I’ve been seeing some news stories recently describing stem cell therapy. This week I will describe what they are, where they come from, how they might be used to treat disease and finally, touch briefly on the social and ethical challenges surrounding their use.
Stem cells are critical to our development and tissue repair. They have the potential to change into other more specialized cells in the body through a process known as differentiation. By definition, stem cells have to exhibit two properties: (1) they must be able to divide multiple times while remaining unchanged and (2) they have to have “potency,” the ability to differentiate into other cell types.
Stem cells have a hierarchy of potency that is important in determining what they are capable of doing. Totipotent cells sit at the top of the stem cell pyramid. They have the capacity to differentiate into any of the approximately 200 cell types in our bodies. Pluripotent cells can differentiate into nearly all cells, while multipotent cells can become only cells of a closely related family of cells. There are additional levels of potency that produce even fewer cell types.
There are two types of stem cells in humans – embryonic and adult. Embryonic stem cells are derived from embryos that are 4-5 days old. Embryos at this stage contain about 50 to 150 cells, some of which are pluripotent and can propagate indefinitely.
Adult stem cells are found in most tissues in the body and are multipotent. They are generally able to produce all of the cell types of the particular organ where they reside. They can also be found in umbilical cord blood. These cells exist to replace and repair tissues. It is hypothesized these cells may bear responsibility for development of some cancers since they have the capacity to divide almost indefinitely, one of the hallmarks of cancer cells.
Stem cells can be removed from either embryos or various tissues of the body and then grown in laboratory tissue culture. This is a very tricky business, as stem cells naturally want to differentiate into other cells. The stem cells need to be kept in a very specific biochemical soup to prevent them from differentiating. One of the most challenging facets of stem cell research is discovering the complex biochemical signaling that controls stem cell differentiation.
The excitement surrounding the use of stem cells arises from the potential they have to replace damaged or genetically defective cells and tissues in the body. Currently, the only stem cell treatments approved by the Food and Drug Administration (FDA) are products that treat certain cancers and disorders of the blood and immune system.
There are no currently approved embryonic stem cell treatments. However, the first human study using embryonic stem cells was started a few years ago. It involved placing stem cells around damaged nerves in patients with spinal cord injuries.
There are a number of potential challenges when using embryonic stem cells. One is discovering how to program them to differentiate into the desired cell type. Stem cells can differentiate into masses of different cell types and tissues if allowed to grow on their own. If a patient receives stem cells from a human donor, there is also the risk of rejection by the recipient’s immune system.
Finally, embryonic stem cell research presents social and ethical challenges in regard to obtaining stem cells from human embryos. Removing embryonic stem cells usually resulted in destruction of the entire embryo. A technique was developed in 2006 that allows the embryo to survive following stem cell removal.
More recent research has developed a methodology to produce pluripotent stem cells from fully differentiated cells. These reverse-engineered stem cells are called induced pluripotent stem cells. Scientists have taken skin cells and added various proteins to alter DNA expression to cause the mature skin cells to revert to stem cells. This discovery is very exciting and has the potential to obviate the need to use embryonic stem cells.
You may run across medical clinics that claim to offer stem cell therapies. This a very questionable practice since using stem cells for treatment of diseases is in its infancy and there is no good evidence to indicate they are safe and effective. The FDA is really cracking down on stem cell clinics. Consumer Reports had an excellent review in January 2018 that you can read at goo.gl/vfZfvv.
-Dr. John Roberts is a retired member of the Franciscan Physician Network specializing in Family Medicine.