Stem Cells

Abstract

Stem cells can be classified as embryonic stem (ES) cells or adult stem cells considering their origin. If plasticity is considered, stem cells can be classified as totipotent, when stem cells retain the ability to give rise to an entire new organism. When stem cells lose this capacity, cells are named pluripotent stem cells, which can give rise to almost all mature cell types that compound an organism. Totipotent and pluripotent stem cells can be obtained from developing early-stage embryos. Multipotent is the group of adult stem cells with restricted plasticity. These cells can differentiate into a defined cell type related with a specific organ or tissue. ES cells can be propagated in vitro under undifferentiated system or with a series of protocols to induce cell differentiation. On the other hand, multipotent adult stem cells cannot be maintained in vitro in an undifferentiated form, except for a special class of adherent adult stem cells named mesenchimal stem cells, which can be expanded in vitro conserving their undifferentiated characteristics. Considering the ability to gen-erate teratomas, ES cells were not used in experimental in vivo cell transplant. On the other hand, several experimental adult stem cells transplants have been performed with controversial results.