Cancer cell apoptosis. Coloured scanning electron micrograph (SEM) of a cancer cell undergoing programmed cell death, or apoptosis. Apoptosis occurs when a cell becomes old or damaged. The cell becomes spherical as its cytoskeleton, which holds cell shape, is digested, and blebs, or vesicles, form on its surface. Eventually the cell breaks into several vesicles, now known as apoptotic bodies, and is phagocytosed (engulfed and digested) by specialist cells. The breakdown into vesicles prevents the leakage of potentially toxic or immunogenic substances from the cell. Breakdown in apoptosis pathways often causes cancer, the uncontrolled proliferation of cells. Magnification: x670 at 10 centimetres wide.