The cells that retain their proliferative capacity thought out life are regarded as stem cells.According to differentiation potential they can be classified as pluripotent, multipotent, unipotent cells. Major potential sources of stem cells are blastocyst embryos, foetal tissues, umbilical cord blood, and adult tissues. ES cells are an extra ordinary class of stem cells that can proliferate indefinitely in culture, as they posses very high developmental potential. Cord blood, donated by mothers after the birth of their children, has become an accepted source of related and unrelated hematopoietic stem cells for marrow reconstitution. Adult stem cells also referred to as somatic stem cells or mesenchymal stem cells are those mature, adult cells that are undifferentiated and found in a specific tissue or organ. Stem cells derived from adipose tissue and bone marrow show great promise as an alternate source of cells for adipose tissue engineering. The purified muscle-derived cells can differentiate into osteogenic lineages and proceed to mineralization and possible bone tissue formation. This concluded that the muscle tissue may become a valuable resource for the isolation of osteoprogenitor cells capable of improving bone healing. Adult neural stem cells (aNSCs) represent an attractive source for the production of specific types of neurons in degenerative CNS diseases and for the development of new regenerative gene therapies. Embryonic and adult stem cells also appear to be the precursors of hair cells the bone marrow derived mesenchymal stem cells/cord blood stem cells may also differentiate into hair cells when grown in suitable medium condition to restore the nearly normal hearing. A potential problem with stem cell therapy is immune rejection of transplanted stem cells. It has been suggested that this might be avoided using therapeutic cloning.
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