Measuring committed preadipocytes in human adipose tissue from severely obese patients by using adipocyte fatty acid binding protein

To understand the significance of the reported depot differences in preadipocyte dynamics, we developed a procedure to identify committed preadipocytes in the stromovascular fraction of fresh human adipose tissue. We documented that adipocyte fatty acid binding protein (aP2) is expressed in human preadipocyte clones capable of replication, indicating that can be used as a marker of committed preadipocytes. Because aP2 expression can be induced in macrophages, stromovascular cells were also stained for the macrophage marker CD68. We found aP2⁺CD68^- cells (designated as committed preadipocytes) that did not have lipid droplets (true preadipocytes) and that did have lipid droplets <6.5 μm in diameter (very immature adipocytes). Adipose tissue from subcutaneous, omental, and mesenteric depots was obtained from nine patients undergoing bariatric surgery for measurement of stromovascular cell number, the number of committed preadipocytes (aP2 ⁺CD68^-), aP2⁺ macrophages (aP2 ⁺CD68⁺), and aP2^- macrophages (aP2 ^-CD68⁺). The number of committed preadipocytes did not differ significantly between depots but varied >20-fold among individuals. Total cell number, stromovascular cell number, and the number of aP2^- macrophages was less (P < 0.05) in subcutaneous than in omental fat (means ± SE, in millions: subcutaneous, 2.3 ± 0.3, 1.4 ± 0.3, and 0.17 ± 0.08; and omental, 4.8 ± 0.7, 3.8 ± 0.5, and 0.34 ± 0.06); mesenteric depot was intermediate. These data indicate that the cellular composition of adipose tissue varies between depots and between individuals. The ability to quantify committed preadipocytes in fresh adipose tissue should facilitate study of adipose tissue biology.