Dear Pu,

Lipid droplets or lipid bodies are found in all types of cells including bacteria. As highlighted by Dr. Ruiz-Vela, accumulation of LDs can be found in different situations. We have been studying LB formation during inflammatory conditions. In response to inflammation, including infections diseases, LDs are formed in the cytoplasm, mainly in cells from the immune system. Interestingly, not only the number but also the size of these organelles increase and may be indicative of cell activation. In your pictures, I noticed different sizes of LDs in addition to accumulation. Please, see a recent review. Best, Rossana

As mentioned already, all cell types have LDs. They can be very small or taking up the entire cytoplasmic space (adipocytes). Their number and size differs between cell types and cell lines and, like their intracellular position (perinuclear or cell periphery)  is dependent on media conditions, energy source and energy homeostasis. There are different types of LDs. Some store preferentially cholesterol esters (CE) feeding into steroidogenesis (steroidogenic cell types) and membrane synthesis, others contain mainly TGAs feeding into phospholipid synthesis and beta-oxidation. I am assuming that you would like to confirm the presence of LDs in your samples. A good fluorescent live stain to simultaneously image LDs (neutral lipids) and membranes (phospholipids) is Nile Red. We also use C12- and C16-Bodipy labels which are taken up by cells and accumulate in LDs. Similarly, cholesterol-Bodipy or other CE fluorescent labels can be used in multiplex quantitative image analysis of CE-type and TGA-type LDs.  Small molecule inhibitors of ATGL and HSL (lipases) or DGAT (TAG synthetase) can inhibit release of fatty acids and glycerol from LDs or uptake of TAGs into LDs.

Are the pattern of liquid droplets in the cytoplasm correlated to cell type or cyto-physiological process? 

I wouldn't call it pattern. Most certainly their degree of interaction with / closeness to mitochondria, ER, Golgi and probably late endosomes/lyposomes are representative of their cyto-physiological status. A recent elegant publication highlight some of these aspects and shows that AMPK is a critical regulator of LD-mitochondria interaction:

Herms et al. 2015. AMPK activation promotes lipid droplet dispersion on detyrosinated microtubules to increase mitochondrial fatty acid oxidation. doi: 10.1038/ncomms8176