Stem cells are creators

You read that right. Creators of what? All organs and tissues as well as every specialized cell in your body. To understand what stem cells do, let’s first look into how tissues are constructed.

Tissues consist of cells surrounded by a network of proteins and molecules, an extracellular matrix, that provide the cells with structure and support. This group of cells all have similar structure and functionality and in this formation, they are able to carry out a specific function together. Multiple tissues of different cells with different functions then work together to form our organs.

So how do stem cells play into this?

The role of the stem cell

There are a large variety of different cell types in the body specialized to perform a specific task. The role of the stem cells is to create this diversity and generate, or even regenerate, tissues. Stem cells are unspecialized cells with the power to differentiate, divide into, other cell types. 

There are two different types of stem cells. Let’s go through them!

1. Embryonic stem cells exist in embryos that are no more than three to five days old. At this point there are about 150 cells in the embryo and they hold the power to differentiate into any other cell type, even other stem cells. We say that they are pluripotent. These stem cells are the origin to all other cells in the body, this is how a single fertilized egg cell can grow into extremely complex multicellular creatures with internal structures with different functions. Since these embryonic stem cells possess this versatility they may be used to regenerate damaged tissues or organs that don’t regenerate on their own. 

2. Adult stem cells are found in small numbers in most tissues. In contrast to the embryonic stem cells, the adult stem cells have been assigned a role and may only divide into certain types of cells, we call them multipotent. For example, stem cells in the bone marrow are limited to blood cells, but may not create more stem cells or for example muscle cells.

The potential of stem cells in medicine is enormous with their regenerative properties. Let’s have a closer look at what could and already can be achieved with stem cells!

Stem cell therapy…

… to treat heart disease

Heart disease is the number one leading cause of death in the US and is a group of conditions that ultimately might lead to stroke or heart attacks. When the heart has completed its development as it has in an adult, it is incapable of repairing large areas of damage such as that caused by a stroke. The irreversible damage is a result of heart cells dying of oxygen depletion when the blood supply to the heart is stopped or reduced. This often results in cardiac dysfunction and heart failure and the common symptoms we associate with heart disease, such as chest pain and shortness of breath. 

A variety of stem cells have been studied for treatment of heart disease. The most common approach is to use stem cells from the patient, these are called autologous stem cells in contrast to stem cells from other individuals which are called allogeneic. The stem cells are derived from for example the bone marrow or the heart area. By introducing stem cells to the heart tissue, these can differentiate into muscle tissue to strengthen the heart or endothelial cells (inner lining of blood vessels) to promote formation of new blood vessels in the damaged tissue which facilitates the recovery of the heart tissue and improves cardiac function.

… to treat spinal cord injury

Another example of the potentiel of the stem cells is the case of spinal cord injury. When the spinal cord is damaged impossible to the body to heal and recover. It result in the definitive loss of nerve connections between the brain and the body and thus loss of sensibility and mobility.

Thanks to stem cell therapy, it is possible to induce regeneration of new neurons from injected stem cells and new connections can then appear. Damaged connections are also myelinate by some stem cells that differentiate into dendrocyte. All this has as an effect to improve the general condition of the injured people.

Turning back the clock on cells

One of the major problems with stem cell therapies is how to attain the stem cells needed for the treatments. Adult stem cells are usable but come with limitations. They cannot divide into all cell types, and they are also hard to cultivate as well as separate from other cells. Embryonic stem cells on the other hand may divide into any cell type, however, they might instead be rejected by the immune system because they are foreign to the body. Most embryonic cells come from fertilized eggs left over from the IVF but another important point to consider that has raised a lot of debate is the ethical aspect of using fertilized eggs for treatments.

Induction of pluripotent stem cells from adult cells

A solution to the ethical issues has been found in 2006 by Shinya Yamanaka and his team of searchers. Their idea? Take a cell of an adult and turn it into a stem cell. To do it, they only put 4 transcription factors called Yamanaka’s factors. They are used to reactivate the expression of some genes involved in embryonic stage and repress genes involved in the cell determination. This method is surprising by its simplicity but has a great efficiency and open to many opportunities.

However, in order to introduce these factors, viral vectors are commonly used. The fact is that retrovirus are implement to provide the factors. As this method can interfere with DNA, it’s not so rare to observe some mutations that can in some case lead to tumorigenic cells. Other ways to introduce Yamanaka’s factors like messenger RNA can be used to reduce the risk of tumor development even if they are less efficient.

This technique is very promising. There is no need to use and destroye an embryo to get some stem cells. This have several adventages. First it solves the ethical problems of using these embryos, it just need some adult cells generally taken in the skin. Secondly, as the cells come from adult cells, those ones are genetically identical to the transplant recipient. This way, graft rejections are avoid.

On the other hand, this new method brings its own ethical issues. With this, it should theoretically be possible to clone an entire human or create chimeric animals. Clear limits must be put in place to avoid any abuse.

Conclusion

To summarize, stem cells have great potential in medicine and are a big field of research given their regenerative properties. However there are still many things to improve like the tumoricity. Moreover, the enormous potential of applications implies a good management of its use and its drifts to avoid any abuse against ethics.

Sources

https://www.mayoclinic.org/tests-procedures/bone-marrow-transplant/in-depth/stem-cells/art-20048117#:~:text=Researchers%20hope%20stem%20cell%20studies,how%20diseases%20and%20conditions%20develop

https://www.sciencedirect.com/science/article/pii/B978012809657465740X

https://www.nih.gov/news-events/news-releases/covid-19-was-third-leading-cause-death-united-states-both-2020-2021#:~:text=Heart%20disease%20was%20the%20number,fifth%20leading%20causes%20of%20death

https://www.mayoclinic.org/diseases-conditions/heart-attack/symptoms-causes/syc-20373106

https://ki.se/en/research/now-stem-cells-will-build-our-health

Induced Pluripotent Stem Cell – an overview | ScienceDirect Topics

https://fr.wikipedia.org/wiki/Cellule_souche_pluripotente_induite

Narihito Nagoshi, Osahiko Tsuji, Masaya Nakamura, Hideyuki Okano (2019). Cell therapy for spinal cord injury using induced pluripotent stem cells. Regenerative Therapy, 11, 75-80.