True about stem cells-

a stem cell is a cell with a unique capacity to produce unaltered daughter cells (self-renewal) and to generate specialized cell types (potency). Stem cells are characterized by two impoant propeies: self-renewal capacity and asymmetric replication As mature cells die, the tissue is replenished by the differentiation of cells generated from stem cells. In human embryo in about 3rd week of development stem cells appear in the yolk sac. ref robbins basic pathology 9th ed page 60