Stem Cell Primer

What are “stem cells?”

Stem cells are the body’s “master cells” that can renew themselves. It is possible for some stem cells to become a variety of different kinds of other cells. Stem cells have been used for many years to treat leukemia and other blood disorders. 

While there are 5 main categories of stem cells – which we’ll get to momentarily – stem cells are classified as either Embryonic stem cells or Adult stem cells. 

Stem cells are classified in one of two ways:

  1. differentiation potential, or what the cells can potential turn into and 
  2. By origin, or source tissue.

Embryonic stem cells are derived from embryos that are typically 3-5 days old. 

Embryonic stem cells are set up to reproduce themselves until they can be “coaxed” into creating a specific type of cell.

While embryonic stem cells are the most potent, there are scientific issues and concerns beyond the moral hazard of using embryos. Until recently, the only kinds of stem cells known to have caused tumors have been embryonic stem cells. It has recently come to be known that Induced Pluripotent stem cells (iPS) have caused tumors. (Read more about iPS below.)

Adult stem cells are stem cells that come from the tissues that are no longer fetal tissue. 

The job of adult stem cells is to “replenish” other tissues. They renew themselves periodically to make sure that the body has a storage of stem cells in times of bodily need (the fighting of damaged or diseased tissues). However, as our bodies age, we lose a larger number of stem cells.

Scientists have identified several stem cell storage centers, or caches, in the body. Adult stem cells can be found in:

    • placental tissue 
    • umbilical tissue 
    • bone marrow
    • the brain
    • adipose (fat) tissue
    • the liver
    • reproductive systems (male and female)
    • skeletal muscle
    • skin
    • teeth

Adult stem cells do not have the ability to change into as other kinds of cells and are not nearly as potent as embryonic stem cells. However, adult stem cells do not come with the moral hazard that comes with embryonic stem cells. Even though not as potent as embryonic stem cells, adult stem cells offer tremendous treatment options that can help the body heal itself. So far, there are no known cases of tumors caused by adult stem cells.

Adult stem cells, minimally manipulated, have a tremendous advantage over embryonic stem cells: 

There is no guarantee that embryonic stem cells are going to stop. This is likely one of the reasons why embryonic stem cells have caused tumors, and adult stem cells have not.

Adult stem cells, on the other hand, are limited to their “normal” environments. What does this mean? Let’s say that you were to receive an adult stem cell transplant. For this example, let’s assume the stem cells are pulled from adipose (fat) tissue from your own body, and then, after the stem cells are extracted from the adipose tissue, transplanted into your knee. With the use of adult stem cells, you have no concern of an ear (or spleen or any other organ) growing in your knee joint. 

This may seem like an extreme example and, to a degree, it is. However, embryonic stem cells require the use of other “coaxing” agents and anti-rejection drugs.

To further explain, let’s discuss the 5 different categories of stem cells.

As stated earlier, stem cells are classified in one of two ways:

    1. differentiation potential, or what the cells can potential turn into and 
    2. by origin, or source tissue

Because there are potentially hundreds of different sources of stem cells in the body, it makes the most sense to understand stem cells based on their capacity to differentiate, or change into other cells.

    • Totipotent Stem Cells- Carry the most potential to change into something else. The best example of a totipotent cell is a fertilized egg. 
    • Pluripotent Stem Cells – The best example of a pluripotent stem cell is an embryonic stem cell. These cells can turn into several different kinds of other cells with the potential to form all kinds of tissues and organs in the human body. The concern here is that scientists have to manipulate the cells to change them into the specific type of tissue they want. There are a variety of different chemicals and biomaterials that are used. In addition, if a patient were to receive an embryonic stem cell transplant, it is highly likely that the patient will also have to take anti-rejection drugs, much like when a patient receives an organ transplant. There is a pluripotent stem cell not derived from embryonic stem cells, but adult stem cells. Induced pluripotent stem cells, or iPS cells, come from adult stem cells. These cells can be reprogrammed to function like embryonic stem cells. While this does eliminate the moral issue regarding the use of embryos, the study and, more importantly, the use of any pluripotent stem cells is currently limited due to safety, policy, and cost issues and concerns. 
    • Multipotent Stem Cells – While not having the same potential potency as pluripotent stem cells, multipotent stem cells can have the ability to give rise to several different kinds of tissues, as long as in their normal environments. What does this mean? Liver cells don’t create heart cells and brain cells don’t generate kidney cells. 

Scientists have identified several stem cell storage centers or caches in the body. Adult stem cells can be found in:

        • placental tissue 
        • umbilical tissue 
        • bone marrow
        • the brain
        • adipose (fat) tissue
        • the liver
        • reproductive systems (male and female)
        • skeletal muscle
        • skin
        • teeth

An excellent example of multipotent cells are something called mesenchymal stem cells (MSC). While these cells have the ability to differentiate, or turn into other kinds of cells, some relatively new evidence (about 2017) has shown that “MSC” should actually stand for “medicinal signaling cells.” This means that these cells did not differentiate. These cells actually “nurse” the damaged/diseased tissue back to health.

    • Oligopotent stem cells – These stems cells are similar to mulipotent stem cells, but have less ability to differentiate and can only change into closely related cell types.
    • Unipotent Stem Cells – the least potent of all stem cells, the cells can only change into one other kind of cell type. That is, muscle cells can only turn into muscle cells.