Regenerative medicine has been an emerging field that represents replacement of lost or damaged tissues in the human body through either cellular transplantation or endogenous repair. It has been considered that stem cells are the potential mediators that function as reservoir for repair and regeneration of damaged tissues, because of its ability of differentiation into many different lineages of specialized cells [123]. Recently, the discovery of induced pluripotent stem cells (iPSC) which have differentiation capacity into all kind of cells in the body, called pluripotency, encourages the synthesis of scientific principles and technology in applications in regenerative medicine.

iPS cells can be achieved by reprogramming somatic cells using reprogramming factors and can undergo self-renewal as well as differentiate into all three germ layers [45]. The in vitro generation of pluripotent cells without the use of embryonic material has been rendered a more suitable source for regenerative medicine compared with nuclear transfer-embryonic stem cells (NT-ESC) [67] or somatic cell nuclear transfer (SCNT) [89]. Moreover, the one of advantages of iPSC is to overcome immune rejection that occurs when one uses cells of an individual to generate pluripotent stem cells. These features make iPSC a useful tool for drug screening, the human disease model and cell therapy including autologous transplantation [31011]. Thereby, since the initial report of mouse and human iPSCs by Takahashi and Yamanaka group, other groups recapitulated and found the iPSC technology to be more reproducible, effective, safe and a therapeutically compatible iPSC generation for the clinical setting. More studies about recent trends will be further discussed.

The challenge of iPSC technology remains similar to those faced in the human embryonic stem cells (hESC) field, such as appropriate differentiation of the cells and the risk of tumor formation following cellular transplantation, although the latter concerns regarding tumor development have been overcome by selective drugs to induce cell death of remaining iPSC after differentiation [1213]. There are still many unresolved issues mainly associated with the maturation of cells to a fully functional state. To overcome these limitations, recent research focuses on using devices and materials to help make iPSC into miniature organs to produce more functional cells [1415]. This may allow production of more appropriate cells for transplantation, meanwhile other goals for higher reproducibility and efficient differentiation protocol currently remain homework. Discussed here are recent advancements in this field related to regenerative medicine, with an emphasis on iPSC generation applicable in the clinical setting and creating more functional iPSC-derivates.

The primary component of reprogramming factors consists of klf4, Oct4, Sox2, and c-Myc which were effective throughout several types of mouse and human cells. Another set including Oct4, Sox2, Nanog, and Lin28 were also enough to reprogram human fibroblasts [16]. Traditionally, iPS cells could be induced from mouse or human fibroblasts through transduction of reprogramming factors mediated by virus systems, which might bring about insertional mutagenesis. In addition, upon re-differentiation of iPSC into target cells, reactivation of reprogramming factors integrated in the donor genome have raised concerns regarding the clinical application due to induction of tumorigenicity [17]. Many studies have been developed to overcome this issue by approaching geno-me footprint free-/non-integrating iPSC for clinically applicable iPSC generation [1819]. Table 1 shows the classified reprogramming techniques based on efficiency, value of footprint, and number of donor types to be reprogrammed as previously reported.

After obtaining reprogrammed cells, identification of iPSC colonies is necessary to determine whether selected the colony is fully reprogrammed by investigating hESC-like morphological criteria, expression of hESC markers endogenously (Tra1-81, Nanog, Oct4, Sox2), loss of transgenes, and ability of differentiation potential as teratoma formation in the case of human iPSC.

The first generated iPSC was selected with ESC-specific gene, Fbx15, which induces partially reprogrammed cells, not yet fully reprogrammed. It was later found that selected colonies with nanog or Oct4, ESC-specific genes, were much more similar to properties of ESC [45]. The identification of human iPSC colony was often based upon morphological features such as a cobblestone appearance with prominent nucleoli and pronounced individual cell borders. Of note is that non-iPSC colonies similar morphologically also appeared during cell reprogramming. Additional methods to identify true hiPSC have been using hESC surface antigen expression and loss of transgene dependence as strategies to identify fully reprogrammed cells. In humans, live staining of cultures for Tra 1-81, hESC-specific surface marker has become a useful tool to distinguish between true or false iPSC [35]. The loss of transgene expression related to full reprogramming, can be evaluated by using reporter gene including green or red fluorescence protein (GFP or RFP) that are silenced in the pluripotent state [36]. Finally, whether a fully reprogrammed state has been achieved by assessing the functional properties of iPSC including expression of other hESC surface markers (SSEA4), alkalinposphatase activity, endogenous pluripotent markers (Nanog, Oct4, Sox2) on a molecular level, and the ability to differentiate into lineages cells derived three embryonic germ layer on a functional level. Fig. 2 shows, at molecular levels, the functional properties of iPSC generated from human fibroblasts using Cre-loxP mediated reprogramming methods.

One of the major challenges for cell therapy using stem cells is host immune rejection of the transplanted cells. To facilitate clinical application of stem cells, it is important to develop therapeutically valuable cells that do not induce an immune rejection. Currently, iPS cell lines can be generated from patients with several genetic disorders [35]. Theses patient-specific derived target cells can serve as easily accessible autologous source for cell replacement therapies as well as drug screening and in vitro disease models [10]. Thereby, iPSC technology providing potentially unlimited autologous cells is especially attractive for regenerative medicine. Despite the advantages of iPSC-derivates, there still remain many obstacles for transplantation including whether iPSC-derivates are mature functional cells that can be used clinically. Recent progress on iPSC technology including detailed differentiation required to fulfill the promises (or dreams) of stem cell-based therapeutics for human disease will be discussed.

Recent progress of iPSC strategy for generation and differentiation has provided a great opportunity to accelerate translation of iPSC technology into clinical therapy. The clinically relevant reprogrammed cells such as integration free-, footprint free- and small molecule based-iPSC will contribute to the use of iPSC in regenerative medicine. To cell replacement therapies, patient-specific iPSC-derivates based on the 3D organogenesis system that produce functionally mature target cells by inducing the process of self-organization would have superior resources to applicative autologous transplantation. In the future, the challenges to enable/improve the generation of high survival and engraftable cells in vivo from iPSC are undoubtedly unveiled leading to answers in any unmet medical needs.