Every cell in the body goes through a life cycle. Cells grow and divide to replace cells that are lost because of normal wear and tear or injury. Different cells grow and die at different rates. Some cells, such as epithelial<span>epithelialA thin layer of epithelial cells that makes up the outer surfaces of the body (the skin) and lines hollow organs, glands and all passages of the respiratory, digestive, reproductive and urinary systems.</span> cells, reproduce quickly. Other cells, like nerve cells, grow slowly. Both normal cells and cancer cells go through a sequence of steps, or phases, when they form new cells. This is called the cell cycle.
