I’m off to the Vatican on Monday, so I figured I could take some time for an extremely brief introduction (EBI… perhaps this will be a series here – any opinions?) to adult vs. embryonic stem cells (ESCs) as a precursor to the information that will be presented at the conference. I’m intentionally excluding a recent stem cell discovery, namely induced pluripotent stem cells (iPSCs), because they blur some of the distinctions I’ll make between adult and embryonic stem cells. Suffice it to say that iPSCs have (generally) the same properties as ESCs without the moral conundrums since they are derived from adult tissues.
Here’s the 10,000 foot view:
- ESCs are pluripotent while adult stem cells are lineage restricted. This means that ESCs can become any cell type in the body if pushed properly in the right direction, while adult stem cells have a limited number of cell types that they can become. This is also known as “differentiation capacity” – stem cells that can become many different cells have a high differentiation capacity. Since embryonic stem cells are taken from a very early stage embryo, these are the same cells which would have gone on to divide and make every single cell in the fully developed human body – thus the “pluripotent” capacity of these cells.
- ESCs are “immortal” while adult stem cells undergo “senescence.” What this means is that ESCs are capable of growing forever in a plastic dish. They do not ever stop dividing and producing more ESCs. This is very desirable because the cells are relatively easy to culture and because the therapies that these cells would be used for generally require a large number of cells. Adult stem cells, on the other hand, undergo senescence, which is to say that they simply stop growing in a culture dish after a certain number of divisions. This immortal capacity of ESCs leads us to…
- Teratoma formation! The major practical hurdle (as opposed to the moral hurdles) with using ESCs is their uncanny ability to form cancers when injected into living animals. They form very specific kinds of tumors called teratomas. During embryonic development, all the cells in the organism divide into one of three “germ layers” that will go on to develop into certain parts of the body. For example, the “endoderm” layer will make things like your digestive tract, the “mesoderm” will make things like your muscles and bones, and the “ectoderm” layer will make things like your skin and nerves. These tissues under normal development, of course, are well organized and make a functional organism. When you inject ESCs into an organism, however, they differentiate into various cells of all three germ layers in a very disorganized fashion and you end up with a chaotic mass of teeth, hair, skin, and even at times primitively developed organs like eyes or even limbs. Because of the adult stem cells’ limited differentiation capacity and because they are not immortal, this does not happen when they are injected into animals.
- The other major practical obstacle with stem cell therapies is immune rejection. When someone receives an organ transplant (like a heart for example) you must have a “matched” donor. This means that certain proteins known as human leukocyte antigens (HLA) must be the same or at least similar between the donor and the recipient, otherwise the recipient’s immune system will recognize the new heart as foreign and attack it, leading to rejection. Embryonic stem cells express HLA antigens and are therefore susceptible to immune rejection. Because of this, the cells would have to be from an HLA matched cell line in order to prevent immune rejection, but even with matching, the patient would likely have to be on immunosuppressive drugs for the rest of their lives. In order to circumvent this problem, many scientists have been attempting “therapeutic cloning” (which in reality is exactly the same thing as regular cloning) in order to harvest ESCs from an embryo that has exactly the same genetic makeup of the person the cells will be used in. With this approach, there would not be any immune rejection. Adult stem cells, on the other hand, generally don’t express HLA antigens so there is no chance of immune rejection! These cells have been used in thousands of patients already without any donor/recipient matching and there is not immune rejection! Even if immune rejection were to be a problem with adult stem cells, you can isolate the cells from your own body, expand them, and then give them back to yourself thereby circumventing immune rejection.
- Finally, ESC production involves the destruction of human persons in order to harvest the stem cells. Some would say that the embryo is not really a “person,” but there is no scientific reason to conclude otherwise – the topic of a future post. Adult stem cells, on the other hand, are isolated from adult patients and do not involve their death or destruction. So long as the patient has given informed consent, there are no moral quandaries with the isolation of adult stem cells.