The ability of cells from the stromal-vascular fraction of rat brown adipose 
tissue to develop into adipocytes in primary cell cultures was investigated. 
Comparison was made with precursor cells isolated by the same procedure from the 
white adipose tissue of the same animals and cultured in parallel under 
identical conditions. The culture procedure used allowed the cells isolated from 
both tissues to rapidly proliferate and differentiate. During the first week in 
culture the brown fat cells grew to confluence and accumulated fat in a 
multilocular way. During the second week, further fat was accumulated, but the 
cells remained multilocular. Analysis of the parallel white fat cell cultures 
revealed clear differences between the two adipocyte types, although the rates 
of cell growth were identical. Measurement of the size of the cellular lipid 
inclusions as a function of the time in culture indicated a much higher number 
of fat droplets larger than 30 micron in the white adipocytes. Moreover, after 
isolation of pelleting fractions of both cultured cell types, comparative 
functional analysis of their mitochondria by oxygen consumption measurement, as 
well as direct cytochrome-c-oxidase determinations, showed a significantly 
higher amount of mitochondria in the brown fat cell fractions than in the white 
fat cell fractions. It was concluded that mature brown fat contains precursor 
cells which can proliferate and develop into adipocytes in monolayer cell 
culture and which have inherent characteristics distinct from those of white fat 
precursor cells.