**2. Stem cells**

During early life and growth, SCs have a spectacular potential to develop into several cell types in the body. In many tissues, SCs behave as a kind of internal repair system, dividing essentially without limit to replenish other cells (Weissman, 2000). Stem cells are distinguished from other cell types by two important characteristics: First, they are unspecialized cells and, sometimes after long periods of inactivity, they can renew themselves through cell division; second, under certain physiologic or experimental conditions, they are naturally sensitive to their environment, responding to chemical, physical, and mechanical features of their matrices or substrates (Discher*, et al.*, 2009, Solis*, et al.*, 2012).

Until recently, scientists primarily worked with two kinds of SCs from animals and humans: embryonic SCs and non-embryonic "somatic" or "adult" SCs (Feng*, et al.*, 2009).

In 1981, scientists discovered ways to derive embryonic SCs from mouse embryos. In 1998, a detailed study of the biology of mouse SCs led to the discovery of a method to derive SCs from human embryos and grow the cells in the laboratory, and these cells are called human embryonic SCs.

In 2006, genetically "reprogrammed" stem-cell-like cells were identified by using specialized adult cells. This new type of stem cell is called **induced pluripotent SCs (iPSCs)** (Krishna*, et al.*, 2011).
