Adipose tissue development: the role of precursor cells and adipogenic factors. Part II: The regulation of the adipogenic conversion by hormones and serum factors.

Cell culture systems have proven to be valuable models for the study of the processes involved in the formation of new fat cells. Two separate steps may be distinguished in adipocyte development. First, the determination of a mesenchymal stem cell into a preadipocyte, second, its conversion into a mature fat cell. In cloned cell lines adipose conversion depends on at least one postconfluent mitosis possibly induced by insulin-like growth factors or by as yet unknown mitogens. In addition growth hormone, glucocorticoids, and insulin are needed for conversion to take place. The adipose conversion of preadipocytes originating from the stromal vascular fraction of adipose tissue does not depend on postconfluent mitoses and needs only insulin and glucocorticoid hormones in physiological concentrations. However, the ability to undergo adipose conversion is not stable in these cells, but gets lost after repeated subcultures or seeding at low densities. In addition to stimulating hormones an increasing number of factors inhibiting the conversion process have also been detected, the physiological function of which remains unclear at the moment.