Monocyte serum withdrawal apoptosis. (A) Time course of monocyte serum withdrawal apoptosis in the presence and absence of zVAD-fmk was assayed by flow cytometry. After 12 hours, more than 70% of monocytes were shown to be annexin V positive and propidium iodide (PI) negative, and therefore in the early stage of apoptosis. Secondary necrotic cells were 5% or less of cells, as indicated by annexin V-positive, PI-positive cells. The specificity of the apoptotic process was further shown by marked apoptosis inhibition in the presence of 20 μM of the pan-caspase inhibitor zVAD-fmk. The dot plots represent viable cells (bottom left quadrant), early apoptotic cells (bottom right quadrant), and secondary necrotic cells (top right quadrant). The percentage of viable, early apoptotic, and secondary necrotic cells is indicated within the respective quadrant. Data are representative of 6 different experiments. (B) Viable and apoptotic monocyte morphology at transmission electron microscopy (TEM). Viable and apoptotic monocytes were prepared for TEM as described in “Transmission electron microscopy.” Apoptotic cells show the typical morphology of condensed cytoplasm and chromatin with cellular membrane blebbing.