Adipose tissue

There are two types of adipose tissue: white and brown. White adipose tissue (WAT) is the main site of fat deposition in the animal body. Its main function is as an energy store, which accumulates in times of positive energy balance and is mobilized in times of negative energy balance. In addition, it protects certain internal organs against physical damage and provides thermal insulation. The main WAT depots are subcutaneous, perinephric (perirenal), pericardial, abdominal (mesenteric and omental, sometimes also called gut and  hannel fat), intermuscular and intramuscular. In newborn animals there is very little WAT. It is a late- eveloping tissue that accumulates as animals approach their mature body size. The main cell type found in adipose tissue is the adipocyte. Adipocytes range in size from 20–200 m. The size and number of adipocytes vary between adipose tissue depots. Intermuscular adipose tissue contains a large number of small adipocytes whereas perinephric adipose tissue contains a small number of large adipocytes.

    The main metabolic processes in adipose tissue are: (i) fatty acid synthesis and (ii) triacylglycerol synthesis, jointly known as lipogenesis; and (iii) lipolysis, the breakdown of triacylglycerols to yield glycerol and non- sterified fatty acids (NEFA). Adipose tissue is the major site of de novo fatty acid synthesis in ruminant species. In non-ruminant mammals, fatty acid synthesis occurs in both adipose tissue and liver; whereas in avian species, adipose tissue is not an important site of fatty acid synthesis and triacylglycerols are synthesized from fatty acids of dietary origin or synthesized in the liver. In ruminant adipose tissue, acetate is the primary substrate for fatty acid synthesis. In non-ruminant mammals and birds, glucose is the major  ubstrate.

    

    Brown adipose tissue (BAT) is a specialized form of adipose tissue. Its function is the generation of heat by the oxidation of fatty acids by the process of non-shivering thermogenesis. It is particularly important in eonatal animals. In some species (e.g. lambs) the ability to enerate heat by non-shivering thermogenesis is lost within 2–3 days of birth; in others (e.g. rats) this property persists into adult life. Some species, such as the pig, do not have BAT and are particularly susceptible to cold immediately after birth. BAT is pale brown in appearance, due to the well-developed blood supply and to the presence of numerous mitochondria in adipocytes. It is found in a number of anatomical locations, e.g. in interscapular, axillary and perinephric regions. Its ability to generate heat is due to the ‘uncoupling’ from ATP synthesis of mitochondrial electron transport by uncoupling proteins (UCPs). These proteins cause the disruption of the proton gradient across the inner mitochondrial membrane. (JRS)