What is "brown adipose tissue"

n. One of the two types of adipose tissue present in many newborn or hibernating mammals which primary serves to generate body heat, and which contains several small vacuoles and a relatively high number of mitochondria and capillary.

Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). BAT 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' .

BAT 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, BAT 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 BAT its brown appearance. Brown fat also contains more capillaries than white fat, to supply the tissue with oxygen and nutrients and distribute the produced heat throughout the body.