HeLa cell dying, coloured scanning electron micrograph (SEM). This cell appears spherical because it is 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. HeLa cells are a continuously cultured cell line of human cancer cells, which are immortal and so thrive in the laboratory. They are widely used in biological and medical research.