Academia Arena - Marsland Press
Renal Stem Cell
Hongbao Ma *, ***, Shen Cherng **, Yan Yang ***
* Bioengineering Department, Zhengzhou University, Zhengzhou, Henan 450001, China, mahongbao@zzu., 01186-137-8342-5354
** Department of Electrical Engineering, Chengshiu University, Niaosong, Taiwan 833, China, cherngs@csu.edu.tw; 011886-7731-0606 ext 3423
*** Brookdale University Hospital and Medical Center, Brooklyn, NY 11212, USA, youngjenny2008@, 347-321-7172
Abstract: The renal disease is a common problem in human society. End-stage renal disease is a big heath problem in the United States and in all places of the world. Embryonic stem cells, pluripotent derivatives of the inner cell mass of the blastocyst, are the most primitive cell type likely to find application in cell therapy. Their potential to generate any cell type of the embryo makes them to be the most attractive stem cell therapy. It is possible to introduce stem cells into a damaged adult kidney to aid in repair and regeneration. Transdifferentiation offers the possibility of avoiding complications from immunogenicity of introduced cells by obtaining the more easily accessible stem cells of another tissue type from the patient undergoing treatment, expanding them in vitro, and reintroducing them as a therapeutic agent. Adult stem cells may possess a considerable degree of plasticity in the differentiation. Immunoisolation of heterologous cells by encapsulation creates opportunities for their safe use as a component of implanted or ex vivo devices. [Stem Cell, 2009;x(x):xx-xx]. (ISSN 1545-4570).
Keywords: stem cell; renal; kidney; disease; treatment
1. Introduction
Stem cells could develop into nearly all cell types in the animal body during early life and growth. In many tissues stem cells serve as an internal repair system to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell could remain a stem cell or become another type of cell with a more specialized function, such as a nerve cell, blood cell or a sperm cell. Stem cell is the origin of an orgnism’s life (Ma, 2005).
End-stage renal disease (ESRD) is a big heath problem in the United States and it costs more than $30 billion each year on ESRD therapy in this country (Arnold, 2000; Mai et al., 2006; Ross et al., 2006). The patients suffering from acute renal failure (ARF) are even worse. ARF develops predominantly due to the injury and necrosis of renal proximal tubule cells (RPTCs) as a result of ischemic or toxic insult. The cause of death subsequent to ARF is generally the development of systemic inflammatory response syndrome. The disease state arising from renal failure is the result of many factors. It is important to reveal the kidney's role in reclamation of metabolic substrates, synthesis of glutathione, free-radical scavenging enzymes, gluconeogenesis, ammoniagenesis, hormones, growth factors, and the production and regulation of multiple cytokines critical to inflammation and immunological. There is considerable drive to develop improved therapies for renal failure (Mehta et al., 2007). It is estimated that there are over 2 million patients in USA who suffer from end-stage renal disease. About 60,000 patients in the United States are currently on the waiting list for a kidney transplant, and some patients have waited for several years before an appropriate donor can be found. Despite the advances in kidney transplantation, the shortage of donor organs limits treatment for the ESRD patients and requires many patients to remain on dialysis for extended periods of life time. The alternate methods has resulted in rapid progression of new approaches, such as therapeutic cloning and stem cell therapy (Berzoff et al., 2008; Sirmon, 1990).
Chronic kidney disease is increasing at the rate of 6-8% per year in the Unite States. At present, dialysis and transplantation are the common treatment options. However, it is possible to use stem cells and regenerative medicine as the additional choices for kidney disease treatment. Such new treatments might involve induction of repair using endogenous or exogenous stem cells or the reprogramming of the organ to reinitiate development (Hopkins et al. 2009).
In the 20th century, an efficient treatment was given to patients with renal failure through the development of kidney dialysis and transplantation. These techniques have proved successful, but are marred by inflammation and limited organ availability and graft survival due to immune rejection. More recently, hope has been placed in the development of stem cell-based therapies, in which the function of the failing organ is restored by injected multipotent cells. Possible sources for these cells include differentiated embryonic stem (ES) cells and adult renal stem cells, and circulating multipotent cells, such as bone marrow-derived stem cells. Using the patient's own stem cells to repair kidney damage could circumvent the problems of immune rejection and organ availability.
Kidneys are possible to regenerate, which varies among species. Some bony and cartilaginous fish continue to form new nephrons during adult life. Adult mammals cannot form new nephrons but, to a certain extent, tubules and glomeruli may recover structure and function after limited injury such as acute tubular necrosis. Severe or prolonged injury results in replacement of functional parenchyma by scar tissue, i.e. fibrosis, which correlates clinically with the development of renal failure.
An effective treatment of renal disease is renal cell regeneration, or replace of damaged renal cells, and discourage fibrosis. The origins for renal parenchymal cells could be: (1) the re-entry into cell cycle of differentiated cells; (2) direct transdifferentiation of one cell type into another, such as tubular cells into interstitial cells or vice versa; (3) differentiation from stem cells of the kidney or the bone marrow.
Embryonic stem cells are different to adult or tissue-specific stem cells. Embryonic stem cells are the stem cell that can be grown in large numbers in the laboratory and retain the ability to grow into any type of cells including renal, nerve, heart muscle, bone and insulin-producing cells. It is difficult for the tissue-specific adult stem cells to grow in a great number, hard to isolate and are difficult to grow outside the body. Adult stem cells, such as skin and bone marrow stem cells, normally grow into a limited number of cell types (Snykers et al. 2008).
The role of embryonic or adult stem cells, in particular bone marrow-derived stem cells, in regenerating the kidney after injury has been the subject of intensive investigation. Bone marrow-derived stem cells have been shown to give rise to small numbers of most renal cell types, including tubular cells, mesangial cells, podocytes, vascular cells and interstitial cells. Injections of bone marrow-derived cells do improve renal function in many animal models of renal disease. Many stages of nephrogenesis can be studied using cultured embryonic kidneys, but there is no efficient technique available to readily knockdown or overexpress transgenes for rapid evaluation of resulting phenotypes. Embryonic stem cells have unlimited developmental potential and can be manipulated at the molecular genetic level by a variety of methods. ES cell technology may achieve the objective of obtaining a versatile cell culture system in which molecular interventions can be used in vitro and consequences of these perturbations on the normal kidney development program in vivo can be studied (Steenhard et al. 2005).
Stem cells and progenitor cells are necessary for repair and regeneration of injured renal tissue. Infiltrating or resident stem cells can contribute to the replacement of lost or damaged tissue. However, the regulation of circulating progenitor cells is not well understood. Many factors influence the stem cell growth in damaged kidney. For example, low levels of erythropoietin induce mobilization and differentiation of endothelial progenitor cells and erythropoietin ameliorates tissue injury. Full regeneration of renal tissue demands the existence of stem cells and an adequate local milieu, a so-called stem cell niche. It was reported that in the regenerating zone of the shark kidney, stem cells exist that can be induced by loss of renal tissue to form new glomeruli. Stem cell may eventually contribute to novel therapies of the kidney disease (Perin et al. 2008).
Recently researchers used a rat model of chronic renal failure in which one kidney is excised so as to increase the load of the remaining kidney, thus causing a chronic deterioration that resembles the clinical situation of renal failure (Alexandre et al. 2008). In Alexandre’s project, the rats were divided into 4 groups: Group 1 were sham operated and both kidneys left in place; Group 2 had a kidney removed but were not administered cells; Group 3 were administered 2x106 lineage negative bone marrow cells on day 15 after one of the kidneys was removed; Group 4 were administered 2x106 lineage negative bone marrow cells on days 15, 30, and 45 after one of the kidneys was removed. They found: (1) Expression of inflammatory cytokines was reduced on day 16 in the kidneys of rats receiving stem cells as compared to rats that were nephrectomized but did not receive cells. (2) On day 60 rats receiving stem cells had decreased proteinuria, glomerulosclerosis, anemia, renal infiltration of immune cells and protein expression of monocyte chemoattractant protein-1, as well as decreased interstitial area. (3) Injured rats had higher numbers of proliferating cells in the kidney, whereas rats receiving stem cells had less. (4) Protein expression of the cyclin-dependent kinase inhibitor p21 and of vascular endothelial growth factor increased after nephrectomy and decreased after stem cell treatment. (5) On day 120, renal function (inulin clearance) was improved in the rats which were administered bone marrow cells compared to controls. This study supports the possibility of using bone marrow cells for various aspects of kidney failure. Other studies have demonstrated that administered stem cells promote kidney repair by secretion of insulin growth factor-1 (Cornelissen et al. 2008).
Bone marrow stromal cells, also known as mesenchymal stem cells or fibroblastic colony-forming units, are multipotent non-hematopoietic stem cells adhering to culture plates (Abdallah and Kassem 2009). Mesenchymal stem cells of the bone marrow have the ability to renew and differentiate themselves into multiple lineages of conjunctive tissues, including bone, cartilage, adipose tissue, tendon, muscle, and bone marrow stroma. Those cells have been first described by Friedenstein et al., who found that mesenchymal stem cells adhere to culture plates, look like in vitro fibroblasts, and build up colonies (Friedenstein et al. 1987).
Bone marrow is the site of hematopoiesis and bone marrow transplant has been successfully used for decades as a means of treating various hematological malignancies in which the recipient hematopoietic compartment is replaced by donor-derived stem cells. Progenitor cells in bone marrow are capable to differentiate into other tissues, such as cardiac tissue. Clinical trials have been conducted demonstrating beneficial effects of bone marrow infusion in cardiac patients. It is believed that injured tissue, whether neural tissue after a stroke, or injured cardiac tissue, has the ability to selectively attract bone marrow stem cells, perhaps to induce regeneration. Bone marrow has therapeutic effect in conditions ranging from liver failure, to peripheral artery disease, and the possibility of using bone marrow stem cells in kidney failure has been relatively understudied (Ma et al. 2009).
Mesenchymal stem cells have been brought to the attention of many researchers, because these cells are of great interest for treating various human diseases. Many studies have isolated mesenchymal stem cells and controlled, in vitro, its differentiation into cartilaginous tissue and bone using specific growth factors, with the objective of using this technology for repairing injured tissues of mesenchymal origin (Xian and Foster 2006; Kurdi and Booz 2007).
2.1 Embryonic Stem Cells (ES cells)
ES cells are pluripotent cells derived from the inner cell mass of blastocysts, and are in theory able to give rise to all the cell types of the body. ES cells can be directed into forming renal progenitor cells, and eventually differentiated renal cells. Ureteric bud epithelial cells and metanephric mesenchymal cells that comprise the metanephric kidney primordium are capable of producing nephrons and collecting ducts through reciprocal inductive interaction. Once these cells are induced from pluripotent ES cells, they have the potential to become powerful tools in the regeneration of kidney tissues. However, there is risk to use stem cells in clinical practice. In vivo injection of ES cells can give rise to teratomas, which are tumours containing cells of all three lineages (ectoderm, endoderm and mesoderm). ES cell-derived teratomas in vivo, renal primordial structures can be detected histochemically, and genes involved in metanephrogenesis are expressed. the potential of ES cells to produce renal primordial duct structures and provides an insight into the regeneration of kidney tissues (Yamamoto et al. 2006). This same potential was reported when ES cells were injected into embryonic mouse kidneys in vitro, and gave rise to ES cell-derived tubules, in this case without forming teratomas (Steenhard et al. 2005). In vitro, transfection of murine ES cells with renal developmental gene Wnt4, as well as the addition of hepatocyte growth factor and activin-A, both promote the formation of renal tubule-like structures, with expression of tubular marker aquaporin-2. Cultured Wnt4-EBs have an ability to differentiate into renal tubular cells; and second, that Wnt4, HGF, and activin A may promote the differentiation of ES cells to renal tubular cells (Kobayashi et al. 2005). The Wnt4-transfected cells can be transplanted into mouse renal cortex, where they also express aquaporin-2 and formed tubular structures. According to Kim et al reported, murine ES cells primed in vitro with retinoic acid, activin-A and BMP-7 (Kim and Dressler 2005), activin-A alone (Vigneau et al. 2007), or BMP-4, differentiate into cells expressing markers of the intermediate mesoderm, early kidney development and/or renal tubule-specific markers (Bruce et al. 2007). After injection of these primed murine ES cells into embryonic kidney cultures, ES cells are incorporated into developing renal tubules, without cell fusion, or into the nephrogenic zone. The primed cells are enriched for renal progenitor cells by FACS and injected in vivo into the kidneys of newborn mice, where they are integrated as proximal tubular cells, without teratoma formation (Vigneau et al. 2007). Human ES cells differentiate in vitro into WT1- and renin-expressing cells following treatment with a combination of specific growth factors (Schuldiner et al. 2000). However, research of the role for ES cells in renal regeneration is still in its infancy (Roufosse and Cook 2008).
2.2 Native Renal Stem Cells and Renal Regeneration
In the embryo, most types of renal parenchymal cells are derived from metanephric mesenchymal cells, which are multipotent and are in addition self-renewing, making them attractive candidates as the stem cells of the embryonic kidney.
In animal models, embryonic metanephroi transplanted into the abdominal cavity of adult animals are colonized by host vasculature, undergo nephrogenesis and produce urine, even if the operation is carried out across species barriers, and with a surprising lack of rejection (Little 2006). Human and porcine embryonic kidney progenitor cells have been isolated and, when injected into mice, can lead to the formation of miniature kidneys producing urine (Dekel et al. 2003), or protect against acute renal failure (Lazzeri et al. 2007). However, there are ethical issues to deal with human ES cells.
In adult mammals, a range of methods have been used to identify potential multipotent precursor cells, including label retention in slow cycling cells, identification of a side population, and expression of stem cell markers such as CD133. This has led to the identification of several candidate renal stem cells, which, depending on the study, are located amongst the tubular cell population (Dekel et al. 2006; Gupta et al. 2006), in the Bowman's capsule, papillary region or cortical interstitium (Bussolati et al. 2005; Sagrinati et al. 2006; Rad et al. 2008). Of note, other studies have not confirmed the presence of a large pool of precursor cells amongst the tubular population and instead argue that regeneration occurs through proliferation of differentiated tubular cells (Vogetseder et al. 2008; Witzgall 2008). Some of the candidate renal stem cells have been shown to enhance recovery after tubular injury, possibly by integration in the tubular epithelium (Rad et al. 2008).
2.3 Bone Marrow-Derived Stem Cells and Renal Regeneration
Bone marrow stem cells would be an ideal source of multipotent cells: they are easy to harvest and are in theory an unlimited source of expandable autologous cells. They display an unexpected plasticity which has been the subject of extensive research over the last few years. The plasticity has been observed both for the haematopoietic stem cell, which gives rise to all differentiated blood cell types, as well as for the bone marrow mesenchymal stem cells, which provide stromal support for haematopoietic stem cell in the bone marrow, and also give rise to various mesenchymal tissues, such as bone, cartilage and fat.
There are important discrepancies in the literature addressing the role of bone marrow cells in renal regeneration. These are partly explained by the methods involved in this research.
The technique most commonly used to study bone marrow cell plasticity is bone marrow transplantation. The host bone marrow is replaced by donor bone marrow, and after bone marrow chimerism is established, donor cells are tracked down in the kidney. The donor bone marrow cells are distinguished from host cells by virtue of their chromosome content (male Y chromosome-positive cells in a female host), the expression of a reporter molecule ([pic]-galactosidase, luciferase, enhanced green fluorescent protein), or the performance of a function (re-establishment of a function in a knockout mouse model). The type of host cell that the bone marrow-derived cell has given rise to (tubular, mesangial, etc.) is ascertained most often using immunohistochemistry.
Discrepancies between studies are attributable to several factors: (1) observations in different species (mouse, rat, human); (2) use of different models of renal damage (ischaemia/reperfusion, toxic, immunological); (3) different protocols for bone marrow transplantation (irradiation doses, quantity of cells injected); (4) injection of different subgroups of bone marrow cells (whole bone marrow, haematopoietic stem cell, mesenchymal stem cell); (5) sensitivity and specificity of the detection method for bone marrow cell origin (in situ hybridization for the Y chromosome, detection of reporter molecules, functional assays), and (6) sensitivity and specificity of the detection method of the renal cell type (immunohistochemistry for specific cell types such as tubular cell, mesangial cells, etc.).
Renal failure can be the result of an initial insult directed against the tubular epithelium, the glomerular cells or the vascular compartment. In the search for remedies for these varied renal diseases, studies have therefore addressed potential bone marrow origin for various renal cell types. It is useful to bear in mind these technical variations when analysing results reported in the literature (Roufosse and Cook 2008).
2.4 Tubular Epithelium
Although initial studies suggested a high contribution of bone marrow to tubular regeneration, the current view is that only a small proportion of tubular cells are bone marrow-derived, and there is disagreement over whether mesenchymal stem cells, haematopoietic stem cells or both are contributing (Humphreys and Bonventre 2008). The current consensus view is that the predominant source of tubular regeneration is through the proliferation of differentiated tubular cells (Lin et al. 2005). A few authors have not found any bone marrow cells engrafted in tubules, and propose that positive observations of bone marrow-derived tubular cells are the result of artifact (Bussolati et al. 2009). There may also be a progenitor slow-cycling cell population contributing to tubular repair. Does this abolish the hope of harnessing the regenerative power of bone marrow-derived cells? The answer is not necessarily.
Firstly, under certain circumstances, bone marrow engraftment in tubules can be dramatically increased. Held et al. made use of a transgenic fumarylacetoacetate (FAH)-/- mouse, in which discontinuation of the rescue drug NTBC leads to acute tubular necrosis (Held et al. 2006). After transplanting bone marrow from wild-type mice into FAH-/- mice, a few bone marrow-derived tubular cells are noted. In a subset of the FAH-/- mice, there is, in addition, loss of heterozygosity (LOH) in the liver for homogentistic acid hydrogenase, which induces a more severe, ongoing form of acute tubular necrosis. In FAH-/- animals with additional hepatic LOH, up to 50% of tubular cells are bone marrow-derived cells. Engraftment of these wild-type bone marrow-derived cells leads to morphological resolution of ATN and to disappearance of the aminoaciduria present in control mice. In this model, the bone marrow cells have a strong survival advantage over native tubular cells, due to their ability to metabolise toxic products. It is possible that this strong positive selective pressure is necessary for regeneration to occur through wild-type bone marrow cells. Interestingly, most of the bone marrow-derived tubular cells are derived from cell fusion between bone marrow cells and tubular cells. This is supported by a study by Li et al. in which fusion of bone marrow cells to tubular cells account for part of bone marrow-derived tubular cells after ischaemia/reperfusion (I/R) injury, but not all. In this model without selective pressure, the percentage of bone marrow-derived tubular cells is low (1.8%) (Li et al. 2007b).
Secondly, although there is disagreement concerning the underlying mechanism, injection of bone marrow cells, particularly mesenchymal stem cells, has repeatedly been shown to improve renal function in ATN, whether induced by toxins (cisplatin and glycerol) or I/R (Imai and Iwatani 2007). With the role of actual engraftment of bone marrow cells as tubular cells thought to be minimal or absent, mesenchymal stem cells may exert their beneficial effects through their antiapoptotic, mitogenic, immunomodulatory and angiogenic properties, or through the contribution of the bone marrow cells to endothelial cell replacement in the peritubular capillaries. It is important to know the nature of the mediators involved in these properties, and the mechanisms governing the homing of mesenchymal stem cells to the kidney (Imai and Iwatani 2007). Imberti et al. confirmed the importance of paracrine mechanisms using co-culture of mesenchymal stem cells with tubular cells in a Transwell® culture excluding contact between the two cell types, which led to less cisplatin-induced tubular cell death. mesenchymal stem cells have been shown to produce vascular endothelial growth factor, basic fibroblast growth factor, monocyte chemoattractant protein-1, hepatocyte growth factor, and insulin-like growth factor, as well as immunomodulators TGF-[pic] and PGE2 (Imai and Iwatani 2007; Imberti et al. 2007). In a recent study, administration of conditioned medium from cultured stromal cells provided the same renoprotective effects as injection of mesenchymal stem cells, suggesting that systemic administration of the beneficial mediators may be just as good as mesenchymal stem cell injection, and safer (Imberti et al. 2007). It is a concern that there have been a few observations of adipogenesis associated with fibrosis and osteogenesis after injection of mesenchymal stem cells (Imai and Iwatani 2007).
Mesenchymal stem cell homing to the kidney has been linked to interactions between molecules upregulated in the injured kidney (SDF-1, hyaluronic acid and PDGF) and ligands expressed on mesenchymal stem cells (respectively, CXCR4, CD44 and PDGF-R) (Imai and Iwatani 2007). Similar beneficial effects on renal function may be induced by mobilizing bone marrow cells from the patient's own bone marrow by administration of growth factors (GF) such as granulocyte colony-forming factor, granulocyte/monocyte colony-forming factor, monocyte colony-forming factor, and stem cell factor. Possible explanations for improved renal function include increased numbers of bone marrow-derived tubular cells, a decrease in neutrophilic infiltrate, or increased cell proliferation and decreased apoptosis in kidneys of GF-treated mice (Roufosse and Cook 2008).
In summary, most but not all authors agree that a small proportion of tubular cells (at most a few percent) are bone marrow-derived after renal injury. The role these bone marrow-derived tubular cells play in improved renal function is probably insignificant, with intrinsic renal cells, either stem cells or differentiated, more likely to play the predominant role in regeneration. However, administration of bone marrow cells or mobilization of bone marrow cells using GF may be used to protect against renal injury. This may be due to paracrine / immunomodulatory effects or endothelial regeneration. In addition, there may be a therapeutic role for bone marrow-derived cells engineered to replace a defective gene, due to a local strong positive selective pressure. mesenchymal stem cells have emerged as the most promising candidate for stem cell therapy, and appear safe, such that phase I clinical trials of mesenchymal stem cell injection for the treatment of acute kidney injury are scheduled to begin shortly (Imai and Iwatani 2007).
2.5 Mesangial Cells
Mesangial cells are modified smooth muscle cells in the glomerular tuft, and provide structural support for the complex of glomerular capillaries. They may be injured by immune complex deposition, toxins and in diabetes. Although mesangial cells have regenerative potential, persistent mesangial damage can lead to glomerulosclerosis. In cell culture, bone marrow cells treated with PDGF-BB in the presence of collagen IV convert to cells with many mesangial characteristics (Suzuki et al. 2004). In rodent models of bone marrow transplantation, there is also support for partial bone marrow derivation of mesangial cells, whether glomeruli are injured or not. In models where mesangial damage has been induced, infusion of bone marrow cells may be associated with improved function, which has been attributed to mesangial and endothelial regeneration or, in the case of mesenchymal stem cells, paracrine mechanisms. Conversely, a deleterious mesangial phenotype responsible for mesangial sclerosis, such as in Os/- or db/db mice, can be induced by transplanting wild-type mice with transgenic mouse bone marrow. Some studies have further illustrated functionality of the bone marrow-derived mesangial cells by harvesting the bone marrow-derived mesangial cells, growing them in culture, and showing angiotensin-II induced contraction in vitro, a typical mesangial function (Kunter et al. 2006).
2.6 Podocytes
Podocytes are epithelial cells with complex interdigitating foot processes which create the slit diaphragm, and contribute to the synthesis of the glomerular basement membrane. Both the slit diaphragm and the glomerular basement membrane are implicated in creating a filtration barrier between blood and urine. Initial studies identified rare bone marrow-derived cells at the periphery of the glomerular tuft, in the location of podocytes. Two recent studies have suggested integration of bone marrow-derived cells as functional podocytes, with production of matrix protein. These studies involved the use of a mouse model of Alport's disease, in which the animals suffer from defective synthesis of the alpha-3 chain of collagen type IV, with glomerular basement membrane abnormalities and progression to glomerulosclerosis and renal failure. Using whole bone marrow transplantation from wild-type animals, both Prodromidi et al. and Sugimoto et al. showed the presence of bone marrow-derived podocytes and mesangial cells, accompanied by re-expression of the defective collagen chains, and improved renal histology and function. Although the bone marrow-derived cells were not numerous, their presence was sufficient to re-establish synthesis of the defective collagen chain. However, the improvement in renal function was substantial raising the possibility that there may be mechanisms involved other than replacement of podocytes. A similar experiment using mesenchymal stem cells only rather than whole bone marrow also led to a reduction in interstitial fibrosis, but without engraftment of bone marrow cells in the kidney, and with no beneficial effect on survival or renal function (Sugimoto et al. 2006).
2.7 Vascular Cells
Endothelial cells are present in the glomerular capillaries, in large vessels and in the abundant network of peritubular capillaries. Endothelial cells are attractive candidates for progeny of bone marrow-derived cells in view of their immediate contact with circulating cells, the existence of known circulating endothelial precursors, and the existence of a known endothelial precursor in the bone marrow: the haemangioblast.
In a rat model of glomerulonephritis, where glomerular endothelial cells are injured, culture-modified bone marrow mononuclear cells injected into the renal artery boosted renal regeneration. This was attributed both to incorporation of bone marrow-derived cells into the endothelial lining and to production of angiogenic factors by the injected cells. Similarly, following acute tubular necrosis, the peritubular capillaries are damaged. The return of blood flow, which depends on endothelial cell integrity, is essential for renal recovery. Duffield et al. contend that bone marrow cells boost renal function after I/R by participating in endothelial cell regeneration. Li et al. observed bone marrow-derived vWF+ and CD31+ endothelial cells in a mouse model of adriamycin-induced nephrosis with subsequent renal fibrosis (Li et al. 2007a).
2.8 Interstitial Cells
The kidney contains a complex population of interstitial cells serving several functions, such as providing a scaffold for renal structure and producing several hormonal substances such as erythropoietin. It may even contain a population of adult native renal stem cells which play a role in renal regeneration. There is also evidence that bone marrow-derived cells could be a source for up to 30% of [pic]-SMA-positive interstitial myofibroblasts, which have been incriminated in the production of extracellular matrix in renal fibrosis. If the bone marrow is indeed a source for such cells, the use of bone marrow cell injections for the treatment of renal failure would run the risk of enhancing fibrosis (Broekema et al. 2007).
2.9 Adipose Stem Cells
Adipose, also known as fat tissue, is the richest and most accessible known source of stem cells. It contains a specialized class of stem cells comprised of multiple cell types that promote healing and repair. Adipose stem cells have been shown to differentiate into multiple cell types including muscle, bone, fat, cartilage and nerve, etc. Beyond differentiation, regenerative cells may provide therapeutic benefit through the release of growth factors and other therapeutic healing mechanisms. The major advantages of adipose tissue as a source of regenerative cells, which distinguish it from alternative cell sources, include: (1) Yield: A therapeutic dose of regenerative cells can be isolated in approximately one hour without cell culture. (2) Safety: Patients receive their own cells (autologous-use) so there is no risk of immune rejection or transmission. (3) Versatility: Stem cells from adipose tissue benefit from multiple mechanisms-of-action.
There is crucial disagreement on the issue of functionality of these cells. Do the bone marrow-derived cells actively participate in extracellular matrix synthesis? Roufosse et al. in a mouse model of unilateral ureteric obstruction, detected bone marrow-derived [pic]-SMA-positive cells. Using two reporter molecules under the control of the promoter and enhancer elements of the collagen I [pic]2 chain gene, we did not however observe any functional bone marrow-derived fibroblasts or myofibroblasts producing collagen I. On the other hand, Iwano et al. in a mouse model of unilateral ureteric obstruction, and Broekema et al. in a rat model of unilateral I/R injury, demonstrated double immunostaining positivity of [pic]-SMA-positive interstitial cells with pro-collagen I protein (Broekema et al. 2007).
In this project, with the mouse model, we aim to find the practical conditions to induce ES cells differentiating into renal stem cells and to find the ways using the renal stem cells to repair and regenerate obstructed kidney. For these, we will explore the techniques to induce ES cells to adopt a renal fate using co-culture with cell lines, metanephroi and novel growth factors. Meantime, we will characterize the expression profile of different renal subcompartments so as to identify the secreted proteins involved in renal differentiation and to isolate the specific cell surface markers identifying renal stem cells.
The theoretical background justifying the pursuit of the potential of bone marrow cells to participate in renal regeneration has been laid. Stem cells, both embryonic and from the adult bone marrow, in the right conditions, can express renal markers in vitro and give rise to renal cells in vivo.
In addition, injection of stem cells into the kidney or the bloodstream can lead to an improvement of renal function, although this does not always seem to be mediated by transdifferentiation into renal cells. Current views favour a predominant role for the delivery of a cocktail of angiogenic and immunomodulatory mediators as the main means by which bone marrow cells enhance epithelial and endothelial cell survival. As far as engraftment of bone marrow cells as renal parenchymal cells is concerned, proving functionality of the engrafted bone marrow-derived cells is crucial in order to assign to them a role in improved renal function, rather than relying on morphological observations alone.
The kidney is a complex organ with over 30 different cell types, and present technology does not envisage constructing a whole kidney from stem cells. However, within existing kidneys where the basic scaffolding is intact, stem cells may contribute to a variety of specialised cell types, either promoting more efficient repair or correcting genetic defects. These would include: (1) acute tubular necrosis (ATN) caused by toxins or ischaemia/reperfusion (associated with kidney transplantation); (2) mesangial damage, often associated with immune complex deposition and diabetes; (3) defective podocyte function (Alport’s disease); (4) vascular endothelial damage (e.g. in glomerulonephritis) (Alison et al. 2007; Alison 2009).
Whether stem cell injections will ever be used for the treatment of renal failure is at this stage still unknown. There is certainly some hope to be found in the numerous animal models that have been developed and analysed over the last few years.
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Some of the most notable recent findings are as follows: (1) the 'stemness' profile may be determined by approximately 250 genes; (2) organ-specific stem-cell growth and differentiation are stimulated during the reparative phase following transient injury; (3) two bone marrow stem-cell types show a remarkable degree of differentiation potential; (4) some organs contain resident marrow-derived stem cells, and their differentiation potential may only be expressed during repair; (5) the metanephric mesenchyme contains pluripotent and self-renewing stem cells; (6) marrow-derived cells invade the kidney and differentiate into mesangial and tubular epithelial cells, and these processes are increased following renal injury; and (7) epithelial-to-mesenchymal transition generates renal fibroblasts (Oliver, 2004).
Cell therapy is dependent on cell and tissue culture methodologies to expand specific cells to replace important differentiated functions lost or deranged in various disease states. Cell-based therapeutics is the question of cell sourcing, and advance of stem cell research is powerful on the resolving of this problem. Stem cell is at present a great deal of speculation over the extent to which stem cell populations traditionally considered distinct may in fact be interchangeable. Stem cells can be self-renewal to differentiate into specialized cell types and they are classified as totipotent, pluripotent and multipotent. Progenitor cells are more lineage-restricted than stem cells but retain the proliferative capacity lacking in terminally differentiated cells (Lin et al., 1996).
Embryonic stem (ES) cells, pluripotent derivatives of the inner cell mass of the blastocyst, are the most primitive cell type likely to find application in cell therapy. Their potential to generate any cell type of the embryo makes them to be the most attractive stem cell therapy. However, the political and ethical debates surrounding the use of human ES cells make the application of stem cells complicated. These factors have combined to intensify the focus on. Neural stem cells (NSCs) are self-renewing, multipotent cells that generate the neuronal and glial cells of the nervous system. In mammals, contrary to long-held belief, neurogenesis occurs in the adult brain, and NSCs reside in the adult central nervous system (Taupin, 2006; Winkler, 2003).
It is possible to introduce stem cells into a damaged adult kidney to aid in repair and regeneration. Transdifferentiation offers the possibility of avoiding complications from immunogenicity of introduced cells by obtaining the more easily accessible stem cells of another tissue type from the patient undergoing treatment, expanding them in vitro, and reintroducing them as a therapeutic agent. Adult stem cells may possess a considerable degree of plasticity in the differentiation. However, the differentiation of stem cells is normally unresolved. Pluripotent cells can be derived from fibroblasts by ectopic expression of defined transcription factors. A fundamental unresolved question is whether terminally differentiated cells can be reprogrammed to pluripotency (Hanna et al., 2008).
Developing nephrons are derived from renal stem cells and transplantation of fetal kidneys may be thought of as a therapeutic stem cell application. There are two bioengineering programs with the aim of producing a device providing full renal replacement therapy in the short to medium term. Both employ biomaterial scaffold structures to overcome the as yet insurmountable difficulties inherent in marshalling cells into organized three-dimensional structures capable of coordinated filtration, resorption / meta- / catabolism / secretion, collection, and disposal of waste. Initial experiments involved adult rabbit renal cortex harvested and fractionated into glomeruli, distal, and proximal tubules, expanded separately in vitro, and seeded onto biodegradable polyglycolic acid sheets for subcutaneous implantation into syngenic hosts. The potential impact of advances in stem cell technology on all the prospective cell-based therapeutic approaches for the treatment of renal failure discussed above is enormous. The kidney has a dramatic capacity to regenerate after injury. Whether stem cells are the source of the epithelial progenitors replacing injured and dying tubular epithelium is an area of intense investigation. Many surviving renal epithelial cells after injury become dedifferentiated and take on mesenchymal characteristics. These cells proliferate to restore the integrity of the denuded basement membrane, and subsequently redifferentiate into a functional epithelium. An alternative possibility is that a minority of surviving intratubular cells possess stem cell properties and selectively proliferate after damage to neighboring cells. Some evidence exists to support this hypothesis but it has not yet been rigorously evaluated (Vigneau et al., 2007).
In recent years, it has been shown that functional stem cells exist in the adult bone marrow, and they can contribute to renal remodelling or reconstitution of injured renal glomeruli, especially mesangial cells, and hMSC found in renal glomeruli differentiated into mesangial cells in vivo after glomerular injury occurred (Wong et al., 2008).
2. Isolation of Kidney Stem Cells
It is difficult to find a definitive marker for kidney stem cells that makes it difficult to isolate and define kidney stem cells. However, kidney stem cells have been isolated from other organs using 4 different ways. For the first method, when the DNA of the cells is labeled with a marker such as bromodeoxyuridine, the cells retain the label for a long period of time. This label retention is used to identify and isolate stem cells. The second method references the side-population (SP) cells that extrude Hoechst dye through the activity of multidrug resistance proteins, which are part of the ATP-binding cassette transporter superfamily. The third method isolates kidney stem cells referencing specific cell surface markers that have been used to identify stem cells in other organs or the metanephric kidney. The markers used to isolate kidney stem cells include Oct-4, Nanog, CD24, CD133 and stem cell antigen-1 (Sca-1). The fourth method uses culture conditions that select stem cells in other organ systems ().
As Zheng et al described in 2008, any unique characteristic can be used to isolate a pure population of stem cell is still lacking. There is few specific biomarker found for epidermal stem cells alone, but epidermal stem cells and transient-amplifying cells share some biomarkers (Bickenbach, 2003) (Zheng, et al, 2008).
3. Culture of Renal Stem Cell
Shimony et al characterized a new model of renal, stromal and mesenchymal stem cell (MSC) matrix deprivation, based on slow rotation cell culture conditions (ROCK). This model induces anoikis using a low shear stress, laminar flow. The mechanism of cell death was determined via FACS (fluorescence-activated cell sorting) analysis for annexin V and propidium iodide uptake and via DNA laddering. Their results showed while only renal epithelial cells progressively died in STCK, the ROCK model could induce apoptosis in stromal and transformed cells; cell survival decreased in ROCK versus STCK to 40%, 52%, 62% and 7% in human fibroblast, rat MSC, renal cell carcinoma (RCC) and human melanoma cell lines, respectively. Furthermore, while ROCK induced primarily apoptosis in renal epithelial cells, necrosis was more prevalent in transformed and cancer cells [necrosis/apoptosis ratio of 72.7% in CaKi-1 RCC cells versus 4.3% in MDCK (Madin-Darby canine kidney) cells]. The ROCK-mediated shift to necrosis in RCC cells was further accentuated 3.4-fold by H(2)O(2)-mediated oxidative stress while in adherent HK-2 renal epithelial cells, oxidative stress enhanced apoptosis. ROCK conditions could also unveil a similar pattern in the LZ100 rat MSC line where in ROCK 44% less apoptosis was observed versus STCK and 45% less apoptosis versus monolayer conditions. Apoptosis in response to oxidative stress was also attenuated in the rat MSC line in ROCK, thereby highlighting rat MSC transformation. They concluded that the ROCK matrix-deficiency cell culture model may provide a valuable insight into the mechanism of renal and MSC cell death in response to matrix deprivation (Shimony et al., 2008)
Celprogen Company has human kidney stem cell derived from human adult and fetal kidney consented tissue. The following is the information and technique of the kidney stem cells:
(1) Positive markers: CD34, Nestin & CD133
(2) Morphology and proliferation: Mixed population of cells with approximately 70% attached cells and the other 30% in suspension; need to change cell culture media every day after 48 hours of initial cell culture or when the media starts changing color to slight yellow for pink. Fast growing cell culture. Change media with Celprogen’s Human Kidney Stem Cell Complete Growth Media with the appropriate Human Stem Cell Extracellular matrix and tissue culture media for differentiation, expansion or maintaining stem cells in their un-differentiated stage. Temperature 370C in 5% CO2 humidified incubator.
(3) Subculturing:
A. Thaw the vial with gentle agitation in a 370C water bath or a dry 370C shaking incubator. For water bath thawing
B. keep the O-ring out of the water, thawing time 2-3 minutes.
C. Remove the thawed vial and wipe with 70% ethanol. Then transfer to the tissue culture hood.
D. Transfer the vial contents to a 15 ml sterile centrifuge tube, and gently add 7ml of pre-warmed Human Kidney Stem Cell Complete Media to the centrifuge tube. Use an additional 0.5 ml of Human Kidney Stem Cell complete media to rinse the vial and transfer the liquid to the centrifuge tube repeat this once more to ensure you have all the cells transferred to the 15 ml centrifuge tube. Add 1 ml of Human Kidney Stem Complete Media to bring the final volume to 10 ml in the 15 ml centrifuge tube.
E. Centrifuge the cells at 100 g for 5 minutes. Remove the supernatant and resuspend the cell pellet in 500 ul of Human Kidney Stem Cell Complete Growth Media.
F. Add the 500 ul of cells to T75 flask pre-coated with Human Kidney Stem Cell Extracellular matrix with 15 ml of Human Stem Cell Complete Growth Medium.
G. Incubate the cells in the T75 flask in a 370C in 5% CO2 humidified incubator. Perform 100% Media Change every 24 to 48 hours.
H. Medium renewal every day, and recommended sub-culturing ratio: 1:3.
(4) Freezing Medium: Human Stem Cell Complete growth Medium supplemented with 90% Fetal Bovine Serum with 10% DMSO.
(5) Storage temperature: liquid nitrogen vapor phase (San Pedro, CA 90731, USA, ; .
In mice with cisplatin-induced acute kidney injury, administration of bone marrow-derived mesenchymal stem cells (MSC) restores renal tubular structure and improves renal function (Imberti et al., 2007).
4. Application of Kidney Stem Cells
Stem cell has powerful potential application purpose in science, medicine and industry, but it is also potentially danger for its enexpected plasticity. The evidence for bone marrow-derived stem cell contributions to renal repair has been challenged. The research and application for adult renal stem cells are also debated. The use of embryonic tissue in research continues to provide valuable insights but will be the subject of intense societal scrutiny and debate before it reaches the stage of clinical application. Embryonic stem cells, with their ability to generate all of kind of cell in living body, are great chance for our human civilization but have ethical and political hurdles for human use (Brodie and Humes, 2005). Stem cell research has attracted great attention because it could be used for the regeneration of damaged organs that are untreatable by conventional medical techniques, and stem cells (such as endothelial stem cells and neural stem cells) have been discovered to be practical useful in clinical applications. The potential for stem cell gene therapy has increased and many therapeutic applications have already been done. Chronic renal failure is a candidate for stem cell gene therapy. In the application of renal stem cell in medical treatment, mesenchymal stem cells could be transplanted, and in contrast, hematopoietic stem cells may be used for gene delivery for diseases, which need foreign cytokines and growth factors, such as glomerulonephritis. The stem cell gene therapy for chronic renal failure and the potential of the novel strategy and the major practical challenges of its clinical application are big targets for the stem cell researches (Yokoo et al., 2003). Ectopic expression of the human telomerase reverse transcriptase gene in human mesenchymal stem cells can reconstitute their telomerase activity and extend their replicative life-spans (Li, et al, 2007).
Discussion
Kidney is derived from the ureteric bud and metanephrogenic mesenchyme, and these two progenitor cells differentiate into more than 26 different cell types in adult kidney. The ureteric bud contains the precursor of the epithelial cells of the collecting duct and the renal mesenchyme contains precursors of all the epithelia of the rest of the nephron, endothelial cell precursors and stroma cells, but the relatedness among these cells is unclear. A single metanephric mesenchymal cell can generate all the epithelial cells of the nephron, indicating that the kidney contains epithelial stem cells. These stem cells also are present in the adult kidney. Embryonic renal epithelial stem cells can generate other cell types (Al-Awqati and Oliver, 2002). The key important target in kidney stem cell research and application is to get kidney stem cells from other types of the cells, and it is also important to find the better way to change kidney stem cells to other cell types.
As the nature will, to live eternally is an extracting dream in all the human history. Stem cell is the original of life and all cells come from stem cells. Germline stem cell (GSC) is the cell in the earliest of the cell stage. It is possible to inject the GSC into adult human body to get the eternal life. This article is to try to describe the stem cell and to explore the possibility of the eternal life with the stem cell strategy The production of functional male gametes is dependent on the continuous activity of germline stem cells. The availability of a transplantation assay system to unequivocally identify male germline stem cells has allowed their in vitro culture, cryopreservation, and genetic modification. Moreover, the system has enabled the identification of conditions and factors involved in stem cell self-renewal, the foundation of spermatogenesis, and the production of spermatozoa. The increased knowledge about these cells is also of great potential practical value, for example, for the possible cryopreservation of stem cells from boys undergoing treatment for cancer to safeguard their germ line (Ma, et al, 2007).
• Adult stem cell—see somatic stem cell.
• Astrocyte—a type of supporting (glial) cell found in the nervous system.
• Blastocoel—The fluid-filled cavity inside the blastocyst, an early, preimplantation stage of the developing embryo.
• Blastocyst—A preimplantation embryo of about 150 cells produced by cell division following fertilization. The blastocyst is a sphere made up of an outer layer of cells (the trophoblast), a fluid-filled cavity (the blastocoel), and a cluster of cells on the interior (the inner cell mass).
• Bone marrow stromal cells—A population of cells found in bone marrow that are different from blood cells, a subset of which are multipotent stem cells, able to give rise to bone, cartilage, marrow fat cells, and able to support formation of blood cells.
• Cell-based therapies—Treatment in which stem cells are induced to differentiate into the specific cell type required to repair damaged or destroyed cells or tissues.
• Cell culture—Growth of cells in vitro in an artificial medium for research or medical treatment.
• Cell division—Method by which a single cell divides to create two cells. There are two main types of cell division depending on what happens to the chromosomes: mitosis and meiosis.
• Chromosome—a structure consisting of DNA and regulatory proteins found in the nucleus of the cell. The DNA in the nucleus is usually divided up among several chromosomes.The number of chromosomes in the nucleus varies depending on the species of the organism. Humans have 46 chromosomes.
• Clone— (v) To generate identical copies of a region of a DNA molecule or to generate genetically identical copies of a cell, or organism; (n) The identical molecule, cell, or organism that results from the cloning process.
• In reference to DNA: To clone a gene, one finds the region where the gene resides on the DNA and copies that section of the DNA using laboratory techniques.
• In reference to cells grown in a tissue culture dish:a clone is a line of cells that is genetically identical to the originating cell. This cloned line is produced by cell division (mitosis) of the original cell.
• In reference or organisms: Many natural clones are produced by plants and (mostly invertebrate) animals. The term clone may also be used to refer to an animal produced by somatic cell nuclear transfer (SCNT) or parthenogenesis.
• Cloning—See Clone.
• Cord blood stem cells—See Umbilical cord blood stem cells.
• Culture medium—The liquid that covers cells in a culture dish and contains nutrients to nourish and support the cells. Culture medium may also include growth factors added to produce desired changes in the cells.
• Differentiation—The process whereby an unspecialized embryonic cell acquires the features of a specialized cell such as a heart, liver, or muscle cell. Differentiation is controlled by the interaction of a cell's genes with the physical and chemical conditions outside the cell, usually through signaling pathways involving proteins embedded in the cell surface.
• Directed differentiation—The manipulation of stem cell culture conditions to induce differentiation into a particular cell type.
• DNA—Deoxyribonucleic acid, a chemical found primarily in the nucleus of cells. DNA carries the instructions or blueprint for making all the structures and materials the body needs to function. DNA consists of both genes and non-gene DNA in between the genes.
• Ectoderm—The outermost germ layer of cells derived from the inner cell mass of the blastocyst; gives rise to the nervous system, sensory organs, skin, and related structures.
• Embryo—In humans, the developing organism from the time of fertilization until the end of the eighth week of gestation, when it is called a fetus.
• Embryoid bodies—Rounded collections of cells that arise when embryonic stem cells are cultured in suspension. Embryoid bodies contain cell types derived from all 3 germ layers.
• Neurons—Nerve cells, the principal functional units of the nervous system. A neuron consists of a cell body and its processes—an axon and one or more dendrites. Neurons transmit information to other neurons or cells by releasing neurotransmitters at synapses.
• Oligodendrocyte—A supporting cell that provides insulation to nerve cells by forming a myelin sheath (a fatty layer) around axons.
• Parthenogenesis—The artificial activation of an egg in the absence of a sperm; the egg begins to divide as if it has been fertilized.
• Passage—In cell culture, the process in which cells are disassociated, washed, and seeded into new culture vessels after a round of cell growth and proliferation. The number of passages a line of cultured cells has gone through is an indication of its age and expected stability.
• Pluripotent—Having the ability to give rise to all of the various cell types of the body. Pluripotent cells cannot make extra-embryonic tissues such as the amnion, chorion, and other components of the placenta. Scientists demonstrate pluripotency by providing evidence of stable developmental potential, even after prolonged culture, to form derivatives of all three embryonic germ layers from the progeny of a single cell and to generate a teratoma after injection into an immunosuppressed mouse.
• Polar Body—A polar body is a structure produced when an early egg cell, or oogonium, undergoes meiosis. In the first meiosis, the oogonium divides its chromosomes evenly between the two cells but divides its cytoplasm unequally. One cell retains most of the cytoplasm, while the other gets almost none, leaving it very small. This smaller cell is called the first polar body. The first polar body usually degenerates. The ovum, or larger cell, then divides again, producing a second polar body with half the amount of chromosomes but almost no cytoplasm. The second polar body splits off and remains adjacent to the large cell, or oocyte, until it (the second polar body) degenerates. Only one large functional oocyte, or egg, is produced at the end of meiosis.
• Preimplantation—With regard to an embryo, preimplantation means that the embryo has not yet implanted in the wall of the uterus. Human embryonic stem cells are derived from preimplantation-stage embryos fertilized outside a woman's body (in vitro).
• Proliferation—Expansion of the number of cells by the continuous division of single cells into two identical daughter cells.
• Regenerative medicine—A field of medicine devoted to treatments in which stem cells are induced to differentiate into the specific cell type required to repair damaged or destroyed cell populations or tissues. (See also cell-based therapies).
• Reproductive cloning—The process of using somatic cell nuclear transfer (SCNT) to produce a normal, full grown organism (e.g., animal) genetically identical to the organism (animal) that donated the somatic cell nucleus. In mammals, this would require implanting the resulting embryo in a uterus where it would undergo normal development to become a live independent being. The first animal to be created by reproductive cloning was Dolly the sheep, born at the Roslin Institute in Scotland in 1996. See also Somatic cell nuclear transfer (SCNT).
• Signals—Internal and external factors that control changes in cell structure and function. They can be chemical or physical in nature.
• Somatic cell—any body cell other than gametes (egg or sperm); sometimes referred to as "adult" cells. See also Gamete.
• Somatic cell nuclear transfer (SCNT)—A technique that combines an enucleated egg and the nucleus of a somatic cell to make an embryo. SCNT can be used for therapeutic or reproductive purposes, but the initial stage that combines an enucleated egg and a somatic cell nucleus is the same. See also therapeutic cloning and reproductive cloning.
• Somatic (adult) stem cells—A relatively rare undifferentiated cell found in many organs and differentiated tissues with a limited capacity for both self renewal (in the laboratory) and differentiation. Such cells vary in their differentiation capacity, but it is usually limited to cell types in the organ of origin. This is an active area of investigation.
• Stem cells—Cells with the ability to divide for indefinite periods in culture and to give rise to specialized cells.
• Stromal cells—Non-blood cells derived from blood organs, such as bone marrow or fetal liver, which are capable of supporting growth of blood cells in vitro. Stromal cells that make the matrix within the bone marrow are also derived from mesenchymal stem cells.
• Subculturing—Transferring cultured cells, with or without dilution, from one culture vessel to another.
• Surface markers—Proteins on the outside surface of a cell that are unique to certain cell types and that can be visualized using antibodies or other detection methods.
• Teratoma— A multi-layered benign tumor that grows from pluripotent cells injected into mice with a dysfunctional immune system. Scientists test whether they have established a human embryonic stem cell (hESC) line by injecting putative stem cells into such mice and verifying that the resulting teratomas contain cells derived from all three embryonic germ layers.
• Therapeutic cloning—The process of using somatic cell nuclear transfer (SCNT) to produce cells that exactly
• match a patient. By combining a patient's somatic cell nucleus and an enucleated egg, a scientist may harvest embryonic stem cells from the resulting embryo that can be used to generate tissues that match a patient's body. This means the tissues created are unlikely to be rejected by the patient's immune system. See also Somatic cell nuclear transfer (SCNT).
• Totipotent—Having the ability to give rise to all the cell types of the body plus all of the cell types that make up the extraembryonic tissues such as the placenta. (See also Pluripotent and Multipotent).
• Transdifferentiation—The process by which stem cells from one tissue differentiate into cells of another tissue.
• Trophectoderm—The outer layer of the preimplantation embryo in mice. It contains trophoblast cells.
• Trophoblast—The outer cell layer of the blastocyst. It is responsible for implantation and develops into the extraembryonic tissues, including the placenta, and controls the exchange of oxygen and metabolites between mother and embryo.
• Umbilical cord blood stem cells—stem cells collected from the umbilical cord at birth that can produce all of the blood cells in the body (hematopoietic). Cord blood is currently used to treat patients who have undergone chemotherapy to destroy their bone marrow due to cancer or other blood-related disorders.
• Undifferentiated—A cell that has not yet developed into a specialized cell type.
• Embryonic germ cells—Pluripotent stem cells that are derived from early germ cells (those that would become sperm and eggs). Embryonic germ cells (EG cells) are thought to have properties similar to embryonic stem cells.
• Embryonic stem cells—Primitive (undifferentiated) cells derived from a 5-day preimplantation embryo that are capable of dividing without differentiating for a prolonged period in culture, and are known to develop into cells and tissues of the three primary germ layers.
• Embryonic stem cell line—Embryonic stem cells, which have been cultured under in vitro conditions that allow proliferation without differentiation for months to years.
• Endoderm—The innermost layer of the cells derived from the inner cell mass of the blastocyst; it gives rise to lungs, other respiratory structures, and digestive organs, or generally "the gut."
• Enucleated—having had its nucleus removed.
• Epigenetic—having to do with the process by which regulatory proteins can turn genes on or off in a way that can be passed on during cell division.
• Feeder layer—Cells used in co-culture to maintain pluripotent stem cells. For human embryonic stem cell culture, typical feeder layers include mouse embryonic fibroblasts (MEFs) or human embryonic fibroblasts that have been treated to prevent them from dividing.
• Fertilization—The joining of the male gamete (sperm) and the female gamete (egg).
• Fetus—In humans, the developing human from approximately eight weeks after conception until the time of its birth.
• Gamete—An egg (in the female) or sperm (in the male) cell. See also Somatic cell.
• Gastrulation—the process in which cells proliferate and migrate within the embryo to transform the inner cell mass of the blastocyst stage into an embryo containing all three primary germ layers.
• Gene—A functional unit of heredity that is a segment of DNA found on chromosomes in the nucleus of a cell. Genes direct the formation of an enzyme or other protein.
• Germ layers—After the blastocyst stage of embryonic development, the inner cell mass of the blastocyst goes through gastrulation, a period when the inner cell mass becomes organized into three distinct cell layers, called germ layers. The three layers are the ectoderm, the mesoderm, and the endoderm.
• Hematopoietic stem cell—A stem cell that gives rise to all red and white blood cells and platelets.
• Human embryonic stem cell (hESC)—A type of pluripotent stem cell derived from the inner cell mass (ICM) of the blastocyst.
• Induced pluripotent stem cells—Somatic (adult) cells reprogrammed to enter an embryonic stem cell–like state by being forced to express factors important for maintaining the "stemness" of embryonic stem cells (ESCs). Mouse iPSCs were first reported in 2006 (Takahashi and Yamanaka), and human iPSCs were first reported in late 2007 (Takahashi et al. and Yu et al.). Mouse iPSCs demonstrate important characteristics of pluripotent stem cells, including the expression of stem cell markers, the formation of tumors containing cells from all three germ layers, and the ability to contribute to many different tissues when injected into mouse embryos at a very early stage in development. Human iPSCs also express stem cell markers and are capable of generating cells characteristic of all three germ layers. Scientists are actively comparing iPSCs and ESCs to identify important similarities and differences.
• In vitro—Latin for "in glass"; in a laboratory dish or test tube; an artificial environment.
• In vitro fertilization—A technique that unites the egg and sperm in a laboratory instead of inside the female body.
• Inner cell mass (ICM)—The cluster of cells inside the blastocyst. These cells give rise to the embryo and ultimately the fetus. The ICM cells are used to generate embryonic stem cells.
• Long-term self-renewal—The ability of stem cells to replicate themselves by dividing into the same non-specialized cell type over long periods (many months to years) depending on the specific type of stem cell.
• Mesenchymal stem cells—Cells from the immature embryonic connective tissue. A number of cell types come from mesenchymal stem cells, including chondrocytes, which produce cartilage.
• Meiosis—The type of cell division a diploid germ cell undergoes to produce gametes (sperm or eggs) that will carry half the normal chromosome number. This is to ensure that when fertilization occurs, the fertilized egg will carry the normal number of chromosomes rather than causing aneuploidy (an abnormal number of chromosomes).
• Mesoderm—Middle layer of a group of cells derived from the inner cell mass of the blastocyst; it gives rise to bone, muscle, connective tissue, kidneys, and related structures.
• Microenvironment—The molecules and compounds such as nutrients and growth factors in the fluid surrounding a cell in an organism or in the laboratory, which play an important role in determining the characteristics of the cell.
• Mitosis—The type of cell division that allows a population of cells to increase its numbers or to maintain its numbers. The number of chromosomes remains the same in this type of cell division.
• Multipotent—Having the ability to develop into more than one cell type of the body. See also pluripotent and totipotent.
• Neural stem cell—A stem cell found in adult neural tissue that can give rise to neurons and glial (supporting) cells. Examples of glial cells include astrocytes and oligodendrocytes.
Correspondence to:
Hongbao Ma
Bioengineering Department
Zhengzhou University
Zhengzhou, Henan 450001, China
mahongbao@zzu.; hongbao@
01186-137-8342-5354
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