Development of brown fat cells in monolayer culture. I. Morphological and biochemical distinction from white fat cells in culture.

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.