an undifferentiated cell that is able to renew itself and produce specialized cells. Embryonic stem cells at the *blastocyst stage of development can differentiate into almost any cell type (except placental cells); they are described as pluripotent. Embryonic cells preceding the blastocyst, produced by the first 3–4 divisions of the fertilized egg, are capable of producing all the different cell types required by the developing embryo (i.e. they are totipotent). Adult stem cells (also known as somatic stem cells) occur in many tissues and organs, including bone marrow (see haemopoietic stem cell), muscle, liver, pancreas, etc., and can produce the specialized cells needed in the particular tissue or organ in which they arise (i.e. they are multipotent). See also umbilical cord blood banked stem cells.... stem cell
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