HeLa cell. Transmission electron micrograph (TEM) of a section through a HeLa cell undergoing apoptosis (programmed cell death), showing the nucleolus, cajal bodies, DNA (deoxyribonucleic acid) fragmentation, interchromatin granules, shrinkage and increased electron density of the cytoplasm, including an abundance of free ribosomes. HeLa cells are a continuously cultured cell line of human cervical cancer cells. They are immortal and so thrive in the laboratory and are widely used in biological and medical research. Apoptosis occurs when a cell becomes old or damaged.