Cellular structure and function of rat fat cells in the primary culture.

A primary culture of undigested fat tissue fragments was used to obtain fat cells in vitro. On day 2 of culture, immature fat cells, which are fibroblast-like fat cells containing fine lipid droplets, appeared, surrounding the fat tissue fragments, and began to proliferate extensively. Afterwards, these fibroblast-like fat cells grew to become multilocular fat cells containing larger intracytoplasmic lipid droplets, and differentiated further into unilocular fat cells containing a single large intracytoplasmic lipid droplet. Treatment with dibutyryl-cAMP, which is a second messenger of the lipolytic factor, caused the cultured fat cells to retract, and the intracytoplasmic lipid droplets of those fat cells became finely granulated and decreased along with an increase of hormone-sensitive lipase activities. Conversely, administration of insulin caused the lipid droplets in the fat cells to increase and become larger along with an increase of alpha-glycerophosphate dehydrogenase activities. These findings indicate the occurrence of lipolysis and lipogenesis of fat cells in vitro. Immuno-cytochemistry revealed that vimentin surrounded intracytoplasmic lipid droplets, and became distinct with an increase of lipid droplets through lipogenesis in the fat cells. Vimentin seems to be correlated to the behavior of lipid droplets in the fat cells. Fat cells in this study showed the appropriate cellular structures and functions in response to stimulation of lipolysis and lipogenesis under culture conditions. It is expected that in vitro culture of fat cells will facilitate cell biological elucidation of obesity in the future.