Stem cells are developmentally immature cells from which more differentiated daughter cells arise. The quintessential stem cell is the fertilized egg. It is totipotent, meaning that all cells in the body may be derived from it. Pluripotent stem cells, such as from the inner cell layer of the blastula of an embryo, give rise to many types of cells, but not all of them. Multipotent stem cells give rise to specialized types of cells, typically in a single organ or system. For example, some bone marrow-derived hematopoetic stem cells are multipotent and give rise to all cell types in blood, but not those of other tissues, such as the liver or lung.
In addition to serving as progenitors for mature differentiated cells, stem cells have the important added capacity for maintenance in culture through clonal expansion. This arises from the fact that stem cells can divide and grow to produce a potentially large supply of identical stem cells in culture. The ability to produce large numbers of cells from a far smaller number of donor cells is a critical advantage for stem cell therapies. In contrast, multiple fetal donor sources must be combined for each non-stem cell transplant, a feature fraught with procurement difficulties and moral disagreement.
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