Fig. 3. TPA induces macropinocytosis in A431 cells. (A,B) EM micrographs of control A431 cells (Cont) serum-starved for 4 hours and then incubated with HRP (10 mg/ml) for 15 minutes at 37°C before processing for EM. Note the irregular membrane on the free surface (FS) of the cell in A. Moderate endocytosis of HRP into endosomes (En) has taken place. B shows the lateral membrane of a cell facing the intercellular space (IS). Here a few microvilli-like structures are seen. Arrowheads indicate the presence of caveolea. (C-F) Serum-starved A431 cells incubated with HRP as above but in the presence of 1 µM TPA. C and D show closure of membrane ruffles (arrows) and subsequent formation of large amounts of HRP-containing macropinosomes (Mp) at the free surface (FS). E and F show that TPA treatment also leads to an increased complexity of the lateral membranes at the intercellular spaces (IS) and that caveolea are present at the lateral membrane facing the intercellular space (arrowheads in F). Bars, 1 µm (A,C-E); 0.5 µm (B,F).