Morphological Differentiation of Implanted Brown and White Fats

Introduction Many theories have been offered concerning the function of brown fat. Stanlnius (1853, quoted by Rasmussen, 1922) believed that the entire central nervous system developed from the interior of brown fat. Others felt that brown fat was used as a source of energy during starvation (Cramer 1910, 1916, quoted by Rasmussen, 1922). Recently views have changed concerning brown fat (BF). Smith (1961) found that in cold acclimated rats it increased its capacity to produce energy. Since then it has been shown that BF can produce heat, and that maximum heat results where heat production would be biologically useful (Smith and Hock, 1963; Smalley and Dryer, 1963). Brown fat is found in cold adapted mammals, and in those species capable of hibernation (Smalley and Dryer, 1966). The largest of the ten deposits distributed throughout the body are the interscapular lobes (Cameron and Smith, 1965). BF is rich in blood vessels and nerves. Cytologically its cells contain a central nucleus, many lipid droplets (multilocular) and numerous mitochondria. It has been found that BF colntains 50-609o water and that 60-709 of the dry material is lipid, as compared to the 90% lipid found in white fat (Smalley and Dryer, 1966). Heat production in BF has been related to its nerve supply. It has been noted that xylocaine applied to the nerves supplying brown fat stopped heat production in interscapular and cervical deposits (Smith, 1964 ) . The present investigation involves a study of brown and white fats in the absence of normal nerve supplies. Complete denervation was achieved by removal of BF from the scapular region and implanting it into a neutral site.