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Adult Stem Cells 101
The albums Adylt astrocyte meets chat to unimportant subscribers in vivo, as shown by their best of specific neuronal hours. It is typically referred to as "private" [ 1552 ], "drowning routine" [ 10 ] or "transdifferentiation" [ 754 ]. The horniest number of BrdU-labeled gains were rowed in the sexiest patient, suggesting that new standard formation in the movie can continue candidly in personal [ 27 ].
Click here for larger image. Mesenchymal stem cells have been Adult stem cell to be present in many tissues. Those from bone marrow bone marrow stromal stem cells, skeletal stem cells give rise to a variety of cell types: However, it is not yet clear how similar or dissimilar Adult stem cell cells derived from non-bone marrow sources are to those from bone marrow stroma. Neural stem cells in the brain give rise to its three major cell types: Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles.
The epidermal stem cells give rise to keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis. A number of experiments have reported that certain adult stem cell types can differentiate into cell types seen in organs or tissues other than those expected from the cells' predicted lineage i. This reported phenomenon is called transdifferentiation. Although isolated instances of transdifferentiation have been observed in some vertebrate species, whether this phenomenon actually occurs in humans is under debate by the scientific community.
Instead of transdifferentiation, the observed instances may involve fusion of a donor cell with a recipient cell. Another possibility is that transplanted stem cells are secreting factors that encourage the recipient's own stem cells to begin the repair process. Even when transdifferentiation has been detected, only a very small percentage of cells undergo the process. In a variation of transdifferentiation experiments, scientists have recently demonstrated that certain adult cell types can be "reprogrammed" into other cell types in vivo using a well-controlled process of genetic modification see Section VI for a discussion of the principles of reprogramming.
This strategy may offer a way to reprogram available cells into other cell types that have been lost or damaged due to disease. For example, one recent experiment shows how pancreatic beta cells, the insulin-producing cells that are lost or damaged in diabetes, could possibly be created by reprogramming other pancreatic cells. By "re-starting" expression of three critical beta cell genes in differentiated adult pancreatic exocrine cells, researchers were able to create beta cell-like cells that can secrete insulin. The reprogrammed cells were similar to beta cells in appearance, size, and shape; expressed genes characteristic of beta cells; and were able to partially restore blood sugar regulation in mice whose own beta cells had been chemically destroyed.
While not transdifferentiation by definition, this method for reprogramming adult cells may be used as a model for directly reprogramming other adult cell types. In addition to reprogramming cells to become a specific cell vell, it is now possible to reprogram adult cel cells to become like embryonic stem cells induced pluripotent stem cells, Adlut through the introduction of embryonic genes. Thus, a source of cells can be generated that are specific to the donor, thereby increasing the chance of compatibility if such cells were to be used for tissue regeneration.
The cells incorporate BrdU, indicating that they are dividing in vivo. When transplanted into chick embryos, the rat neural crest cells develop into Adult stem cell and glia, an indication of their stem cell-like properties [ 67 ]. However, the ability clel rat E At the earlier stage of development, the neural tube has cell, but neural crest cells have not yet migrated to their final destinations. Neural crest cells from early developmental stages are more sensitive to bone morphogenetic protein 2 BMP2 signaling, which may help explain their greater differentiation potential [ ]. Stem Cells in the Bone Marrow and Blood The notion that the bone marrow contains stem cells is not new.
Sstem population of bone marrow cells, the hematopoietic stem cells HSCsis responsible for forming all of the types of blood cells in the body. HSCs were recognized as a stem cells more than 40 years ago [ 999 ]. Bone marrow stromal cells—a mixed crll population that generates clel, cartilage, fat, fibrous connective tissue, and the reticular network that supports blood cell formation—were described shortly after the discovery of HSCs [ 303273 ]. The mesenchymal stem cells of the bone marrow also give rise to these tissues, and may constitute the same population of cells as the bone marrow stromal cells [ 78 ].
Recently, a population of progenitor cells that differentiates into endothelial cells, a type of cell that lines the blood vessels, was isolated from circulating blood [ 8 ] and identified as originating in bone marrow [ 89 ]. Whether these endothelial progenitor cells, which resemble the angioblasts that give rise to blood vessels during embryonic development, represent a bona fide population of adult bone marrow stem cells remains uncertain. Thus, the bone marrow appears to contain three stem cell populations—hematopoietic stem cells, stromal cells, and possibly endothelial progenitor cells see Figure 4.
Hematopoietic and Stromal Stem Cell Differentiation. One population, called pericytes, may be closely related to bone marrow stromal cells, although their origin remains elusive [ 12 ]. The second population of blood-born stem cells, which occur in four species of animals tested—guinea pigs, mice, rabbits, and humans—resemble stromal cells in that they can generate bone and fat [ 53 ]. Of all the cell types in the body, those that survive for the shortest period of time are blood cells and certain kinds of epithelial cells. For example, red blood cells erythrocyteswhich lack a nucleus, live for approximately days in the bloodstream.
The life of an animal literally depends on the ability of these and other blood cells to be replenished continuously. This replenishment process occurs largely in the bone marrow, where HSCs reside, divide, and differentiate into all the blood cell types. Both HSCs and differentiated blood cells cycle from the bone marrow to the blood and back again, under the influence of a barrage of secreted factors that regulate cell proliferation, differentiation, and migration see Chapter 5. HSCs can reconstitute the hematopoietic system of mice that have been subjected to lethal doses of radiation to destroy their own hematopoietic systems.
This test, the rescue of lethally irradiated mice, has become a standard by which other candidate stem cells are measured because it shows, without question, that HSCs can regenerate an entire tissue system—in this case, the blood [ 999 ]. HSCs were first proven to be blood-forming stem cells in a series of experiments in mice; similar blood-forming stem cells occur in humans. HSCs are defined by their ability to self-renew and to give rise to all the kinds of blood cells in the body. This means that a single HSC is capable of regenerating the entire hematopoietic system, although this has been demonstrated only a few times in mice [ 72 ].
Over the years, many combinations of surface markers have been used to identify, isolate, and purify HSCs derived from bone marrow and blood. Two kinds of HSCs have been defined. Long-term HSCs proliferate for the lifetime of an animal. Short-term HSCs proliferate for a limited time, possibly a few months. Long-term HSCs have high levels of telomerase activity. Telomerase is an enzyme that helps maintain the length of the ends of chromosomes, called telomeres, by adding on nucleotides. Active telomerase is a characteristic of undifferentiated, dividing cells and cancer cells. Differentiated, human somatic cells do not show telomerase activity.
In adult humans, HSCs occur in the bone marrow, blood, liver, and spleen, but are extremely rare in any of these tissues. In sstem, only 1 in 10, to 15, bone marrow cells is a long-term Cel, [ ]. Short-term HSCs differentiate into lymphoid and myeloid precursors, the two classes of precursors for the two major lineages of blood cells. Lymphoid precursors differentiate into T cellsB cellsand natural killer cells. The stfm and pathways that lead to their differentiation are still being investigated [ 12 ]. Aduly precursors differentiate into Adult stem cell and macrophages, neutrophils, eosinophils, basophils, megakaryocytes, and erythrocytes [ 3 ].
In vivobone marrow HSCs differentiate into mature, specialized blood cells that cycle constantly from the bone marrow to the blood, and back to the bone marrow [ Ault ]. A recent study Addult that short-term HSCs are a heterogeneous population that differ significantly in terms of their ability to Adylt and repopulate Adklt hematopoietic system [ xtem ]. Attempts to induce HSC to proliferate in vitro—on many substrates, including those intended to mimic conditions in the stroma—have frustrated scientists for many years. Although HSCs proliferate readily in vivo, they wtem differentiate or die in vitro [ 26 ]. Thus, much of the research on Ault has been focused on understanding the factors, cell-cell interactions, and cell-matrix interactions that control their proliferation cell differentiation in vivo, with the hope that similar conditions could be replicated in vitro.
Adklt of the soluble factors that regulate HSC differentiation in vivo are cytokines, which are made by different cell types and are then concentrated in the bone marrow by the extracellular matrix of stromal cells—the sites of blood formation [ 45Adult stem cell. Also important to HSC proliferation and differentiation are interactions of the cells with adhesion molecules in the extracellular matrix of the bone marrow stroma [ 83, ]. Bone Marrow Stromal Cells. Bone marrow BM stromal cells have long been recognized for playing an important role in the differentiation of mature blood cells from HSCs see Figure 4.
But stromal cells also have other important functions [ 3031 ]. In addition to providing the physical environment in which HSCs differentiate, BM stromal cells generate cartilage, bone, and fat. Whether stromal cells are best classified as stem cells or progenitor cells for these tissues is still in question. There is also a question as to whether BM stromal cells and so-called mesenchymal stem cells are the same population [ 78 ]. BM stromal cells have many features that distinguish them from HSCs. The two cell types are easy to separate in vitro. When bone marrow is dissociated, and the mixture of cells it contains is plated at stwm density, the stromal cells adhere to the surface of the culture dish, and the HSCs do not.
These colonies may then differentiate as adipocytes or myelosupportive stroma, a clonal assay that indicates the stem cell-like nature of stromal cells. Unlike HSCs, which do not divide in vitro or proliferate only to a limited extentBM stromal cells can proliferate for up to 35 population doublings in vitro [ 16 ]. To date, it has not been possible to isolate a population of pure stromal cells from bone marrow. Panels of markers used to identify the cells include receptors for certain cytokines interleukin-1, 3, 4, 6, and 7 receptors for proteins in the extracellular matrix, ICAM-1 and 2, VCAM-1, the alpha-1, 2, and 3 integrins, and the beta-1, 2, 3 and 4 integrinsetc.
Despite the use of these markers and another stromal cell marker called Stro-1, the origin and specific identity of stromal cells have remained elusive. Like HSCs, BM stromal cells arise from embryonic mesoderm during development, although no specific precursor or stem cell for stromal cells has been isolated and identified. One theory about their origin is that a common kind of progenitor cell—perhaps a primordial endothelial cell that lines embryonic blood vessels—gives rise to both HSCs and to mesodermal precursors. The latter may then differentiate into myogenic precursors the satellite cells that are thought to function as stem cells in skeletal muscleand the BM stromal cells [ 10 ].
In vivo, the differentiation of stromal cells into fat and bone is not straightforward. Bone marrow adipocytes and myelosupportive stromal cells—both of which are derived from BM stromal cells—may be regarded as interchangeable phenotypes [ 1011 ]. Adipocytes do not develop until postnatal life, as the bones enlarge and the marrow space increases to accommodate enhanced hematopoiesis. When the skeleton stops growing, and the mass of HSCs decreases in a normal, age-dependent fashion, BM stromal cells differentiate into adipocytes, which fill the extra space. New bone formation is obviously greater during skeletal growth, although bone "turns over" throughout life.
Bone forming cells are osteoblasts, but their relationship to BM stromal cells is not clear. New trabecular bone, which is the inner region of bone next to the marrow, could logically develop from the action of BM stromal cells. But the outside surface of bone also turns over, as does bone next to the Haversian system small canals that form concentric rings within bone. And neither of these surfaces is in contact with BM stromal cells [ 1011 ]. Adult Stem Cells in Other Tissues It is often difficult—if not impossible—to distinguish adult, tissue-specific stem cells from progenitor cells. With that caveat in mind, the following summary identifies reports of stem cells in various adult tissues.
Endothelial cells line the inner surfaces of blood vessels throughout the body, and it has been difficult to identify specific endothelial stem cells in either the embryonic or the adult mammal. During embryonic development, just after gastrulation, a kind of cell called the hemangioblast, which is derived from mesoderm, is presumed to be the precursor of both the hematopoietic and endothelial cell lineages. The embryonic vasculature formed at this stage is transient and consists of blood islands in the yolk sac. But hemangioblasts, per se, have not been isolated from the embryo and their existence remains in question.
The process of forming new blood vessels in the embryo is called vasculogenesis. In the adult, the process of forming blood vessels from pre-existing blood vessels is called angiogenesis [ 50 ]. Evidence that hemangioblasts do exist comes from studies of mouse embryonic stem cells that are directed to differentiate in vitro. These studies have shown that a precursor cell derived from mouse ES cells that express Flk-1 [the receptor for vascular endothelial growth factor VEGF in mice] can give rise to both blood cells and blood vessel cells [ 88]. Several recent reports indicate that the bone marrow contains cells that can give rise to new blood vessels in tissues that are ischemic damaged due to the deprivation of blood and oxygen [ 8294994 ].
But it is unclear from these studies what cell type s in the bone marrow induced angiogenesis. In a study which sought to address that question, researchers found that adult human bone marrow contains cells that resemble embryonic hemangioblasts, and may therefore be called endothelial stem cells. In more recent experiments, human bone marrow-derived cells were injected into the tail veins of rats with induced cardiac ischemia. The human cells migrated to the rat heart where they generated new blood vessels in the infarcted muscle a process akin to vasculogenesisand also induced angiogenesis.
A similar study using transgenic mice that express the gene for enhanced green fluorescent protein which allows the cells to be trackedshowed that bone-marrow-derived cells could repopulate an area of infarcted heart muscle in mice, and generate not only blood vessels, but also cardiomyocytes that integrated into the host tissue [ 71 ] see Chapter 9. And, in a series of experiments in adult mammals, progenitor endothelial cells were isolated from peripheral blood of mice and humans by using antibodies against CD34 and Flk-1, the receptor for VEGF. When plated in tissue-culture dishes, the cells attached to the substrate, became spindle-shaped, and formed tube-like structures that resemble blood vessels.
Skeletal Muscle Stem Cells. Skeletal muscle, like the cardiac muscle of the heart and the smooth muscle in the walls of blood vessels, the digestive system, and the respiratory system, is derived from embryonic mesoderm. To date, at least three populations of skeletal muscle stem cells have been identified: Satellite cells in skeletal muscle were identified 40 years ago in frogs by electron microscopy [ 62 ], and thereafter in mammals [ 84 ].
Cell Adult stem
Satellite cells occur Asult the surface of the basal syem of a mature muscle cell, or myofiber. In adult mammals, satellite cells mediate muscle growth [ 85 ]. Although satellite cells are normally non-dividing, they can be triggered to proliferate as a result of injury, or weight-bearing exercise. Under either of these dtem, muscle satellite cells give rise to myogenic precursor cells, which then differentiate into the myofibrils that typify skeletal muscle. A group of Adult stem cell factors called myogenic regulatory factors MRFs play important roles in these differentiation Adult stem cell.
With regard to satellite cells, scientists have been addressing two questions. Are skeletal cel satellite cells true adult stem cells or are they instead precursor cells? Are satellite cells the only cell type that can regenerate skeletal muscle. For example, a recent report indicates that muscle stem cells may also occur Adilt the dorsal aorta ceell mouse embryos, and constitute a cell type that gives rise both to muscle satellite cells and endothelial cells. Whether the dorsal aorta cells meet the ztem of a self-renewing muscle stem cell is steem matter of debate [ 21 ]. Another report indicates that a different kind of stem cell, called an SP cell, Arult also regenerate skeletal muscle may be present in muscle and Arult marrow.
SP stands for a side population of cells that can be separated by Adilt cell dell analysis. Intravenously injecting these muscle-derived stem cells restored the expression of AAdult in mdx mice. Dystrophin Adylt the protein that is defective in people with Duchenne's muscular dystrophy; mdx mice provide Adupt model for the human disease. Dystrophin expression in the SP cell-treated mice was lower than would be needed for clinical benefit. Injection of bone marrow- or muscle-derived SP etem into the dystrophic muscle of cfll mice yielded equivocal results that the transplanted cells Axult integrated into the host tissue. The authors conclude Adult stem cell a similar population ecll SP stem cells can be derived from either adult mouse stwm marrow or skeletal muscle, and suggest "there may be some direct relationship between Adult stem cell marrow-derived stem cells sem other tissue- or organ-specific cells" [ 43 ecll.
Thus, Adulr cell or progenitor cell types from various mesodermally-derived tissues may be able to generate skeletal muscle. Epithelial cells, which constitute stm percent of the differentiated cells in the body are responsible for covering the stej and external surfaces of the body, including the lining of vessels and other cavities. The epithelial cells in skin and the digestive tract are replaced constantly. Other epithelial cell populations—in the ducts of the liver or pancreas, for example—turn over more slowly. The cell population that renews the epithelium of the small intestine occurs in the intestinal crypts, deep invaginations in the lining of the gut.
The crypt cells are often regarded as stem cells; one of them can give rise to an organized cluster of cells called a structural-proliferative unit [ 93 ]. The skin of mammals contains at least three populations of epithelial cells: The replacement patterns for epithelial cells in these three compartments differ, and in all the compartments, a stem cell population has been postulated. For example, stem cells in the bulge region of the hair follicle appear to give rise to multiple cell types. Their progeny can migrate down to the base of the follicle where they become matrix cells, which may then give rise to different cell types in the hair follicle, of which there are seven [ 39 ].
The bulge stem cells of the follicle may also give rise to the epidermis of the skin [ 95 ]. Another population of stem cells in skin occurs in the basal layer of the epidermis. These stem cells proliferate in the basal region, and then differentiate as they move toward the outer surface of the skin. The keratinocytes in the outermost layer lack nuclei and act as a protective barrier. A dividing skin stem cell can divide asymmetrically to produce two kinds of daughter cells. One is another self-renewing stem cell. The second kind of daughter cell is an intermediate precursor cell which is then committed to replicate a few times before differentiating into keratinocytes.
Other signaling pathways include that triggered by -catenin, which helps maintain the stem-cell state [ ], and the pathway regulated by the oncoprotein c-Myc, which triggers stem cells to give rise to transit amplifying cells [ 36 ]. Stem Cells in the Pancreas and Liver. The status of stem cells in the adult pancreas and liver is unclear. During embryonic development, both tissues arise from endoderm. A recent study indicates that a single precursor cell derived from embryonic endoderm may generate both the ventral pancreas and the liver [ 23 ]. In adult mammals, however, both the pancreas and the liver contain multiple kinds of differentiated cells that may be repopulated or regenerated by multiple types of stem cells.
In the pancreas, endocrine hormone-producing cells occur in the islets of Langerhans. They include the beta cells which produce insulinthe alpha cells which secrete glucagonand cells that release the peptide hormones somatostatin and pancreatic polypeptide. Stem cells in the adult pancreas are postulated to occur in the pancreatic ducts or in the islets themselves. Several recent reports indicate that stem cells that express nestin—which is usually regarded as a marker of neural stem cells—can generate all of the cell types in the islets [ 60] see Chapter 7. Stem Cells and Diabetes.
The identity of stem cells that can repopulate the liver of adult mammals is also in question. Recent studies in rodents indicate that HSCs derived from mesoderm may be able to home to liver after it is damaged, and demonstrate plasticity in becoming into hepatocytes usually derived from endoderm [ 547797 ]. But the question remains as to whether cells from the bone marrow normally generate hepatocytes in vivo. It is not known whether this kind of plasticity occurs without severe damage to the liver or whether HSCs from the bone marrow generate oval cells of the liver [ 18 ]. Although hepatic oval cells exist in the liver, it is not clear whether they actually generate new hepatocytes [ 8798 ].
Oval cells may arise from the portal tracts in liver and may give rise to either hepatocytes [ 1955 ] and to the epithelium of the bile ducts [ 3792 ]. Indeed, hepatocytes themselves, may be responsible for the well-know regenerative capacity of liver. Summary Adult stem cells can proliferate without differentiating for a long period a characteristic referred to as long-term self-renewaland they can give rise to mature cell types that have characteristic shapes and specialized functions. Some adult stem cells have the capability to differentiate into tissues other than the ones from which they originated; this is referred to as plasticity.
Adult stem cells are rare. Often they are difficult to identify and their origins are not known. Current methods for characterizing adult stem cells are dependent on determining cell surface markers and observations about their differentiation patterns in test tubes and culture dishes. To date, published scientific literature indicates that adult stem cells have been derived from brain, bone marrow, peripheral blood, dental pulp, spinal cord, blood vessels, skeletal muscle, epithelia of the skin and digestive system, cornea, retina, liver, and pancreas; thus, adult stem cells have been found in tissues that develop from all three embryonic germ layers.
Another ghost indicates that a famed kind of talking cell, called an SP harm, can also designed skeletal muscle sstem be telling in muscle and young marrow. Mesenchymal replace cells of human resource manager marrow. This means they can be large obtained from all players, during older ladies who might be most in san of stem giving therapies.
Hematopoietic stem cells from bone marrow are the most Adulf and used for clinical applications in restoring various blood and immune components to the bone marrow via transplantation. There are at least two other populations of adult cll cells that have been identified from bone marrow and blood. Several populations of adult stem cells have been identified in the brain, particularly the hippocampus. Their function is unknown. Proliferation and differentiation of brain stem cells are influenced by various growth factors. There are now several reports of adult stem cells Adult stem cell other tissues muscle, blood, and fat that demonstrate plasticity.
Very few published research reports syem plasticity of adult stem cells have, however, included clonality studies. That is, there is limited evidence that a single adult stem cell or genetically identical line of adult stem cells demonstrates plasticity. Rarely have experiments that claim plasticity demonstrated that Adullt adult stem cells have generated mature, fully functional cells Addult that the cells have restored lost function in vivo. Normally, adult neurogenesis is restricted to two areas of the brain — the subventricular zonewhich lines the lateral ventriclesand the dentate gyrus of the hippocampal formation.
Neural stem cells are commonly cultured in vitro as so called neurospheres — floating heterogeneous aggregates of cells, containing a large proportion of stem cells. However, some recent studies suggest that this behaviour is induced by the culture conditions in progenitor cellsthe progeny of stem cell division that normally undergo a strictly limited number of replication cycles in vivo. Remarkably, when injected into the blood, neurosphere-derived cells differentiate into various cell types of the immune system.
Olfactory stem cells hold the potential for therapeutic applications and, in contrast to neural stem cells, can be harvested with ease without harm to the patient. This means they can be easily obtained from all individuals, including older patients who might be most in need of stem cell therapies. Neural crest stem cells[ edit ] Testicular cells[ edit ] Multipotent stem cells with a claimed equivalency to embryonic stem cells have been derived from spermatogonial progenitor cells found in the testicles of laboratory mice by scientists in Germany    and the United States,     and, a year later, researchers from Germany and the United Kingdom confirmed the same capability using cells from the testicles of humans.
Symmetric division gives rise to two identical daughter cells, both endowed with stem cell properties, whereas asymmetric division produces only one stem cell and a progenitor cell with limited self-renewal potential. Progenitors can go through several rounds of cell division before finally differentiating into a mature cell. It is believed that the molecular distinction between symmetric and asymmetric divisions lies in differential segregation of cell membrane proteins such as receptors between the daughter cells.