Morphological changes of the 3T3-L1 fibroblast plasma membrane upon differentiation to the adipocyte form

By quantitative evaluation carried out on freeze-fracture replicas, we have investigated the changes in plasma membrane organization as the 3T3-L1 fibroblast phenotype differentiates into the 3T3-L1 adipocyte form. As differentiation takes place there is a dramatic change in overall appearance of the cell as it acquires large lipid-laden vacuoles. On freeze-fracture replicas we find: (1) a ninefold increase in small invaginations as the cell differentiates from the fibroblast to the adipocyte phenotype; (2) virtually no quantitative change in the larger coated invaginations upon differentiation; (3) a greater density of intramembrane particles in the large invaginations as compared to the uninvaginated membrane in both the fibroblast and adipocyte form. This remains relatively constant with differentiation. By contrast, there is a marked increase of intramembrane particles in the undifferentiated membrane as the cell changes from fibroblast to adipocyte. The functional significance of these changes in plasma membrane organization is unknown, but they significantly correlate with the onset of lipogenesis by the cell.