High-resolution scanning electron microscopic evaluation of cell-membrane porosity by ultrasound

The effect of low-intensity ultrasound on HL-60 cells (human promyelocytic leukemia cells) in the presence of the photo sensitizing drug merocyanine 540 (MC 540) was evaluated morphologically, using a high-resolution scanning electron microscope and a transmission electron microscope. Exposure of HL-60 cells to ultrasound without MC 540 resulted in a decrease of finger-like processes in the cells. The cells showed many undulating ruffles on the surface. Distinct pits or holes in the membrane were not observed in these cells. The surface of HL-60 cells treated only with MC 540 was relatively smooth compared with that of control cells. HL-60 cells exposed to ultrasound in the presence of MC 540 showed apparent surface deformation. Numerous crater-like depressions of heterogeneous dimensions were observed in many cells. In addition, various-sized pores were noted in the cell membranes of more damaged cells. These results indicate that cell degeneration was induced by a rapid change in cell membrane porosity during sonication in the presence of MC540.