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How do adult stem cells replace worn-out or damaged cells?

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Diagram showing the locations of neural stem cells in brain regions such as the hippocampus and ventricles, and hematopoietic stem cells in the bone marrow and blood, with labeled illustrations of the brain, spinal cord, circulatory system, and skeleton. — stem cell Stem cells of neural and hematopoietic origin have tremendous potential in the development of therapies for certain diseases, such as diabetes and Parkinson disease. Neural stem cells occur in the spinal cord and in specific regions of the brain, and hematopoietic stem cells occur in the blood and bone marrow.

Adult stem cells replace worn-out or damaged cells through a process of self-renewal and cell differentiation. These cells reside in specialized niches within tissues, where they receive signals that keep them alive and active.123

The replacement process typically involves asymmetric cell division, where a stem cell divides to produce one daughter cell that remains a stem cell—ensuring the population persists—and a second daughter cell that becomes a progenitor or "transit-amplifying" cell. These progenitor cells divide a finite number of times, becoming increasingly specialized with each round, until they reach a state of terminal differentiation to replace the specific damaged or worn-out cells in the tissue.31

In tissues that undergo constant turnover, such as the skin, small intestine, and bone marrow, this repair mechanism is continuous. In other tissues that show minimal division, such as the liver, regeneration may occur through the participation of existing cells when injury triggers the need for repair.32