Highly proliferative cell types include: stem cells, smooth muscle cells, epithelial cells of the internal organs (such as GI tract, liver, kidneys, pancreas), skin fibroblasts and endothelial cells that line blood vessels. As an example, skin fibroblasts rush to the site of injury to repair damage from a cut or wound. Liver cells rapidly proliferate if they are lost (such as in surgery) to replace the missing tissue. Gastrointestinal epithelial cells also proliferate rapidly as they are consistently exposed to harsh environments. Stem cells have a unique ability to replicate many times unlike other cell types. They stimulate the growth of specific types of cells to produce healing to an area of the body that has been injured. This makes them useful due to the fact that most cells in the body cannot replicate themselves.