Stem cell dying. Coloured scanning electron micrograph (SEM) of a stem cell undergoing apoptosis, or programmed cell death. Apoptosis occurs when a cell becomes old or damaged. Blebs (vesicles) called apoptotic bodies form on its surface, which prevent toxic or immunogenic substances from leaking when it is phagocytosed (engulfed and digested) by specialist cells. Magnification: x4000 when printed 10 centimetres wide.