Centre National de la Recherche Scientifique, Nice, France
German Diabetes Research Institute, Dusseldorf, Germany
The development of white adipose tissue (WAT) represents a dynamic process throughout life. It was long assumed that the acquisition of fat cells was irreversible, but recent observations suggest that apoptosis may also occur in WAT, although to a much smaller extent than in brown adipose tissue (BAT). At the cellular level, there is still limited and partially contradictory information about the characteristics of the cells constituting the adipose tissue organ. This leads in turn to the question of the nature of the factors that regulate the formation of new fat cells from dormant adipose precursor cells or evenmultipotentmesenchymal stem cells. Once adipose tissue is formed, adipocytes represent between one-third and two-thirds of the total number of cells. The remaining cells are blood cells, endothelial cells, pericytes, adipose precursor cells of varying degree of diﬀerentiation, and, most likely, other cell types (1). The existence of very small fat cells in addition to mature adipocytes has also been documented (2). Figure 1 summarizes the various cell types present in this tissue, based on ultrastructural studies in vivo and biological studies in vitro (3).