Regenerative medicine operates on the basis of replacing or repairing damaged or aged tissue that has lost its ability to function naturally. This is achieved in the modern day by growing or culturing living tissue with specific functions to be transplanted in place of the now obsolete tissue.  However, long before the first recorded usage of the term regenerative medicine in scientific literature, which was in 1992 by Leland Kaiser  , the concept of regeneration was not foreign to ancient civilizations. For example, the ancient Greeks named the liver "hÄ“par", which was derived from another Greek word which meant "repairable", indicating an understanding of the regenerative capability of the liver.  Fast forward to December 23 1954 and prior advances in regenerative medicine had paved the way for the first successful kidney transplant in a human being, Richard Herrick who received the donor kidney from his twin brother, Ronald.  Presently, there have been huge leaps of progress in the field such as engineered autologous bladder tissue which showed to improve bladder function postoperatively in patients with bladder disease who required cystoplasty.  The engineered tissue was also found to be highly similar to native bladder tissue, showing the great level of precision that has been achieved.
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Among the factors leading to the success of regenerative medicine in recent years, the main contributor has been stem cell therapy, while gene therapy has great potential to add to future successes of regenerative medicine. Stem cells, by simple definition, are cells that are undifferentiated, in that they do not possess specific characteristics or functions unlike mature cells such as beta-cells in the pancreas which function to release insulin when required. The common types of stem cells involved in regenerative medicine are embryonic stem cells which are obtained from a blastocyst and may develop into all the cells in the body, adult stem cells which are extracted from various tissues and may develop into any cell in the lineage of its tissue of origin as well as, more recently, induced pluripotent stem (IPS) cells which are cells obtained from any tissue that have been 'reprogrammed' to exhibit embryonic stem cell behaviour  . Due to the nature of stem cells, in that they may be manipulated to differentiate into a tissue of interest, they possess huge potential in regenerative medicine (Figure 2).
Figure 1: Sources from which human stem cells may be obtained. 
Figure 2: Potential areas in which stem cell therapy may be applied. 
As seen in Figure 2, one area in which stem cell therapy may be applied is in spinal cord injuries. As shown in a study by K-S Kang et al (2005), a successful operation was performed in which multipotent stem cells derived from human umbilical cord blood (UCB) were transplanted into a patient with spinal cord injury due to damage to the 11th and 12th thoracic vertebrae.  Stem cells derived from UCB were chosen for the procedure in an attempt to confirm results of a prior study which found that UCB-derived stem cells cultured in a neurogenic medium developed into neuronal cells.  Remarkably, the postoperative results indicated improved mobility and sensory perception in the hips and thighs within 41 days in the patient who previously was paraplegic due to the sustained injuries. It is worth noting that preoperatively, the patient obtained a '0/5 score on a manual motor test of her lower extremities and was recorded to have absent range of motion of joint in the lower extremities'. In contrast, by day-15 following the operation, the patient was able to 'elevate to about 1cm both of her legs'. It is also worth noting that within 41 days after the operation which involved transplantation of stem cells and laminectomy, 'the spinal cord which was previously atrophied was observed using MRI and CT to have expanded and enlarged', indicating regeneration of the spinal cord.
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Another potential area of application of stem cells in regenerative medicine is tooth replacement. A problem with current tooth replacement is that the implants involved fail to produce root structures. The consequent result to this lack of a root structure is jaw bone resorption due to forces exerted while chewing. This has been shown in studies published by Maiorana C, et al. (2005)  and by Heineman F, et al. (2010)  . While these studies demonstrated that implant design played a part in reducing bone resorption, it did not eliminate the problem due to the lack of a root structure as mentioned above. In a very recent study by A. Angelova Volponi, et al. (2013)  , they investigated alternative cell sources for bioengineering new teeth. They demonstrated that by combining cells obtained from adult human gingival tissue with mouse embryonic tooth mesenchyme cells, teeth could form. This mixture of cells was cultured for 7 days and it was then transplanted to renal capsules of adult mice with SCID and given a period of 6 weeks to develop. It was found that 1 in 5 transplantations formed teeth which, when analyzed with micro CT, were found to "show typical tooth appearance, with well-developed crowns and roots". While this only translates to a 20% rate of tooth formation, it proves that adult human gingival cells are a viable source for use in tooth regeneration and research will no doubt continue to improve the formation rate.
As mentioned above, gene therapy has the potential to further the success of regenerative medicine, but for the most part, gene therapy has been relegated to experimental status until very recently. Gene therapy involves replacing a mutated or non-functional gene with a correctly-functioning copy of that gene or even introducing a new gene altogether into a patient which confers a specific function to aid in combating a disease.  The main research branch of gene therapy is somatic gene therapy which involves the individual patient and the widely prohibited branch of germ line gene therapy, due to ethical issues.  Most gene therapy research involves the use of viruses as transport systems or vectors which are modified to remove their virulence. The gene of interest is inserted into the virus, which is subsequently injected into the patient to infect target cells and thus introducing the functional gene into the nuclei of the cells.  This is illustrated by the following images (Figure 3 and Figure 4).
Figure 3: Outline of the use of viral vectors in gene therapy. 
Figure 4: The action of an adenovirus vector in infecting a target cell. 
Gene therapy has been the subject of much controversy, a fair amount of which is attributed to the use of viruses as vectors. A concern put forward on this matter is that the virus may recover its virulence and in turn cause disease in the patient.  Another setback of gene therapy is that once the virus is injected into the patient, the immune system is likely to mount an immune response against it.  However, despite of these issues and setbacks, gene therapy has had success such as the recovery of vision in patients with Leber's Congenital Amaurosis, a blindness-causing disease involving a mutation in the gene encoding retinal pigment epithelium.  Also, a recent breakthrough has seen Glybera, the 'first commercial gene therapy in Europe and the USA'  , be approved for marketing by the European Commission. Glybera is intended to be marketed as a therapy for a condition called lipoprotein lipase deficiency (LPLD), the effects of which are a build-up of large amounts of fat in the blood causing symptoms such as abdominal pain, loss of appetite and nausea.  Glybera involves the introduction of a functional copy of the lipoprotein lipase gene to skeletal muscle cells with the use of an adeno-associated virus (AAV) vector and it has been shown in clinical trials that blood triglyceride levels were reduced for 'up to 12 weeks after injection' and the incidence of pancreatitis attacks was 'drastically reduced for up to 2 years after treatment'  , proving it's worth as a viable treatment method.
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In closing, both stem cell and gene therapy are major players in the regenerative medicine field. However, they are not free from having hurdles which need to be overcome with future research. For stem cells, this hurdle comes in the form of ethical issues pertaining to the use of human embryonic stem cells in research. Though, the successful induction of pluripotent adult human stem cells by Shinya Yamanaka et al. (2007)  is a starting point to overcoming this issue. Setbacks in gene therapy, as aforementioned, involve the concern of the risks associated with the use of viruses as gene vectors. While this would require much research, a possible solution may be the use of an inert transport system such as carbon nanotube (CNT)-based structures. As shown by Ciraci S, et al. (2004  ), CNTs may be functionalized to improve cell target specificity while Bianco A (2004) reviewed that functionalized CNTs were able to 'localize into the cytoplasm and nucleus without damaging the cell membrane'  . These characteristics make CNTs a potential candidate as gene transport vectors in future. At the current rate of success and volume of research being carried out, regenerative medicine involving stem cell and gene therapy is likely to become a mainstream therapeutic method in the foreseeable future.