2.2. Induced pluripotent stem cells

Induced pluripotent stem cells (iPS) were first achieved by inducing a forced expression of specific genes that can reprogram human and mouse adult somatic cells into the undifferentiated cell [6, 7]. iPS have the same characteristics of ESCs, such as expression of pluripotency markers and differentiation capability [6].

#### 2.3. Fetal stem cells

Fetal stem cells (FSCs) are derived either from a fetus or from extraembryonic structures. Various subtypes of FSCs were described according to their origin (i.e., amniotic fluid, umbilical cord, Wharton's jelly, amniotic membrane, and placenta). FSCs are ideal sources of cells for use in regenerative medicine. They are easily accessible, having a high proliferation rate. In addition, FSCs do not form teratomas [8] and overcome the ethical problem associated with ESCs [9].

#### 2.4. Adult stem cells

In principle, adult stem cells are unspecialized (undifferentiated) cells. They are found in differentiated tissues and considered to be quiescent, but still capable of self-renewal and differentiation. These cells remain in their undifferentiated state until stimulated [10]. Adult MSCs have been isolated from different sites: bone marrow, adult peripheral blood, tooth pulp and liver [11].
