Scanning electron micrograph of a 3-day-old breast tumor spheroid (cluster of cells). The spheroid is a 3D system used to test the therapeutic activity of anti-cancer drugs and can reduce the need for research on animals. Different colors highlight different layers of cells within the spheroid (outer layer in green and innermost layer in purple) that will be proliferating at different rates and will have varying accessibility to drugs. Diameter of the spheroid is approximately 250 micrometres. Supplied by Khuloud T. Al-Jamal, David McCarthy & Izzat Suffian, courtesy of Wellcome Images.