White adipose tissue, commonly known as fat tissue, is a type of connective tissue composed of adipocytes. It has several functions: primarily it is the main long-term energy storage organ, but it also functions as an endocrine organ, as a mechanical cushion around the eyes and fingertips, and as thermal insulation. Brown adipose tissue, a specialized form of which contains many mitochondria, is used to produce heat, and exists mainly in rodents and hibernating animals, as well as neonate humans.
Fat is stored in adipocytes mainly in the form of triglycerides, a condensation of one glycerol and three fatty acid molecules. The tryglycerides are stored in lipid vesicles within the adipocyte cytoplasm. This form of energy storage is highly efficient, since triglycerides contain a lot of energy. Also, because of their hydrophobicity they do not require the presence of water and therefore can be packed very densely. Under conditions of energy deficiency, adipose tissue can break the triglycerides back into glycerol and free fatty acids, which can then be secreted and transported to the energy-requiring target organs.
Adipose tissue can be found mainly underneath the skin (subcutaneous), surrounding internal organs (viscerla: epididymal, retroperitoneal, perirenal etc.), and dispersed between skeletal muscles. Brown fat is mainly found in the back between the shoulders (interscapular).
Adipose tissue exists in insects, but in other invertebrates most of the fat storage is done by the liver. In vertebrates, adipose tissue is most extensive in birds and mammals. It appears not to be an essential organ, since mice completely lacking adipose tissue are viable. However, these mice have reduced fertility, diabetes, severe liver steatosis and organ enlargement, and suffer a premature death.