What are adult stem cells & their clinical applications?

When someone uses the term “adult stem cells” you might think that simply means stem cells from adults. However, it’s not that simple. The goal of today’s post is to help you get all the key facts about adult stem cells. This includes the different types of adult stem cells and their clinical potential.

What are adult stem cells and how are they defined?

An adult stem cell has often been defined in a negative fashion. For instance, for years adult stem cells were considered by many to be stem cells that were not embryonic stem cells. This way of thinking was too broad. It ended up including induced pluripotent stem cells or iPS cells in the adult category. They don’t belong there as pluripotent stem cells.  And also what about fetal stem cells?

A more specific definition was needed.

Today my definition of adult stem cells is those stem cells derived from differentiated tissues. That can include stem cells from children and even infants. These cells are generally multipotent stem cells.

The best example of adult stem cells is hematopoietic stem cells. Neural stem cells are another great example.

What is the source of adult stem cells?

One of the most common types of adult stem cell goes by the acronym MSC. What MSC stands for is now debated, but at this point I’d say the best explanation of MSC is “mesenchymal stromal/stem cell.” It’s complicated because most preps of MSCs contain a mixture of both stem cells and non-stem cells.

One of the challenges with sourcing adult stem cells is that in isolating them from adult tissues you get a whole bunch of other cell types too. It isn’t always easy or practical to 100% separate out the stem cells from the non-stem cells. As a result, when talking about cells like “MSCs” as mentioned earlier, oftentimes they are not pure populations.

MSCs can also be isolated from many tissues including bone marrow, fat, and the wall of the umbilical cord, just to name three.

Another common type of adult stem cell is hematopoietic stem cells such as from bone marrow or cord blood.

You can isolate adult stem cell-like cells from just about any differentiated tissues. Over the years there have been debates about whether certain tissues have stem cells. For a long time it was thought by most people that the mature brain did not have true stem cells in it. However, the consensus now is that it does. The same goes for lung stem cells.

Probably the most contentious discussion in this area today is whether the heart has any true or meaningful populations of stem cells. I doubt that there are meaningful endogenous populations of heart stem cells.

Skin stem cells are one of the more interesting types in the adult category. They have many potential clinical applications.

Embryonic stem cells vs adult stem cells

Because of the past debate about funding of embryonic stem cell research in the U.S. (and elsewhere), there ended up being this false dichotomy. Opponents of embryonic stem cell research created an “us vs. them” mentality. Embryonic stem cells vs. adult stem cells. The reality is that both adult stem cells and embryonic stem cells have clinical promise.

I hope that both types of cells are the basis for therapies that help many people.

Other types, such as iPS cells, have real promise too.

As mentioned earlier, the main difference between these types of stem cells is their “potency”. This means how many other kinds of cells they can turn into, which is called differentiation. Surprisingly, some researchers claim that there are pluripotent adult stem cells, but the research in this area is not convincing to me. If such cells existed in adult tissues, why don’t they form stem cell-related tumors called teratomas? Also, why would adult tissues have pluripotent stem cells in them?

For instance, why would a lung have cells in it with the ability to make neurons, kidney, heart, bone, etc.? It just doesn’t make sense.

One long-shot possibility is that while these cells do not naturally exist that the harsh isolation process of trying to get cells out of adult tissues causes some cells to seem like pluripotent cells for a time. This brings to mind the whole STAP cell fiasco. Even so, cellular stress is a very interesting area of research.

Adult stem cell therapy clinical trials

There is a vast array of clinical trials that in some way utilize adult multipotent stem cells. For MSCs alone, this week I found 709 trials for just one type of search on Clinicaltrials.gov. Because MSCs go by different names this likely an underestimation. On the other hand, some of these trials may be non-interventional. Other MSC trials don’t use actual cells but instead are based on products derived from those cells like exosomes.

The most talked-about property of adult stem cells today is their ability to modulate immune responses. Many clinical trials including cell therapy trials for COVID-19 are based on this rationale.

Another therapeutic angle is that an adult stem cell sometimes can turn into another kind of cell. It may, for example, differentiate into fat, muscle, bone, cartilage, or skin cells. This differentiation property could be very useful too in the clinic. This means that we shouldn’t get tunnel vision on the immunomodulation properties alone.

Overall, I see adult stem cells as promising for some specific diseases. We need strong, convincing data from clinical trials to be sure for any specific application.

References

• Search on Clinicaltrials.gov on June 14, 2021 for MSCs or mesenchymal stem cells.
• Special section in Nature in April 2021 on adult stem cells.