Brown adipose tissue

Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals. Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals. Classification of brown fat refers to two distinct cell populations with similar functions. The first shares a common embryological origin with muscle cells, found in larger 'classic' deposits. The second develops from white adipocytes that are stimulated by the sympathetic nervous system. These adipocytes are found interspersed in white adipose tissue and are also named 'beige' or 'brite' (for 'brown in white'). Brown adipose tissue is especially abundant in newborns and in hibernating mammals. It is also present and metabolically active in adult humans, but its prevalence decreases as humans age. Its primary function is thermoregulation. In addition to heat produced by shivering muscle, brown adipose tissue produces heat by non-shivering thermogenesis. In contrast to white adipocytes, which contain a single lipid droplet, brown adipocytes contain numerous smaller droplets and a much higher number of (iron-containing) mitochondria, which gives the tissue its color. Brown fat also contains more capillaries than white fat. These supply the tissue with oxygen, nutrients and distribute the produced heat throughout the body. The presence of brown adipose tissue in adult humans was discovered during FDG-PET scans to detect metastatic cancers. Using these scans and data from human autopsies, several brown adipose tissue depots have been identified. In infants, brown adipose tissue depots include, but are not limited to: interscapular, supraclavicular, suprarenal, pericardial, para-aortic and around the pancreas, kidney and trachea. These deposits gradually get more white fat-like during adulthood. In adults, the depots that are most often detected in FDG-PET scans are the supraclavicular, paravertebral, mediastinal, para-aortic and suprarenal ones. It remains to be determined whether these depots are 'classical' brown adipose tissue or beige/brite fat.