Adult Stem Cells

Adult▶ stem cells are defined as undifferentiated tissue-specific stem cells with extensive selfrenewal capacity, which can proliferate to generate mature cells of the tissue of origin. The primary roles of adult stem cells are to maintain and/or regenerate the cells of damaged tissues. Stem cells may remain quiescent for long periods of time until they are activated by a need for more cells. Adult stem cells were first described in organs and tissues characterized by high cell turnover, such as blood, gut, testis, and skin, but have to date also been isolated from many other organs and tissues including brain, bone marrow, liver, heart, lung, retina, ovarian epithelium, teeth, mammary cells, and skeletal muscle. Adult stem cells mainly possess two key properties: (1) self-renewal, which is the ability to allow the cells to go through many cell divisions while remaining in an undifferentiated state, and (2) multipotency or multidifferentiative potential, which is the ability to generate progeny of several distinct cell types of the tissue or organ such as glial cells, neurons, etc. Stem cells differ from somatic cells with their different potentials and their proliferation ability. There are three kinds of stem cells – embryonic, germinal, and adult stem cells – that are classified according to their developmental potential ranging from totipotency to unipotency. The fertilized oocyte and the blastomere up to the eight-cell stage are considered as totipotent (totipotent stem cells) as they can differentiate to generate a complete organism. ▶Embryonic stem cells, the cells derived from the inner cell mass of the blastocyst, are pluripotent (pluripotent stem cells) and have the ability to differentiate into cells and tissues from all three germ layers: the endoderm, the ectoderm, and the mesoderm. Germinal stem cells are also pluripotent and are derived from so-called primordial germ cells