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Blood Adult Stem Cell: Stem and Progenitor Cells in Adults
Blood Adult Stem Cell: Stem and Progenitor Cells in Adults
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Blood Adult Stem Cell: Stem and Progenitor Cells in Adults
Adult stem cells are defined as undifferentiated, self-renewing cells that can further differentiate into needed cell types. Maintenance and restoration of tissue types necessitates these stem cells, especially in adult organisms that accrue tissue damage over time. Among the major adult stem cell types are hematopoietic, mesenchymal, and endothelial cells. These cell types occupy a major portion of adult stem cell research, as the actual origins of these cells are not yet fully understood. Even so, exploitation of adult stem cell technology has already led to years of successful bone marrow (adult hematopoietic stem cells) transplant, and paved the way for clinical testing of stem cell therapy. Controlled differentiation of these adult stem cells may hold the key for advanced stem cell transplantation.
Multi-Potential Hematopoietic Stem Cells
Hematopoietic Stem Cells (HSCs) are highly undifferentiated, and can become any kind of blood cell. They are thus defined as multi-potent stem cells. Due to the high demand for both red and white blood cells, HSCs must frequently undergo self-renewal and differentiation. Self-renewal occurs at a pace such that the HSC pool maintains a constant approximate size. Differentiation removes cells from the HSC pool and creates other cell types in variable quantities.
HSCs can immediately undergo two differentiated fates, into the common myeloid progenitor cells, or the common lymphoid progenitor cell. Erythrocytes (RBCs) and thrombocytes (platelets) are among the differentiated myeloid cell types. B and T lymphocytes are among the further differentiated lymphoid cell types. Many factors influence the fate of a differentiating HSC, including hormone and protein signaling. HSCs may also choose to undergo apoptosis or programmed cell death in the event that their presence is unneeded or harmful. The actual signaling system that causes apoptosis in these cells remains a mystery. It was found, however, that the protein BCL-2 promotes life in HSCs and thus helps prevent apoptosis. Lastly, HSCs may leave bone marrow and enter the bloodstream, now able to travel across the body. It is noted, however, that many of the cells that leave the marrow eventually return, for reasons that are still unknown. Research has shown that many of these migrating cells are non-dividing, or at least have limited self-renewing potential.
In human adults, a comparatively high concentration of HSCs is found in the bone marrow, especially in the epiphysis (ends) of long bones. Current hematopoietic stem cell transplantation typically utilizes the pelvic bone as the source of bone marrow. It is also known that HSCs can be found in circulating blood, though in a much smaller quantity. As it offers a less invasive procedure, research is being done seeking to use peripheral blood as a source of HSCs for transplantation. With less significance to adults, it is known that HSCs are found abundantly in umbilical cord blood. A growing field of research seeks to find uses for umbilical cord-extracted HSCs in adults.
While a defining characteristic of HSCs is self-renewal, these cells seem to be more able to divide earlier in life. Research has linked progressive telomere shortening with reduced self-renewal capacity. Thus, aged HSCs have a lower ability to self-renew and commonly adopt other fates (apoptosis, differentiation, migration). This is coupled with the fact that self-renewal in adults is a slow process to begin with. The presence of HSCs as found in mouse bone marrow is approximately 1 in 10,000 blood cells. This poses a significant challenge to researchers seeking to work with large quantities of cells.
Progenitor Cells
A progenitor cell is similar to a stem cell in that it has a variety of fates it can adopt, thus also making it multi-potent. It differs, however, in that it can self-renew only a selected number of times. By contrast, a HSC can theoretically divide indefinitely. Also unlike HSCs, progenitor cells are biased to further differentiate into target cells, rather than maintain growth or divide. In this regard, a progenitor cell is more like an undifferentiated intermediate cell type between HSCs and target cells
The common myeloid progenitor cell is still undifferentiate, and may adopt a wide variety of cell types. These progenitor cells have the capacity to become megakaryocytes, erythrocytes, mast cells, and myeloblasts. Megakaryocytes eventually form thrombocytes, while myeloblasts can form basophils, neutrophils, eosinophils, monocytes, and macrophages. Many of these pathways have intermediate cell types, some of which are also capable of self-renewal. Myeloid cells are especially important in that their products supply oxygen to the body (erythrocytes), are integral in the blood clotting cascade (thrombocytes), and form a critical response for the immune system (neutrophils, etc.). These functions continue until death and thus make myeloid progenitor cells critical for normal adult function.
