Stem cells. Coloured light micrograph of human embryonic stem cells (HESCs, centre) surrounded by their feeder cells. HESCs are pluripotent, they are able to differentiate into any of the 200 cell types in the human body. The type of cell they mature into depends upon the biochemical signals received by the immature cells. Feeder cells support the growth of HESCs, preventing differentiation until there are a sufficient number present. HESCs are a potential source of cells to repair damaged tissue in diseases such as Parkinson's and insulin-dependent diabetes. However, use of HESCs is controversial as it requires the destruction of an embryo.