles that we annotate in the Cell Atlas. Lipid droplet (LD) proteins fall under the vesicle category, but starting with version 16, they will have their own place in the Atlas? organelle panel.

LDs can be found in animals, plants, fungi and even some bacteria have them. They have a simple and evolutionary conserved structure: A hydrophobic core that contains the lipids is surrounded by a phospholipid monolayer with proteins attached. This phospholipid monolayer makes the LDs unique among organelles; all other membranes consist of a double layer. LDs are perfectly round but they vary greatly in their size, from a few hundred nanometers to the more than 100 µm large, single LDs that fill the cells of the fat tissue (Walther TC & Farese RV (Jr) 2012 ).

The existence of LDs has been known for a long time, however until recently they were considered a rather inert storage place for lipids, analogous to a drop of grease in a soup. This view changed when the first LD-associated protein was discovered 25 years ago. This protein was called perilipin (PLIN1) and soon more similar proteins like the PLIN3 (figure 1) were identified. After more investigation, it was discovered that these proteins are important for the structure of LDs and are involved the regulation of the lipid degradation (Brasaemle DL 2007).

The main function of LDs is of course providing fat that is burned in mitochondria for the generation of energy. Beyond that LDs also serve as organizing centers for synthesizing specific lipids and storage place for proteins. Interestingly, the hepatitis C virus uses the surface of LDs to assemble (Herker E and Ott M 2011).

Almost all cells have the capability to make LDs, but in the human body LDs are mainly found in adipocytes and liver cells. Both adipocytes and liver regulate the amount of fat in the blood to ensure that all tissues are supplied with enough fat to produce energy. But a constant excess of fat leads to the formation of too many LDs as well as in the wrong places. This can eventually cause diseases like diabetes or obesity (Greenberg AS et al. 2011), in which LDs play a prominent role. The work of the HPA helps to identify LD-associated proteins, that might be new target for the treatment of such metabolic diseases.

We would like to thank all the members of the Subcellular Human Protein Atlas who generate these images. A special thanks to Peter Thul for contributing this article about the lipid droplets and congratulations to him on becoming a father this week as well!!!