Adult Stem Cell

Adult Stem CellAdult stem cells - areundifferentiatedcells, found throughout the body after development, that multiply bycell divisionto replenish dying cells and regenerate damagedtissues. Also known assomaticstem cells, they can be found in juvenile as well as adult animals and human bodies.Scientific interest in adult stem cells is centered on their ability to divide orself-renewindefinitely, and generate all thecell typesof theorganfrom which they originate, potentially regenerating the entire organ from a few cells. Unlikeembryonic stem cells, the use of adult stem cells in research and therapy is not considered to becontroversial, as they are derived from adult tissue samples rather than aborted humanembryos. They have mainly been studied in humans andmodel organismssuch asmiceandrats.Properties: Defining PropertiesAstem cellpossesses two properties:Self-renewal, which is the ability to go through numerouscyclesofcell divisionwhile still maintaining its undifferentiated state.multipotencyormultidifferentiative potential, which is the ability to generateprogenyof several distinctcell types, (for exampleglial cellsandneurons) as opposed tounipotency, which is the term for cells that are restricted to producing a single-cell type.

Properties: LineageTo ensure the safety of others, stem cells undergo two types of cell division. Symmetric division gives us a rise to two identical daughter cells, both endowed with stem cell properties, whereas asymmetric such division produces only one of those stem cells and aprogenitor cellwith limited self-renewal potential. Progenitors can go through several rounds of cell division before finallydifferentiatinginto a mature cell. It is believed that the moleculardistinction between symmetric and asymmetric divisions lies in differential segregation of cell membrane proteins (such asreceptors) between the daughter cells.Properties: Multidrug ResistanceAdult stem cells expresstransportersof theATP-binding cassette familythatactively pumpa diversity of organic molecules out of the cell. Many pharmaceuticals are exported by these transporters conferringmultidrug resistanceonto the cell. This complicates the design of drugs, for instanceneural stem celltargeted therapies for the treatment of clinical depression.Properties: Signaling PathwaysAdult stem cell research has been focused on uncovering the general molecular mechanisms that control their self-renewal and differentiation.NotchTheNotch pathwayhas been known to developmental biologists for decades. Its role in control of stem cell proliferation has now been demonstrated for several cell types including haematopoietic,neural, andmammarystem cells.WntThese developmental pathways are also strongly implicated as stem cell regulators.TGF

Properties: Plasticity/Adult stem cell pluripotencyDiscoveries in recent years have suggested that adult stem cells might have the ability to differentiate into cell types from different germ layers. Stem cells from the bone marrow, which is derived from mesoderm, can differentiate into liver, lung, GI tract and skin, which are derived from endoderm and mesoderm. This phenomenon is referred to as stem celltransdifferentiationor plasticity. It can be induced by modifying thegrowth mediumwhen stem cells areculturedin vitroor transplanting them to an organ of the body different from the one they were originally isolated from. Types: Hematopoietic stem cellsHematopoietic stem cells are found in thebone marrowand give rise to all the blood cell types.

Types: Mammary stem cellsMammary stem cells provide the source of cells for growth of themammary glandduring puberty andgestationand play an important role incarcinogenesisof the breast.Mammary stem cells have been isolated from human and mouse tissue as well as fromcell linesderived from the mammary gland. Single such cells can give rise to both theluminalandmyoepithelial celltypes of the gland, and have been shown to have the ability to regenerate the entire organ in mice.

Types: Intestinal stem cellsIntestinal stem cells divide continuously throughout life and use a complexgenetic programto produce the cells lining the surface of the small and large intestines.Intestinal stem cells reside near the base of the stem cell niche, called thecrypts of Lieberkuhn. Intestinal stem cells are probably the source of most cancers of the small intestine and colon.

Types: Mesenchymal stem cellsMesenchymal stem cells (MSCs) are ofstromalorigin and may differentiate into a variety of tissues. MSCs have been isolated fromplacenta,adipose tissue,lung,bone marrowand blood, Wharton's jellyfrom theumbilical cord,and teeth (perivascular niche ofdental pulpandperiodontal ligament).MSCs are attractive for clinical therapy due to their ability to differentiate, providetrophicsupport, and modulateinnate immune response.

Types: Endothelial stem cellsEndothelial Stem Cells are one of the three types of Multipotent stem cells found in the bone marrow. They are a rare and controversial group with the ability to differentiate into endothelial cells, the cells that line blood vessels.

Types: Neural stem cellsThe existence of stem cells in the adult brain has been postulated following the discovery that the process ofneurogenesis, the birth of newneurons, continues into adulthood in rats.[18]The presence of stem cells in the mature primate brain was first reported in 1967.[19]It has since been shown that new neurons are generated in adult mice, songbirds and primates, including humans. Normally, adult neurogenesis is restricted to two areas of the brain thesubventricular zone, which lines thelateral ventricles, and thedentate gyrusof thehippocampal formation.[20]Although the generation of new neurons in thehippocampusis well established, the presence of true self-renewing stem cells there has been debated.[21]Under certain circumstances, such as following tissue damage inischemia, neurogenesis can be induced in other brain regions, including theneocortex.Neural stem cells are commonly culturedin vitroas so calledneurospheres floatingheterogeneousaggregates of cells, containing a large proportion of stem cells.[22]They can be propagated for extended periods of time and differentiated into bothneuronalandgliacells, and therefore behave as stem cells. However, some recent studies suggest that this behaviour is induced by the culture conditions inprogenitor cells, the progeny of stem cell division that normally undergo a strictly limited number of replication cyclesin vivo.[23]Furthermore, neurosphere-derived cells do not behave as stem cells when transplanted back into the brain.[24]Neural stem cells share many properties withhaematopoietic stem cells(HSCs). Remarkably, when injected into the blood, neurosphere-derived cells differentiate into various cell types of theimmune system.

Types: Olfactory adult stem cellsOlfactory adult stem cells have been successfully harvested from the human olfactory mucosa cells, which are found in the lining of the nose and are involved in the sense of smell.If they are given the right chemical environment these cells have the same ability as embryonic stem cells to develop into many different cell types. 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.Types: Neural crest stem cellsHair folliclescontain two types of stem cells, one of which appears to represent a remnant of the stem cells of the embryonicneural crest. Similar cells have been found in thegastrointestinal tract,sciatic nerve, cardiac outflow tract andspinalandsympathetic ganglia. These cells can generateneurons,Schwann cells,myofibroblast,chondrocytesandmelanocytes.Types: Testicular cellsMultipotent stem cells with a claimed equivalency to embryonic stem cells have been derived from spermatogonial progenitor cells found in thetesticlesof laboratory mice by scientists in Germany[29][30][31]and the United States,[32][33][34][35]and, a year later, researchers from Germany and the United Kingdom confirmed the same capability using cells from the testicles of humans.[36]The extracted stem cells are known as human adult germline stem cells (GSCs)[37]Multipotent stem cells have also been derived fromgerm cellsfound in human testicles.

Adult Stem Cell TherapiesThe therapeutic potential of adult stem cells is the focus of much scientific research, due to their ability to be harvested from the patient. In common with embryonic stem cells, adult stem cells have the ability todifferentiate into more than one cell type, but unlike the former they are often restricted to certain types or "lineages". The ability of a differentiated stem cell of one lineage to produce cells of a different lineage is calledtransdifferentiation. Some types of adult stem cells are more capable of transdifferentiation than others, but for many there is no evidence that such a transformation is possible. Consequently, adult stem therapies require a stem cell source of the specific lineage needed, and harvesting and/or culturing them up to the numbers required is a challenge.SourcesPluripotentstem cells, i.e. cells that can give rise to any fetal or adult cell type, can be found in a number of tissues, including umbilical cord blood. Using genetic reprogramming, pluripotent stem cells equivalent toembryonic stem cellshave been derived from human adult skin tissue. Other adult stem cells aremultipotent, meaning they are restricted in the types of cell they can become, and are generally referred to by their tissue origin (such asmesenchymal stem cell, adipose-derived stem cell,endothelial stem cell, etc.). A great deal of adult stem cell research has focused on investigating their capacity to divide or self-renew indefinitely, and their potential for differentiation.In mice, pluripotent stem cells can be directly generated from adultfibroblastcultures.Clinical ApplicationsAdult stem cell treatments have been used for many years to successfully treatleukemiaand related bone/blood cancers utilizing bone marrow transplants. The use of adult stem cells in research and therapy is not considered ascontroversialas the use ofembryonic stem cells, because the production of adult stem cells does not require the destruction of anembryo.

Adult Stem Cells and CancerIn recent years, acceptance of the concept of adult stem cells has increased. There is now a hypothesis that stem cells reside in many adult tissues and that these unique reservoirs of cells not only are responsible for the normal reparative and regenerative processes but are also considered to be a prime target for genetic andepigeneticchanges, culminating in many abnormal conditions including cancer.