An explanation of stem cells and their potential to treat HD
What are the advantages and disadvantages of using embryonic stem cells, embryonic germ cells, and adult stem cells for research?
At first glance, embryonic stem cells, embryonic germ cells, and adult stem cells all present similar possibilities for scientific research. They are all stem cells, after all, and therefore share some key characteristics and hold similar potential. For example, they all have the ability to self-replicate for indefinite periods of time in the human body and can give rise to specialized cells. The overall purpose behind research with all types of stemm cells, therefore, is very similar. (See Part 3 for a more detailed description of stem cell research.) It has also been shown that all three cell types can be isolated from other cells and kept in a specific laboratory environment that keeps them unspecialized. This is crucial for controlled scientific research. Upon experimentation, it has also been shown that all stem cell types will replicate and specialize when transplanted into an animal with a lowered immune system. The cells then undergo “homing,” a process where the transplanted cells are attracted to and travel to an injured site when transplanted into an animal that has been injured or diseased. Homing provides hope that the transplantation of stem cells will be a clinically useful procedure.
Despite these general similarities, there are some important differences between embryonic stem cells, embryonic germ cells, and adult stem cells. The origins of these three cell types define their differences: ES cells are derived from the inner cell mass of the blastocyst in a developing embryo, EG cells are obtained from the primordial germ cells of a fetus, and adult stem cells are found in developed, specialized tissues. The differences between ES, EG, and adult stem cells result in different advantages and disadvantages for each stem cell type in scientific research and development.
ES and EG cells have some clear advantages over adult stem cells concerning research and clinical usefulness. For example, ES and EG cells are pluripotent, meaning they have the potential to give rise to all types of cells in the body. Adult stem cells are multipotent, meaning they only have the potential to give rise to a limited number of cell types. So far, no adult stem cells have proven to be pluripotent. This means that ES and EG cells could potentially provide a renewable source of replacement cells for any tissue in the human body (see Transplantation Research for more details). Adult stem cells, however, would only be clinically useful for the specific adult tissue that the stem cells came from. ES and EG cells are also relatively abundant in the developing organism, especially compared to adult stem cells, which are scarce in the adult body. As a result, ES and EG cells are much easier to identify, isolate, and purify compared to adult stem cells, which are very difficult to identify, isolate and purify in the lab. This makes research with ES and EG cells all around easier than research with adult stem cells.
On the flip side, adult stem cells have some distinct advantages over ES and EG cells. For example, adult stem cells are around for an organism’s lifetime, while ES and EG cells are only found in the developing organism. This allows a longer time frame for adult stem cells to be studied in an individual. Also, removal of stem cells from an embryo will result in the death of the embryo. Removal of adult stem cells, however, does not involve the death of an embryo, and is therefore less ethically complicated. Furthermore, adult stem cells pose no chance of immune rejection after transplantation because they can be transplanted back into the adult that they came from. ES and EG cells are derived from embryos and fetuses, however, and are transplanted into people with different genetic make-ups. Therefore, rejection is an issue only with the use of ES and EG cells.
Finally, ES cells have a strong advantage and disadvantage over the other stem cell types. First, ES cells are able to replicate in the laboratory far better than either EG or adult stem cells. ES cells can self-renew for up to 2 years, doubling up to 300 times. EG cells can only double a maximum of 70-80 times. Meanwhile, adult stem cells only have a limited ability to replicate in the lab. Replication in the laboratory is critical for research to continue. On the other hand, ES cells are the most likely to develop into tumors. If undifferentiated ES cells are taken from the lab and injected into a mouse, a benign tumor can develop. For this reason, scientists do not plan to use undifferentiated ES cells for transplants or other therapeutic applications. EG cells do not form these tumors, however. At this point, it is not known whether tumors will form with transplanted adult stem cells.
The similarities and differences of ES, EG, and adult stem cells are summarized in the chart below:
Last Modified: 1-25-04
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