Dividing HeLa cells. Coloured scanning electron micrograph (SEM) of HeLa cells undergoing cytokinesis (cell division). Cytokinesis occurs after nuclear division (mitosis), which produces two daughter nuclei. The two daughter cells are still connected by a midbody, a transient structure formed from microtubules. 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. Magnification: x2600 when printed at 10 centimetres wide.