Cell division. Coloured transmission electron micrograph (TEM) of the narrow bridge of cytoplasm (centre) between two separating cells. The dark purple areas contain chromosomes and will form the nuclei of the two daughter cells. Division of a cell's cytoplasm (cytokinesis) follows shortly after division of the nucleus (mitosis). A contractile ring of actin filaments gradually narrows the bridge between the two cells until it breaks the microtubule fibres of the spindle (purple lines across bridge) and divides the cells. The spindle is involved in division of chromosomes during mitosis. This cell is from cultured human embryonic kidney tissue. Magnification: x15,000 when printed at 10 centimetres tall.