Cells early in a lineage that can divide and give rise to one or more differentiated cell types. May either remain as stem cells or continue development to become a fully differentiated cell. Unipotent= only produce one type of differentiated cell. Multipotent= give rise to a whole range of differentiated cells. Embryonic stem cells- have the potential to differentiate into any cell type of the body (pluripotency), offering the possibility of treating diseases of multiple tissues from a single stem cell source. Can be from embryonic or adult stem cells. Tissues are maintained by repair and/or replacement throughout life. Tissues may be regenerated by division of differentiated cells or by proliferation from a relatively unspecialised stem cell population. In some tissues, there is no turnover of cells. Most permanent cells are able to repair themselves by turnover of cell components.