Proliferation of unilocular fat cells in the primary culture.

Mature white fat cells (unilocular fat cells) have generally been considered to be in terminal differentiation and, hence, to have no proliferative ability. A new method, referred to as "ceiling culture," has been devised in our laboratory to culture unilocular fat cells in vitro. Under such culture conditions, the fat cells continue to exhibit specific functions of lipid metabolism and proliferate extensively. Intracytoplasmic lipid droplets did not inhibit division of the cells. There were two modes of proliferation of unilocular fat cells: "loculus-dividing" cell division, in which the single loculus of fat in the dividing cell was broken down into multiple droplets and distributed evenly between the daughter cells, and "loculus-preserving" cell division, in which the loculus in the dividing cell was minimally broken down and inherited with its shape preserved by one of the daughter cells with the other getting only a small number of fine lipid droplets. Such findings suggest that unilocular fat cells in mature fat tissue in vivo are probably capable of proliferation in such modes under some conditions.