Stem cells fulfill this function, especially after catastrophic cell loss. A stem cell is a unique type of cell that is defined by two properties. It can give rise to other functional cell types, such as nerve cells or skin cells, through differentiation. And it can renew itself—i.e., give rise to more stem cells, which can, in turn, differentiate to form the functional cells of an organ. A stem cell is somewhat akin to a grandfather that continues to produce children, grandchildren, and great-grandchildren, generation upon generation, without ever losing his own reproductive fecundity. It is
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