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Regenerative medicine is a multidisciplinary field concerned with the replacement, repair or restoration of injured tissues. This field emerged from the need for reconstruction in children and adults in whom tissue has been damaged by diseases, trauma and congenital anomalies. Stem cell research is a promising field with an alluring potential for therapeutic intervention, and thus begs a critical understanding of the long-term consequences of stem cell replacement. Stem cells have unrestricted potential to divide and this strength is used for the regeneration and repair of cells within the body during tissue damage. Research on stem cells is advancing knowledge about how an organism develops from a single cell and how healthy cells replace damaged cells in adult organisms. This promising area of science is also leading scientists to investigate the possibility of cell-based therapies to treat disease. In our present review we tried to provide the information about stem cells and their significant role in regenerative medicine for treatment of various diseases.
Regenerative medicine is an emerging and rapidly evolving field of research and therapeutics to restore, maintain and improve body functions (1). Daar and Greenwood (2) stated that regenerative medicine aims at 'repair, replacement or regeneration of cells, tissue or organs to restore impaired function'. It aids the body to form new functional tissue to replace lost or defective tissue. Ultimately, this will help to provide therapeutic treatment for conditions where current therapies are inadequate. Cell therapy and tissue engineering are part of the broader field of regenerative medicine, whose aim is the delivery of safe, effective and consistent therapies. The human body has an endogenous system of regeneration and repair through stem cells, where stem cells can be found almost in every type of tissue. This process is highly evolved through evolution, and so it is logical that restoration of function is best accomplished by these cells. Therefore, stem cells hold great promise for the future of translational medicine. Regenerative medicine is also a primer for pediatricians (3-6).
In the early 1900's European researchers realized that the various type of blood cells - white blood cells, red blood cells and platelets all came from a particular 'stem cell'. Stem cells were first studied by Becker et al (7), who injected bone marrow cells into irradiated mice and noticed that nodules developed in the spleens of the mice in proportion to the number of bone marrow cells injected. They concluded that each nodule arose from a single marrow cell. Later on, they found by evidence that these cells were capable of infinite self-renewal, a central characteristic of stem cells. Thus, stem cells by definition have two essential properties, i.e. the capacity of self renewal, and the capacity to differentiate into different cell lineages. Under the right conditions, or given the right signals, stem cells can give rise (differentiate) to the many different cell types that make up the organism (Fig-1). Stem cell lineage determination is explained by several ideas, one among is focused on the stem cells microenvironment or 'niche'. A niche consists of signalling molecules, intercellular communication and the interaction between stem cells and their neighboring extracellular matrix. This three-dimensional microenvironment is thought to influence/control genes and properties that define 'stemness' of the stem cells, i.e. self-renewal or development to committed cells. An interesting theory put forward is that stem cells might be terminal differentiation cells with the potential to display diverse cell types, depending on the host niche. Adult stem cells that are implanted into a totally different niche (different germ layer) can potentially differentiate into cell types similar to those found in the new environment. The potential of stem cells and its plasticity are having invaluable properties for regenerative medicine (8). Beneficiaries of regenerative medicine include the increasingly ageing population, people with sports injuries and war casualties. The tremendous technological progress achieved during the last decade in gene transfer methods and imaging techniques, as well as recent increases in our knowledge of cell biology, have opened new horizons in the field of regenerative medicine. Genetically engineered cells are a tool for tissue engineering and regenerative medicine, albeit a tool whose development is fraught with difficulties (9,10). This review summarizes current knowledge of stem cells in regenerative medicine particularly in the treatment of various diseases.
1.1. TYPES OF STEM CELLS:
There are two main types of stem cells, embryonic and non-embryonic. Embryonic stem cells (ESCs) are totipotent and, accordingly, they can differentiate into all three embryonic germ layers. On the other hand, non-embryonic stem cells (non-ESCs), also known as adult stem cells, are just multipotent; their potential to differentiate into different cell types seems to be more limited (11). Embryonic stem cells are derived from the inner cell mass of a blastocyst (a very early embryo) and the adult stem cells are derived from mature tissue. A large variety of cell types have been used for regenerative medicine, including adult cells, resident tissue specific stem cells, bone marrow stem cells, embryonic stem cells(12) and the recent breakthrough discovery of induced pluripotent stem cells from mature/adult cells (iPS) (13).
1.1.1. EMBRYONIC STEM CELLS:
Human embryonic stem cells (ES cells) are primitive (undifferentiated) cells that can self-renew or differentiate into all cell types found in adult human body (14,15). The derivation of mouse ES cells was first reported in 1981 (16,17) but it was not until 1998 that the derivations of human ES cell lines were first reported (18). A new era in stem cell biology began in 1998 with the derivation of cells from human blastocysts and fetal tissue with the unique ability of differentiating into cells of all tissues in the body. Embryonic stem cells are derived from embryos at a developmental stage before the time that implantation would normally occur in the uterus. Each of the cells (blastomeres) of these cleavage-stage embryos is undifferentiated. The first differentiation event in humans occurs at approximately five days of development, when an outer layer of cells committed to becoming part of the placenta (trophectoderm) separates from the inner cell mass (ICM). The ICM cells have the potential to generate any cell type of the body, but after implantation, they are quickly depleted as they differentiate to other cell types with more limited developmental potential. The ICM derived cells can continue to proliferate and replicate them indefinitely and still maintain the developmental potential to form any cell type of the body (Fig-1).
Bongso et al. (19) first described isolation and culture of cells of the inner cell mass of human blastocysts, and techniques for deriving and culturing stable hES cell lines were first reported in 1998 (18). The trophectoderm was removed from 5th day blastocysts consisting ICM of 30-34 cells, was placed into tissue culture. The possible sources of stem cells are embryos created via In vitro Fertilization (IVF) (20), embryos or fetuses obtained through elective abortion and embryos created via somatic cell nuclear transfer (SCNT) or cloning. They can be isolated by immunosurgery from the inner cell mass of the embryo during the blastocyst stage, and are usually grown on feeder layers consisting of mouse embryonic fibroblasts or human feeder cells (21). More recent reports have shown that these cells can be grown without the use of a feeder layer (22), and thus avoid the exposure of these human cells to mouse viruses and proteins. These cells have demonstrated longevity in culture by maintaining their undifferentiated state for at least 80 passages when grown using published protocols (3, 23). The source of ESCs opens a Pandora's box of ethical dilemmas, including the moral status of the embryo, the sanctity of life (24) and the possible use of saviour siblings as a source of ESCs. These add to the long-standing accusation to scientists of tampering with the natural process of life. The ethical debate relates to whether it is right to use human tissue in an abnormal manner. Life-saving situations where the strongest ethical arguments can be made to support the use of cells that are from an embryo that will not become an independent human life. As non-ESCs use becomes more widespread, then acceptance of ESCs treatments may increase. When ethical obstacles are overcome, ESCs might be introduced for treating several conditions, including diabetes (25), spinal cord injuries (26) and liver (27) and heart transplantation (28). Recently Guenou et al (29) demonstrated that human embryonic stem cells (hESCs) can differentiate into mature keratinocytes able to generate a pluristratified epithelium on immunodeficient mice. Jukes et al (30) reviewed on chondrogenic and osteogenic differentiation of mouse and human embryonic stem cells (ESCs) and their potential in cartilage and bone tissue engineering.
Embryonic stem cells have been shown to differentiate into cells from all three embryonic germ layers in vitro (Fig-1). Skin and neurons formed from ectodermal differentiation (31-34), blood, cardiac cells, cartilage, endothelial cells, and muscle formed from mesodermal differentiation (35-37) and pancreatic cells from endodermal differentiation (38). In addition, as further evidence of their pluripotency, embryonic stem cells can form embryoid bodies, the cell aggregations that contain all three embryonic germ layers, while in culture, and can form teratomas in vivo (39,40).
188.8.131.52. Human Embryonic Germ Cells:
Embryonic germ cells are derived from primordial germ line cells in early fetal tissue. Unlike embryonic stem cells, animal experiments on embryonic germ cells have been limited. In 1998 the isolation, culture, and partial characterization of germ cells derived from the gonadal ridge of human tissue obtained from abort uses were reported (41). There are fewer data from animal embryonic germ cell experiments than from ES cell experiments, but it is generally assumed that the range of potential fates will be relatively limited compared to ES cells, because the embryonic germ cells are much further along in development (5-9 weeks). Fetal tissue may provide committed progenitors, but the feasibility of large scale sourcing and manufacturing of products utilizing such cells is questionable. Furthermore, the behavior of these cells in vivo is not well understood; significant research will be required to avoid unwanted outcomes, including ectopic tissue formation i.e., additional, unwanted tissue, tumor induction, or other abnormal development (31).