In 134 out of 180 perirenal fat samples (74%) derived from Japanese necropsy 
cases aged from 1 month to 86 years, the brown fat tissue persisted in variable 
amounts. Brown fat cells were classified into 6 types: Type 1 cells are 
fat-depleted cells filled with granular cytoplasm and are believed to be 
produced after oxidation of fat for heat production. Type 2 cells are 
small-locular cells suitable for rapid oxidation of fat droplets. Type 3 
(middle-locular) and 4 (large-locular) represent fat-storage cells containing 
large amounts of fat. Type 5 cells are thought to be transitional forms between 
multilocular brown fat cells and monolocular white fat cells. Type 6 
(cytoplasm-rich multilocular) cells, usually found together with Type 1 cells, 
are thought to be fat-depleting or -consuming cells, since in them fat droplets 
are reduced in number and size probably in consequence of oxidation of fat, but 
by contrast granular cytoplasm is increased in amount separating the individual 
fat droplets by thick cytoplasmic septa. The occurrence of Types 1 and/or 6 
cells that has been revealed in 65 out of the total 180 samples (36%), suggests 
that the oxidation of fat for the thermogenesis proceeds in the brown fat tissue 
and that brown fat cells partially undergo fat depletion. In the present study, 
the thermogenesis of human brown fat tissue was suggested chiefly with regard to 
the occurrence of Types 1 and/or 6 cells. In the majority of perirenal fat 
samples from infants (1-11 months) relatively numerous Types 1 and 6 cells were 
frequently revealed together with Type 2 cells, suggesting rapid and active heat 
production in support of the view that in human infants the brown fat tissue may 
be thermogenetically active to maintain body temperature. In the same manner, 
marked ability to produce a considerable amount of heat was evidenced in brown 
fat tissue of children and teenagers. In younger and elderly adults the 
frequency of occurrence and the amount of the perirenal brown fat tissue were 
decreased but Types 1 and/or 6 cells could be found in 17-40% of them, 
infrequently together, with Type 2 cells, suggesting persistence of the 
thermogenic activity with occasional large heat production especially in younger 
adults (20-39 years). Thus, the results obtained in this study have clarified 
that the human brown fat tissue can respond to stimuli given to the body by 
oxidation of stored fat even in the latest decades of life. In cases of death 
from burning, drowning, bleeding, drug poisoning etc., numerous Types 1 and/or 6 
cells were found, suggesting that an active fat oxidation would take place in 
brown fat tissue assumedly as the result of the raised noradrenalin level in 
this tissue. The so-called small cytoplasmic cells found in perirenal fats from 
cases of death from liver cirrhosis and other causes were assumed to be atrophic 
fat-depleted brown fat cells.