Figure 1.

Examples of U251 human GBM cells undergoing methuosis triggered by ectopic expression of activated Ras. Conditional expression of H-Ras(G12V) was induced by addition of doxycycline to a stable cell line (U251-C18). A: The electron micrograph shows the initial stage of methuosis, where large electron-lucent cytoplasmic vacuoles bound by a single membrane can be seen forming from lamellipodial extensions of the plasma membrane (asterisks). B: The macropinosome-derived vacuoles remain separate from lysosomes. Lysosomes were prelabeled for 3 hours with LysoTracker Red. Macropinosome-derived vacuoles were then labeled with dextran–Alexa Fluor 488 (green). The cells were examined 4 hours after addition of the labeled dextran. C–F: Electron micrographs show detached U251-C18 cells in various stages of degeneration between 4 and 6 days after induction of H-Ras(G12V) expression. Vacuoles accumulate and coalesce until they fill almost the entire cytoplasmic space. F: The nuclei (N) generally remain intact, with little or no chromatin condensation, even in cells that have undergone lysis. Images are reprinted with permission from Overmeyer et al.²⁶ Scale bars: 1 μm (A); 10 μm (F).