**3. Discussion**

#### **3.1 Stem cells, somatic cells**

Stem cells were first discovered by Ernest Armstrong McCulloch (April 27, 1926–January 20, 2011) and James Edgar Till in 1963 **(**Becker et al., 1963; McCulloch et al., 1964; Siminovitch et al., 1963; Till et al., 1964; Weissman & Shizuru, 2008). Stem cells have the ability to selfrenew and to differentiate into different cell types. Early stage embryonic stem cells can form all types of cells. Adult stem cells only can differentiate and generate specialized cells, like bone cells, liver cells, blood cells, skin cells. The stem cells that produce all the blood cell types are called hematopoietic stem cells (Bordignon, 2006; Spangrude et al., 1988; Thomson et al., 1998; Weissman & Shizuru, 2008).

Stem cells and their differentiated cells (somatic cells) maintain a balance-homeostasis. When under stress, stem cells are activated, and start to produce more differentiated cells (Jiang et al., 2009; Martinez-Agosto et al., 2007; Till et al., 1964; Wilson et al., 2004).

Somatic cells are the end products of stem cells, they are unable to self-renew.
