Instruct: Given a question, retrieve relevant documents that can help answer the question.
Query: Where are stem cells located in the adult body?
Adult stem cells are undifferentiated cells, found throughout the body after development, that multiply by cell division to replenish dying cells and regenerate damaged tissues. Also known as somatic stem cells (from Greek Σωματικóς, meaning of the body), they can be found in juvenile as well as adult animals and humans, unlike embryonic stem cells.
On July 24, 2009, the first publication of a successful breakthrough in Stem cell research was released, where Chinese researchers from the Shanghai Stem Cell Institute, led by Professor Fanyi Zeng, successfully reprogrammed adult stem cells to be able to differentiate into any body cell, as in the case with standard embryonic stem cells, the cells in question known as "induced pluripotent stem cells" (IPS cells). The IPS cells were obtained by genetically reprogramming the skin cells of mice to acts like embryonic stem cells, which then were able to differentiate into all forms of body tissue. The researchers have managed to use the IPS cells to create every type of cell in a mouse, creating entire mouse pups using the technique. This is the first time the technique has been used to make an entire mouse.
In the label-retention assay, the goal is to mark 'immortal' or parental DNA strands with a DNA label such as tritiated thymidine or bromodeoxyuridine (BrdU). These types of DNA labels will incorporate into the newly synthesized DNA of dividing cells during S phase. A pulse of DNA label is given to adult stem cells under conditions where they have not yet delineated an immortal DNA strand. During these conditions, the adult stem cells are either dividing "symmetrically" (thus with each division a new 'immortal' strand is determined and in at least one of the stem cells the immortal DNA strand will be marked with DNA label), or the adult stem cells have "not yet been determined" (thus their precursors are dividing symmetrically, and once they differentiate into adult stem cells and choose an 'immortal' strand, the 'immortal strand' will already have been marked). Experimentally, adult stem cells are undergoing symmetric divisions during growth and after wound healing, and are not yet determined at neonatal stages. Once the immortal DNA strand is labelled and the adult stem cell has begun or resumed asymmetric divisions, the DNA label is chased out. In symmetric divisions (most mitotic cells), DNA is segregating randomly and the DNA label will be diluted out to levels below detection after five divisions. If, however, cells are using an immortal DNA strand mechanism, then all the labeled DNA will continue to co-segregate with the adult stem cell, and after five (or more) divisions will still be detected within the adult stem cell. These cells are sometimes called label-retaining cells (LRCs).
Stem cell treatments are a type of cell therapy that introduce new cells into adult bodies for possible treatment of cancer, somatic cell nuclear transfer, diabetes, and other medical conditions. Cloning also might be done with stem cells. Stem cells have been used to repair tissue damaged by disease.
Pluripotent stem 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.[65] Using genetic reprogramming, pluripotent stem cells equivalent to embryonic stem cells have been derived from human adult skin tissue.[66][67][68][69][70] Other adult stem cells are multipotent, meaning they are restricted in the types of cell they can become, and are generally referred to by their tissue origin (such as mesenchymal stem cell, adipose-derived stem cell, endothelial stem cell, etc.).[71][72] 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.[73] In mice, pluripotent stem cells can be directly generated from adult fibroblast cultures.[74]
Embryonic stem cells of the inner cell mass are pluripotent, meaning they are able to differentiate to generate primitive ectoderm, which ultimately differentiates during gastrulation into all derivatives of the three primary germ layers: ectoderm, endoderm, and mesoderm. These include each of the more than 220 cell types in the adult human body. Pluripotency distinguishes embryonic stem cells from adult stem cells, which are multipotent and can only produce a limited number of cell types.
In Watt's lab, the researchers study the adult mammalian epidermis, the outermost layer of skin, which is maintained throughout life by the differentiation of stem cells into hair follicles, sebaceous glands, and interfollicular epidermis. Generally, these stem cells remain dormant and do not divide, but proliferation is initiated by damage. Professor Watt's lab and others have discovered many stem cells markers including high expression of 1 integrins, but their goal is to determine whether or not stem cell markers are hard-wired or expressed due to certain environmental cues depending on where the stem cells are located.
Adult stem cells can, however, be artificially reverted to a state where they behave like embryonic stem cells (including the associated DNA repair mechanisms). This was done with mice as early as 2006[43] with future prospects to slow down human aging substantially. Such cells are one of the various classes of induced stem cells.
The use of human stem cells, particularly embryonic stem cells, is controversial. Opponents' objections generally are based on interpretations of religious teachings or ethical considerations. Proponents of stem cell research point out that cells are routinely formed and destroyed in a variety of contexts. Use of stem cells taken from the umbilical cord or parts of the adult body may not provoke controversy.
Stem cells are cells found in all multi-cellular organisms. They were isolated in mice in 1981, and in humans in 1998.[1] In humans there are many types of stem cells, each with varying levels of potency. Potency is a measure of a cell's differentiation potential, or the number of other cell types that can be made from that stem cell. Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of the blastocyst. These stem cells can differentiate into all other cells in the human body and are the subject of much scientific research. However, since they must be derived from early human embryos their production and use in research has been a hotly debated topic as the emt introduce new cells into adult bodies for possible treatment of cancer, diabetes, neurological disorders and other medical conditions. Stem cells have been used to repair tissue damaged by disease or age.[2] Cloning also might be done with stem cells. Pluripotent stem cells can also be derived from Somatic cell nuclear transfer which is a laboratory technique where a clone embryo is created from a donor nucleus. Somatic cell nuclear transfer is also tightly regulated amongst various countries.
Adult stem cells are not able to proliferate and differentiate as effectively "in vitro" as they are able to "in vivo". Adult stem cells can come from many different tissue locations, but it is difficult to isolate them because they are defined by behavior and not surface markers. A method has yet to be developed for clearly distinguishing between stem cells and the differentiated cells surrounding them. However, surface markers can still be used to a certain extent to remove most of the unwanted differentiated cells. Stem cell plasticity is the ability to differentiate across embryonic germ line boundaries. Though, the presence of plasticity has been hotly contested. Some claim that plasticity is caused by heterogeneity among the cells or cell fusion events. Currently, cells can be differentiated across cell lines with yields ranging from 10% to 90% depending on techniques used. More studies need to be done in order to standardize the yield with transdifferentiation. Transdifferentiation of multipotent stem cells is a potential means for obtaining stem cells that are not available or not easily obtained in the adult.
Most progenitors are described as oligopotent. In this point of view, they may be compared to adult stem cells. But progenitors are said to be in a further stage of cell differentiation. They are in the “center” between stem cells and fully differentiated cells. The kind of potency they have depends on the type of their "parent" stem cell and also on their niche. Some progenitor cells were found during research, and were isolated. After their marker was found, it was proven that these progenitor cells could move through the body and migrate towards the tissue where they are needed. Many properties are shared by adult stem cells and progenitor cells.
Hair follicle renewal is governed by Wnt signalling that act upon hair follicle stem cells located in the follicle bulge. Although these cells are well characterised by CD34 and cytokeratin markers, there is a growing body of agreement that LGR5 is a putative hair follicle stem cell marker. LGR5 in conjunction with LRG6, is expressed in a remarkable fashion: LRG6 stem cells maintain the upper sebaceous gland whilst LRG5 stem cells fuel the actual hair follicle shaft upon migration of transit-amplifying cells into the dermal papilla. In between these two distinct populations of stem cells are the multipotent LRG5/6 stem cells that ultimately maintain the epidermal hair follicle in adults.
Stem-cell niche refers to a microenvironment, within the specific anatomic location where stem cells are found, which interacts with stem cells to regulate cell fate. The word 'niche' can be in reference to the "in vivo" or "in vitro" stem-cell microenvironment. During embryonic development, various niche factors act on embryonic stem cells to alter gene expression, and induce their proliferation or differentiation for the development of the fetus. Within the human body, stem-cell niches maintain adult stem cells in a quiescent state, but after tissue injury, the surrounding micro-environment actively signals to stem cells to promote either self-renewal or differentiation to form new tissues. Several factors are important to regulate stem-cell characteristics within the niche: cell–cell interactions between stem cells, as well as interactions between stem cells and neighbouring differentiated cells, interactions between stem cells and adhesion molecules, extracellular matrix components, the oxygen tension, growth factors, cytokines, and the physicochemical nature of the environment including the pH, ionic strength (e.g. Ca concentration) and metabolites, like ATP, are also important. The stem cells and niche may induce each other during development and reciprocally signal to maintain each other during adulthood.
The therapeutic potential of adult stem cells is the focus of much scientific research, due to their ability to be harvested from the parent body that is females during the delivery.[59][60][61] In common with embryonic stem cells, adult stem cells have the ability to differentiate 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 called transdifferentiation. 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.[62][63] Additionally, cues from the immediate environment (including how stiff or porous the surrounding structure/extracellular matrix is) can alter or enhance the fate and differentiation of the stem cells.[64]
In mammals, there are two broad types of stem cells: embryonic stem cells, which are isolated from the inner cell mass of blastocysts, and adult stem cells, which are found in various tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing adult tissues. In a developing embryo, stem cells can differentiate into all the specialized cells—ectoderm, endoderm and mesoderm (see induced pluripotent stem cells)—but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.
Great levels of success and potential have been realized from research using adult stem cells. In early 2009, the FDA approved the first human clinical trials using embryonic stem cells. Only cells from an embryo at the morula stage or earlier are truly totipotent, meaning that they are able to form all cell types including placental cells. Adult stem cells are generally limited to differentiating into different cell types of their tissue of origin. However, some evidence suggests that adult stem cell plasticity may exist, increasing the number of cell types a given adult stem cell can become.
Of the types of adult stem cells have successfully been isolated and identified, only mesenchymal stem cells can successfully be grown in culture for long periods of time. Other adult stem cell types, such as hematopoietic stem cells, are difficult to grow and propagate in vitro. Identifying methods for maintaining hematopoietic stem cells in vitro is an active area of research. Thus, while mesenchymal stem cell lines exist, other types of adult stem cells that are grown in vitro can better be classified as primary cells.
In the label-release assay, the goal is to mark the newly synthesized DNA that is normally passed on to the daughter (non-stem) cell. A pulse of DNA label is given to adult stem cells under conditions where they are dividing "asymmetrically." Under conditions of homeostasis, adult stem cells should be dividing asymmetrically so that the same number of adult stem cells is maintained in the tissue compartment. After pulsing for long enough to label all the newly replicated DNA, the DNA label is chased out (each DNA replication now incorporates unlabeled nucleotides) and the adult stem cells are assayed for loss of the DNA label after two cell divisions. If cells are using a random segregation mechanism, then enough DNA label should remain in the cell to be detected. If, however, the adult stem cells are using an immortal DNA strand mechanism, they are obligated to retain the unlabeled 'immortal' DNA, and will release all the newly synthesized labeled DNA to their differentiating daughter cells in two divisions.
Most adult stem cells are lineage-restricted (multipotent) and are generally referred to by their tissue origin (mesenchymal stem cell, adipose-derived stem cell, endothelial stem cell, dental pulp stem cell, etc.).[37][38] Muse cells (multi-lineage differentiating stress enduring cells) are a recently discovered pluripotent stem cell type found in multiple adult tissues, including adipose, dermal fibroblasts, and bone marrow. While rare, muse cells are identifiable by their expression of SSEA-3, a marker for undifferentiated stem cells, and general mesenchymal stem cells markers such as CD105. When subjected to single cell suspension culture, the cells will generate clusters that are similar to embryoid bodies in morphology as well as gene expression, including canonical pluripotency markers Oct4, Sox2, and Nanog.[39]
With the increasing demand of human adult stem cells for both research and clinical purposes (typically 1–5 million cells per kg of body weight are required per treatment) it becomes of utmost importance to bridge the gap between the need to expand the cells in vitro and the capability of harnessing the factors underlying replicative senescence. Adult stem cells are known to have a limited lifespan in vitro and to enter replicative senescence almost undetectably upon starting in vitro culturing.[43]
The use of adult stem cells in research and therapy is not as controversial as the use of embryonic stem cells, because the production of adult stem cells does not require the destruction of an embryo. Additionally, in instances where adult stem cells are obtained from the intended recipient (an autograft), the risk of rejection is essentially non-existent. Consequently, more US government funding is being provided for adult stem cell research.[42]
Facial autologous muscular injections are procedures that target deep multiplane autologous fat. FAMI consists of an autograft of adult stem cells in order to rebuild muscles and bones. The adult stem cells are taken from an area of fatty tissue in a separate area of the body, typically from the patient's hip, knee or abdomen. The collected fat and stem cells are concentrated and refined. The stem cells are then re-injected into the muscles of the face. Specifically, the injections are made in close proximity or within the muscles responsible for facial expression.
Adult stem cell treatments have been used for many years to successfully treat leukemia and related bone/blood cancers utilizing bone marrow transplants.[47] The use of adult stem cells in research and therapy is not considered as controversial as the use of embryonic stem cells, because the production of adult stem cells does not require the destruction of an embryo.
Adult stem cells, also called somatic (from Greek σωματικóς, "of the body") stem cells, are stem cells which maintain and repair the tissue in which they are found.[26] They can be found in children, as well as adults.[27]