Adipose conversion of Ob17 preadipocytes : Relationships between cell division and fat cell cluster formation

Adipose conversion of Ob17 preadipocyte cells proceeds after confluence with the formation of fat cell clusters, due to the coexistence of cells susceptible or not to adipose conversion. In order to determine whether commitment to differentiation occurs after quiescence or during exponential growth, the spatial arrangement of Obl7 cells was destroyed at different times before and after confluence by trypsinization followed by cell reinoculation. The resulting distribution of lipid-filled cells, as compared to non-replated control cells, indicates that both insusceptible and susceptible cells are present during the growth phase in the absence of insulin. It is shown that the formation of fat cell clusters of large size is due to mitoses of susceptible cells during a limited period of time after confluence. Blockade of post-confluent mitoses by selective elimination of cells in the S phase abolishes the formation of clusters of large size, but single differentiated cells and clusters containing a few cells remain present. Therefore post-confluent mitoses are not necessary for the differentiation to occur, but rather they serve to amplify the proportion of adipose cells relative to non-adipose cells.