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Isolation and Clinical Potential of Human Embryonic Stem Cells.
Stem cells, under the right conditions, have the ability to differentiate into many specialised cell types. They can give rise to cardiomyocytes, nerve cells, adipocytes and osteocytes. The most researched stem cell is the adult stem cell, but these cells, except in the treatment of leukaemia, do not often lead to true transdifferentation when used clinically. Much research is still to needed to prove their efficacy in treating other diseases, including heart defects for example. The human embryonic stem (hES) cell is characterised as the most plastic stem cell, capable of differentiating into any cell type. HES cells are said to be pluripotent. This means they are able to differentiate into all the germ layers; ectoderm, mesoderm and endoderm. This would achieve the true transdifferentiation that adult stem cells cannot. Although hES cells have huge potential in the treatment of diseases certain issues, such as control of proliferation and differentiation, must be extensively studied when considering their clinical potential. In addition major ethical issues prevent the mainstream isolation of hES cells.
Isolation of Human Embryonic Stem Cells:
In order to be used in regenerative medicine, hES cells must be generated in sufficient numbers via good manufacturing practice. The culture mediums should be free of animal proteins to prevent the risk of infection. They should also be genetically and epigenetically normal (Skottman et al., 2006). HES cells are derived from the inner cell mass (ICM) of preimplantation embryos. In 1998 the first hES cell lines were derived by Thomson et al. from the ICM of surplus human embryos resulting from assisted reproductive procedures (Young and Carpenter, 2006). Currently embryos are donated with the informed consent of couples undergoing in vitro fertilization therapy (Pera et al, 2003). Many people regard isolation of human stem cells as intentional killing of the embryo; it is a very difficult ethical problem (Medical Hypotheses Editorial, 2007).
The first step to isolation of hES cells is to culture the donated embryos to the blastocyst stage. They are grown on specific culture media before obtaining the ICM via immunosurgery. Immunosurgery involves removing the zona pellucida with an enzyme such as pronase or Acidic Tyrode’s solution (Klimanskaya and McMahon, 2006). Following this, the outer trophoblast layer is selectively removed using rabbit anti-human antibodies (Vugler et al., 2007). The ICM is then washed thoroughly and propagated on the culture medium DMEM (Dulbeco’s modified Eagle’s medium) with a mouse embryonic fibroblast feeder layer (Laslett et al., 2003). The use of alternative methods to culture hES cells, such as feeder-free culture to prevent infection, has proved unsuccessful, producing sub-optimal cells (Skottman et al., 2006). After a number of days the ICM will be ready to subcultivate. Firstly the viable, undifferentiated cells are separated from those that have differentiated, these segments are identified via the use of a stereomicroscope. The undifferentiated cells are then plated onto new culture dishes. Successful propagation of the ICM is associated with the appearance of cells with undifferentiated hES cell morphology (Sartipy et al., 2007).
However, there is a risk that the derivation and culture of hES cells could produce epigenetic alterations, often leaving genomic regions susceptible to DNA methylation, which is known to be involved in tumour progression (Pannetier and Feil, 2007).
Clinical Potential of Human Embryonic Stem Cells:
The potential of hES cells to differentiate into all three germ layers and indefinitely replicate has raised hope that these cells could be a renewable source for cell transplantation in severe degenerative diseases (Hoffman and Carpenter, 2005). Many different avenues of research employ the use of hES cells. These include tissue-engineering, cardiology, treating spinal cord injury, testing drug toxicity, among others.
Tissue engineering requires cells that are easily isolated, sufficient in number, and have a defined and controlled phenotype; hES cells are one of the most promising candidates (Cohen et al., 2006).
It is hoped that embryonic derived cardiomyocytes will be effective in treating disorders such a myocardial infarction. Applying specific growth factors to hES cells allows the generation of cardiomyocytes in vitro for use in cell replacement therapies (Winkler et al., 2004). Due to growing transplant lists there is an increased need to develop new methods to treat people with heart failure and cardiomyopathies (Salmons, 1999). As these ES cell derived cardiomyocytes are highly plastic they appear ideal for cardiac repair applications (Zimmerman and Eschenhagen, 2007). Currently there is no clinical trial in humans but trials in mice have produced promising results. After transplantation of embryonic cardiomyocytes into the damaged myocardium of mice there was significant improvement in left ventricular ejection fraction and cardiac output (Roell et al., 2006).
In relation to the treatment of spinal cord injury (SCI) no human clinical trials have been carried out yet but hES cells have been directed to differentiate into multipotent neural precursors. A study in rats has shown that ES cell derived neurons can survive, integrate and help restore function after transplantation (Coutts and Kirstead, 2007), so there is potential for the use of hES cells to treat SCI.
In the case of drug toxicity testing, the fact that hES cells are pluripotent is highly advantageous. The development of cell-based assays using hES as physiological targets for drug activity should mean that safer and more effective drugs would be introduced into clinical trials and the marketplace (Cezar, 2007).
It is important to realise that even with the great potential hES cells have there are several challenges to be faced before they can be used in clinical trials. These include the formation of teratocarcinomas and graft versus host disease (Winkler et al., 2004).
The use of hES cells in treatment of disease is one of the most exciting prospects for researchers and clinicians at the moment. Due to their highly plastic nature, hES cells are excellent candidates for use in cell replacement therapies. Although their clinical potential is known, the isolation of hES cells being a serious ethical issue, adverse immune reactions and the possibility of teratocarcinoma formation in the recipient means that it will be a long time before they are accepted as a conventional treatment for degenerative diseases.
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