Future Applications and Regenerative Medicine. Embryonic stem cells are a potential source of cells for cell replacement therapy for the treatment of a variety diseases. Type I diabetes, cardiovascular disease, PD and blood cell diseases, are all considered candidates for cell replacement therapy [39]. For this application to advance, several issues need to be resolved. The first is the type and number of cells to be transplanted. One of the most important questions is at what stage of maturation should cells be transplanted. The answer to this will depend to a large extent on the lineage under investigation. A second obstacle to be overcome is the danger of contamination of the graft with residual undifferentiated ES cells. Transplantation of ES cells can result in the development of teratomas [6]. One approach for reducing the number of ES cells in the graft is the selection of lineage to be transplanted [40]. A second approach is the use of strategies to eliminate undifferentiated ES cells from the graft [41]. Thus, with a combination of positive and negative selection, it should be possible to generate grafts free of undifferentiated ES cells. A third concern is donor/recipient compatibility and graft rejection. One solution to this problem is to generate individualized ES cell lines through SCNT. With this approach, the cells used for transplantation would be genetically identical to those of the patient.
In addition to developmental biology and cell-based therapy, the ES cell model could be very important for drug discovery and drug development. Cell types such as cardiomyocytes and hepatocytes generated from human ES cells could be ideal candidates for predictive toxicology, revealing the toxicity of certain drugs that might not be detected using conventional assays that rely on animal models. Embryonic stem cells offer different strategies to develop drugs that can be used for regenerative medicine, that is, therapeutic regrowth and/or repair of damaged cells. Genetically modified cell lines with genotypes characteristic of various genetic diseases, could not only provide novel insights into the mechanisms of the disease process, but also offer powerful screening systems for developing drugs for treating those diseases [39].
Human stem cells are one of the most promising candidates for the correction of a defective gene because they are capable of targeting solid organs and can be isolated using a combination of surface markers [42]. In general, ES cells may be very useful in the future in a variety of areas including human developmental biology, pharmacological testing, tissue engineering, gene- and cell-based therapies. Nevertheless, important knowledge is still lacking in many of these areas, much basic work is required, and several hurdles need to be overcome before any clinical successes can be expected. It is also important to take into account the complex ethical, political and religious issues that surround the subject matter.