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Stem cells are the fundamental cells found within any tissue in a mammalian organism. In the lifetime span of an organism, cells derive from the continuous division of stem cells whenever required. The two types of stem cells are the embryonic and adult stem cell each emerging from different tissues of the body (eg. Embryo and bone marrow). Their role in a human body is to divide (proliferate); thus producing daughter cells (transit amplifying cell) which their fate is to amplify into terminally differentiated cells. This mechanism/pathway gives the ability to our system to repair a damaged tissue or dead/useless cells.
Scientist took advantage of this unique function stem cells have, by isolating stem cells from a particular tissue in order to transfer them onto a damaged tissue and eventually cure it. The critical feature that this type of cells is used is because are unspecialised and of their wide plasticity. Unspecialised cells are the cells which do not have any specific structure or perform specialized functions. Plasticity is the capability to transform in any type of other cell. Thus therapeutic used has been developed where uncured diseases can be now be treated using stem cells. The aim of this paper is to explore the advantages and disadvantages of using adult stem cells (ASC) in therapeutic uses instead of embryonic stem cells (ESC).
Embryonic and Adult Stem Cells
There is a distinct difference between the locations found of these two types of cells. Firstly, ESC are only found in a 5-day pre-implantation embryo which its cell content (blastocytes) gives rise to the whole body. However ASC are only found in adult/grown tissues like bone marrow, muscle and generally any developed tissue. Both ESCs and ASCs have relevant advantages and disadvantages according to their clinical uses. The table below summarizes the most important in relation to therapeutic applications.
Table 1 The table above compares the advantages and disadvantages between ASCs and ESCs (Hmadcha A. et al., 2009)
As seen in the table above and stated in introduction, it can be clearly noticed that ASCs can be isolated from various sources. The types of ASCs that will be discuss by means of therapeutic applications are the Bone marrow which are found in the spinal cord (cord blood), the mesenchymal stem cells (MSCs) which can be extracted from various sources like fat, bone marrow and skin (Hmadcha A., et al. 2009). Each and every stem cell is under exploration to develop safe and effective ways of implanting these types of cells into medical propositions programmes. However advantages and disadvantages exist for each type of cell which at the moment, they hold regenerative medication to stay one step before success.
Clinical Uses of Stem Cells
Over the years scientists kept exploring the capabilities of stem cells, in order to find a safe way to utilize them for clinical purposes and program them to perform specialized tasks. Every achievement and every new characteristic discovered about the functions of stem cells, all lead to promising therapeutic applications by curing untreatable and chronic diseases like cancer, spinal cord injury, heart diseases, tissue transplantation and many others. However limitations exist for both adult and embryonic stem cells, acting as barriers to the implementation of unspecialised stem cells.
Advantages and disadvantages of using ASCs in clinical applications
ASCs are loaded with many advantages which can make them to overcome limitations that ESCs reveal. They are autologous thus bypass the possibility of immune rejection due to the fact that the immune system of the host is not activated. A direct transplantation of ASCs to a patient body by skipping the use of in vitro culture techniques rejects the unwanted outcome of cell differentiation. ASCs can be found is most adult tissues as they are essential to them by supplying new cells when necessary. For example, haematopoietic stem cells which are found in the bone marrow are responsible for the regeneration of blood cells found in the human body. The section below briefly states the advantages and disadvantages of various ASC types and clinical therapies according to the tissue are found:
Skeletal Myoblasts: they are found in muscles tissues (muscle satellite cells) and give rise to Myoblasts in the case of a muscle injury. They have been used for the restoration of myocardial function in the heart tissue from myocardial infractions. The advantage that makes them unique to these types of applications is that are ischemia resistant and can differentiate into myogenic cells.
Bone Marrow Mononuclear Cells (MN): hugely involved in the cardiac cell therapy. MN cells are found in the bone marrow and consist of hematopoietic stem cells, MSCs and endothelial progenitor stem cells (EPCs). Clinical studies for cardiac transplantation in animals showed functional restoration of their heart and rather than those were not exposed to therapy. They can be easily isolated and directly transplanted without wasting time to culture them.
Endothelial Progenitor Cells: can be easily collected from blood and bone marrow. They are responsible for healing injuries in the peripheral circulation. In a case vascular damage, EPCs help the organism during revascularization by angiogenesis. Plasticity of EPCs is limited in heart related diseases because they do not differentiate into myogenic lineages, however can restore function to the damaged area of the heart by providing oxygen and blood nutrients.
Mesenchymal Stem Cells: Mesenchymal stem cells are multipotent cells which have the ability to differentiate into different cells and therefore tissues like cartilage, bone and adipose tissue. This type of cells can be found in high number and isolated from bone marrow, adipocytes, heart tissue and some others. MSCs can be treated under certain conditions to differentiate into a desired culture. Specific growth factors like the transforming growth factor Î² (TGFÎ²) must be present so that transformation of the MSCs into a different cell lineages (Antal S., et al. 2009). MSCs are capable of allogeneic transplantation because they express low levels of human leukocyte antigen class II proteins (Tse WT., et al., 2003). Clinical trials have been made by differentiating MSCs into cardiomyocytes for treating cardiac infraction but because of their large size have shown to cause microinfraction (Vulliet Pr., et al., 2004).
Amniotic Fluid Stem Cells:
Hematopoietic Stem Cells (HSCs): widely used in the reconstruction of blood cells especially to treat diseases like leukaemia however difficulties in the purification of HSCs from adult tissues exists.
Embryonic Stem Cells
ESCs in contrast to ASCs have other privileges by means of therapeutic applications. First of all they have the property of pluripotency which allows them to differentiate into most of the cells in the human body, except placental tissues and are able of self-renewal. Culturing in vitro can occur very fast since their doubling time is from 24 to 48 hours (Hmadcha A., et al. 2009). On the other hand, ESCs have a particular pitfall which reconsiderations are on the table of how to treat these cells. Their drawbacks are that they give rise to teratoma in vivo (Fujikawa T., et al. 2005), difficult to stabilize them and many others (see table 1) which will be stated in context. However models have been created and experiments have been made by genetically modifying these stem cells in order to limit these drawbacks. For example a technique has been developed by scientists (Takahashi and Yamanaka) that adult somatic cells are de-differentiated to produce patient-specific pluripotent stem cells, thus avoiding the consumption of embryos.
Endothelial Stem Cells (EC): peripheral artery disease, ischemic heart diseases and cerebral ischemia therapies using human ESC-ECs are under observation. Experiments showed successful transplantations of mesenchymal precursor cell line into an animalâ€™s vascular tree created newly synthesized vessels and blood was able to flow through. By the means of angiogenesis/vasculogenesis, this implantation of human ESC-EC cells could repair the vascular system of a patient. However, as stated above, some aspects need to be reconsidered in the use of ECs like contamination and teratoma formation (Zongjin L., et al. 2008).
Techniques have been developed to overcome the limitations by transplant rejection of using ESCs for tissue repair and transplantation. This technique involves the emerging of ESCs from nuclear transfer from the target own cells. In general words, the optimization of several factors for human somatic cell nuclear transfer in order to derive cloned blastocysts that will be further cultured and modified to result in the output of human cloned ESCs line. Thus gives rise to autologous ESCs which will be used in clinical uses like transplantation (Hwang S. W., et al 2005).
Additionally, as stated before ESC can differentiate and proliferate into a wide range of cell types from all three germ layers. For example neuronal (ectodermal), endothelial (mesodermal) and pancreatic cells (endodermal).
Outside the scientific drawbacks of embryonic stem cells, some ethical issues are acting as barriers on the uses of embryos for regenerative medicine applications. The fact that ESC lines are obtained from fertilized embryos before their implantation. Political and theological ideologies act against the research and use of embryos in science due to their destruction. On the other hand scientists still need more time to unlock all the features of embryonic stem cells and find alternative ways of reprogramming other stem cells to act like embryonic stem cells. These obstacles slow down the progression of investigating and exploring the uses of ESCs in regenerative medical applications.
Stem cells utilization in regenerative clinical uses sent very promising messages for the treatment of diseases and abnormalities that were impossible to achieve before. Adult stem cells are already being used in circulatory blood reconstruction for blood related diseases, bone marrow cells in spinal cord defects and many others on the way. However, the pluripotent embryonic stem cells which have a great plasticity face ceaseless attacks from our community in order to stop using embryos due to theological and political views. All the above lead to the conclusion that both ASCs and ESCs should be exhaustively investigated. Additionally scientist should be supported by every one of us in order to achieve the best of stem cells and find the light to unanswered questions about diseases.