Department of Histology and Embryology, Medical School, University of Zagreb, Republic of Croatia, Yugoslavia.
Besides having a metabolic and insulatory-supporting function, adipose tissue in endotherms also performs a thermogenic function. Thermogenic adipocytes contain specific UC-mitochondria with uncoupling protein (UCP) and produce heat. Thermogenic adipose tissue has two forms: brown adipose tissue (BAT) and convertible adipose tissue (CAT). Brown adipocytes have UC-mitochondria and express UCP throughout the entire life of small rodents, chiropterans, and insectivores. However, in other endotherms and in humans CAT participates as thermogenic tissue only during early postnatal period. Both BAT and CAT start to develop in utero, although in some animals (hamsters, marsupials) or in some particular areas (thoraco-periaortal and medio-perirenal areas in rats) development of thermogenic adipose tissue starts after birth. Postnatal development of BAT in small endotherms is characterized by quantitative changes (the amount of UC-mitochondria, UCP, and lipids). Postnatal development of CAT causes qualitative changes during which UC-mitochondria in convertible adipocytes are replaced by common, nonthermogenic C-mitochondria; vascularization of adipocytes drops to a low level and, with lipid accumulation, convertible adipocytes appear as lipid-store cells. Postnatal development of CAT can be modulated or reversed by the environmental temperature. The duration of postnatal changes varies between species; i.e., cats, rabbits and sheep, change their thermogenic form of CAT into the lipid-store form within the first postnatal month, while in humans the same process takes up to 15-20 years. In maturity all these large endotherms have CAT in lipid-store form. In light of these results, the question of participation of thermogenic adipose tissue in the regulation of human obesity needs to be answered.