cells that are the precursors to the mature cells found in most tissues. Stem cells, such as epithelial stem cells in the gut, can give rise to all the cells of the tissues in which they are found. Adult stem cells are found in most tissues of the body and under normal circumstances, only give rise to the tissue in which they are found (e.g. blood stem cells give rise to the cells of blood only). The most primitive stem cells, found in early embryos (embryonic stem cells), can give rise to all the tissues of an entire organism.