Definition and classification
A stem cell is an undifferentiated cell characterized by its ability to generate specialized cells by differentiation and its ability to multiply infinitely and identically (self-renewal), in vivo or in vitro (cell expansion). They play a role in integrity conservation of our bodies, in particular through process of tissue regeneration. Human stem cells are classified into three types according to their origin, embryonic, fetal or adult stem cells. Research on embryonic and fetal stem cells currently has been curtailed, mainly for ethical reasons.