There are four types of stem cells undergoing current study. They are embryonic SCs, allogeneic (donor) SCs, induced pluripotential adult SCs, and finally autologous SCs. Of these four, only two, donor SCs and autologous SCs have been used to treat arthritis so far.
However, this discussion would be incomplete without mentioning the other types as well.
Embryonic SCs (ESCs) are derived from embryos and are pluripotential, meaning they can easily differentiate into any body tissue.
Self-renewing cells were first extensively studied in mouse cancer models. These cells showed the ability to not only self-duplicate but to also differentiate into multiple types of tissue. Obviously, though, cells that are capable of self-duplication are also capable of becoming malignant.
Potential pitfalls associated with embryonic stem cells are the following:
• The ethical dilemma which has restricted the amount of government spending towards ESC research.
• The risk of malignancy associated with cells that have not completely differentiated yet and are as potent as ESCs.
• The theoretical problem with a graft versus host reaction. While immunosuppressive drugs can be used to mitigate the effects of this, these drugs are not without significant side effects.
There is actually a variant of embryonic stem cells, the fetal stem cell. For more about this, here’s a video: http://youtu.be/vvklsJs86BI
The second type of SC is the induced pluripotential SC. In 2006, Japanese researchers used retroviruses to insert genes into mouse cells. They were able to take these adult mouse cells and cause them to revert back to a pluripotential embryonic state.
The identical technique was then applied to human skin cells. These “adult turned baby cells” are known as induced pluripotential stem cells (IPSCs). Therefore, it is technically possible to take any adult cell and make it function like an embryonic stem cell.
As one might expect, the primary concern is malignancy. How can these cells get controlled off once they start to multiply?
The third type of SC and one which has been used to treat arthritis in both animals as well as humans is the allogeneic or donor SC. These cells come from healthy donors. Advantages are that a tremendous number of SCs can be cultured. Downsides include the potential transmission of unknown genetic disorders as well as the possibility of infection.
The fourth and most commonly used type of SC in arthritis treatment are the autologous SCs or adult SCs. There are various techniques used to harvest these cells from the adult. Typically, they are obtained from bone marrow, fat, or blood, which is then concentrated to provide a maximum number of cells in the smallest possible volume.
Autologous SCs have the advantage of coming from the host – the patient. This helps avoid the consequences of rejection or graft versus host reaction which may occur with SCs that come from either embryos or donors.
These are multipotent, meaning they can be coaxed into becoming a limited number of different tissue types. This is one of the major differences between adult SCs and embryonic SCs (and induced pluripotential SCs).
For a great video about this topic: http://youtu.be/hky2KB1yrX4
Embryonic SCs and induced pluripotential SCs are pluripotent, meaning they can be converted into any type of tissue.
Nonetheless, the multipotent property of autologous (also known as “mesenchymal” SCs) is sufficient for them to be used to treat disorders involving connective tissue, such as blood, tendon, ligament, cartilage, bone, nerve, muscle, and liver.
These cells are programmed to zero in on areas of tissue injury to help with repair. While it is still not clear what the homing mechanism is, it is suspected that different types of chemical messengers are involved in “calling” mesenchymal SCs. These cells are capable of contributing to both repair as well as regeneration.
One danger is that some techniques using adult SCs involve the use of cell culture ex vivo, meaning outside the body. The possibility of contamination and infection is a concern. Also, when cultured for a lengthy period of time, these SCs may become unstable and the possibility of malignancy developing arises.
Finally, a major weakness of adult SCs is that they are restricted in their capacity to replicate and differentiate. In other words, they go through an aging process and have a limited life span. This is unlike embryonic SCs which have the capacity to multiply and divide forever.
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