This review deals with the comparative observations of brown fat tissue in the 
bat, a hibernator, and in man, a nonhibernator. In both mammals, the brown fat 
is distributed in restricted portions of the body and brought into a 
thermogenetic activity by an acute drop in ambient temperature. Light 
microscopic examination was performed on the interscapular brown fat of bats 
captured monthly throughout one year; electron microscopic observations were 
made using a bat captured in April and another in September. Human perirenal 
brown fat was investigated light-microscopically on tissues derived from 215 
fresh necropsy cases (Japanese) of both sexes aged from one month to 93 years. 
Brown adipose tissue was recognized only in 162 (75%) of the 215 samples, 
because brown fat cells were reduced by transformation into white fat cells. 
Human brown fat cells were classified into six types according to the 
morphological features of their lipid droplets. These reflect different 
functional states of intracellular heat production. The Type 1 (D) cell is a 
fat-depleted brown fat cell with a darkly stained cytoplasm; it is specific to 
humans. Human perirenal brown fat cells begin to show a transformation into 
white fat cells already at the infantile stage. This change occurs from the 
peripheral toward the central portion of the lobule, so that various functioning 
cell types remain only in the central area of the lobules. In contrast to 
humans, bat interscapular brown fat cells exhibit regular seasonal changes in 
size and lipid droplet content, so that the cells could not be classified as in 
humans into definite types according to the features of their lipid droplets. 
The most conspicuous difference between brown fat tissue in the nonhibernator 
and hibernator is that in the latter, white fat cells never occur within the 
brown fat tissue. In the hibernator, thermogenesis in the brown fat is necessary 
for both the arousal from hibernation and the maintenance of hibernation as well 
as rutting. In human perirenal brown fat tissue, darkly stained fat-depleted 
cells (D) occupy, with other cell types (CR, CR'), an important part in the 
reversible heat production cycle of the brown fat tissue. The "hypothermic" or 
"cold syndrome (cold injury)" is a disorder affecting inadequately protected 
infants in severely cold seasons, accompanied by lethargy, hypothermia and 
lactation refusal and revealing hemorrhagic pneumonia in necropsy. The brown fat 
tissue in such infants is composed exclusively of fat-depleted brown fat 
cells.(ABSTRACT TRUNCATED AT 400 WORDS)