Stem cell is defined as a cell with a unique capacity to produce unaltered daughter cellsand to generate specialized cell types (potency). Once cells are differentiated, their phenotypes are stable. However, tissue stem cells, which are thought to be lineage-committed multipotent cells, possess the capacity to differentiate into cell types outside their lineage restrictions called trans-differentiation or stem cell plasticity). For example, hematopoietic stem cells may be conveed into neurons as well as germ cells.