Brown Adipose Tissue and Thermoregulation

During adaptation to extrautenne life the newborn organism has to compensate for great heat loss. Fullterm and premature infants both respond to cold with an increased metabolic rate. The smaller the child, the greater is the heat loss because of its larger surfacevolume ratio and thinner insulation. The neutral temperature rangethat is the zone where the basal metabolic rate alone maintains a normal body temperature-lies between 32 and 36 degrees C. At 2 3 O C infants double their metabolic rate while adults show hardly any increase. Temperature control presupposes an ability to equilibrate heat production and heat loss. Nonshivering thermogenesis, stimulated by catecholamines is more effective than autonomically controlled muscle work (“shivering”) in compensating for heat loss, and thus dominates in neonates. After birth there is a gradual replacement of nonshivering by shivering. Brown adipose tissue, which is found in several species at birth, including Homo, is specialized for nonshivering heat production. Though babies have got only about 15 g of brown fat, there is evidence that the organ is important for thermoregulation. For instance, the venous system of the interscapular part of the tissue is connected with the vertebral sinus, thus influencing thermal receptors in the cervical canal. I t has been shown in guinea pigs that shivering is suppressed as long as the temperature of the cervical canal is maintained above a certain level. About the mechanism of thermogenesis in brown fat it is known