Lesson Explainer: Stem Cells Biology • First Year of Secondary School

In this explainer, we will learn how to recall the location and function of stem cells and explain how they can be used in medicine.

Stem cells are like our body’s own home renovation team. Do your muscle tissues need some maintenance? Perhaps your heart tissues need a bit of repair? There are stem cells in your body that carry out these tasks and many others constantly, keeping your internal systems working to their full potential. Stem cells are a fast-developing area of medical research and their capabilities to treat different diseases are rapidly progressing. They have given scientists methods by which they can treat certain cancers and neurological diseases; they even have the potential to cure blindness. Stem cells are already being used to save lives, and they have the potential to save many, many more.

Every living organism has many different specialized cells. These are cells that have a specific job, or function, to carry out and so must have a particular structure to serve this function. For example, the function of a phagocytic white blood cell, called a macrophage, is to break down pathogenic microbes using enzymes. These cells have adapted to have many lysosome organelles that release these enzymes in order to destroy the pathogenic intruder as shown in Figure 1.

Once upon a time when you were just a small bundle of cells, there were no macrophages in your body. They all specialized, or differentiated, from an originally unspecialized cell. These unspecialized cells are called stem cells, and some have the capability to become any different type of specialized cell: heart, muscle, nerve, skin, retinal, liver, and over 200 other different types that exist in the human body alone!

Stem cells that have the capacity to differentiate in all the cell types of the body are said to be pluripotent, whereas stem cells that can differentiate into a more limited number of cells are said to be multipotent. The hematopoietic (blood) stem cell shown in Figure 2 is multipotent and you can see the variety of specialized cells that derive from it, including the macrophage we discussed earlier.

Stem cells are found in both plants and animals. In plants, stem cells are usually found in constantly growing regions called meristematic tissue, such as the tip of the roots and shoots, as shown in Figure 3 below. These stem cells are called meristem cells, and they are able to differentiate into any specialized cell to form any tissue or organ in the plant.

Human stem cells can be found in both embryos and in adults. They can also be found in the umbilical cord, which links the embryo to the mother and is removed following birth. Embryonic stem cells are present from when the embryo is between 3–5 days old until the late blastocyst stage, which is seen in Figure 4.

Embryonic stem cells have the capability to differentiate into any different type of specialized cell and even to form whole new organisms. This explains why if the embryo splits at any time up until this blastocyst stage in development, it will form two identical twins!

Because of their capacity to generate any type of tissue, embryonic stem cells have generated a great interest in medical research. For example, in animal studies, stem cells can be obtained from mice embryos produced after in vitro fertilization (IVF). IVF is a form of fertilization in which eggs are fertilized by sperm cells “in vitro,” meaning in glass, in reference to the lab petri dish. IVF is a very efficient technique producing lots of embryos that can be conserved for many years in ultracold freezers.

The use of human embryonic stem cells for research could also result in a giant leap forward for modern medicine, but it is a very controversial topic raising a number of ethical issues that we will discuss later in this explainer.

Adult stem cells are found in many different tissues of the body: bone marrow, brain, liver, and blood vessels, among others. Unlike embryonic stem cells that are pluripotent, adult stem cells are multipotent: they cannot form any type of cell. For example, our hematopoietic stem cell in Figure 2, which is found within bone marrow and blood vessels, can become any one of the different types of blood cell, but it cannot differentiate into a brain cell. This limits their function in different medical treatments, but they are still useful for targeted stem cell therapy.

Since 2005, scientists have been developing new ways to obtain human stem cells from body cells, bypassing the need for embryonic stem cells and avoiding the limitations of adult stem cells. Specific proteins called transcription factors can induce an adult body cell to reprogram itself into a stem cell that has properties very similar to an embryonic stem cell. These cells are called induced pluripotent stem cells (iPSC). This discovery, which was awarded a Nobel prize in 2012, was a biomedical revolution as it represented a very promising field in medicine.

People with damaged immune systems, for example those suffering from leukemia (a type of cancer that affects white blood cells), can receive stem cells via an injection. These stem cells could be sourced from blood or voluntary adult bone marrow extraction. These methods also have their drawbacks, however, as procedures like bone marrow extraction are painful and receiving the transplant has its own risks and side effects. Soon, iPSC made from skin cells of patients could become a very effective solution.

Stem cells have an almost unlimited potential to treat diseases. Currently, stem cells are being used both to be differentiated into specific cells, such as retinal cells to treat loss of vision, or to grow organs for transplants. Many diseases destroy cells; for example, heart attacks block blood flow to the heart, so cardiac cells die. As cardiac cells struggle to regenerate quickly enough, often the only option for survival is a highly demanded heart transplant.

Stem cells offer an alternative solution, as they can be stimulated to differentiate into cardiac cells to rebuild the dead heart tissue. This has not yet been tested on humans, but studies on mice have been effective. Studies on rodents have also yielded encouraging results in treating neural diseases such as spinal injuries or Parkinson’s disease, for which there is currently no alternative treatment as brain transplants are not an option.

Other current areas of stem cell research include the development of pancreas stem cells to treat diabetes, blood stem cells for cancer, skeletal muscle stem cells for muscular dystrophy and many others. Some potential uses of stem cells to produce cells to repair various organs are shown in Figure 5.

Another example of stem cell use is treating patients who have undergone chemotherapy treatment for cancer. In chemotherapy, many healthy cells are damaged in an attempt to target and destroy the cancerous cells. Stem cells are helpful in replacing these cells, as they can be stimulated to differentiate into the specific cells that have been damaged. Another major advantage of stem cells, especially iPSCs, is that they can be produced from our own body cells to make new organs or new tissues. This way, our immune system would not reject them after transplantation, as would be the case with organs or stem cells from other donors.

In order to improve the effectiveness of stem cell use in medicine, a technique called cell fractionation can be used to separate the components of a cell. Fractionation can also be used to separate different cells from a mixture. This means stem cells that are more suited to certain treatments can be separated from less useful stem cells for an effective treatment. Different organelle proportions will result in a different density of each stem cell. Cell fractionation involves rotating cells at high speeds in a process called centrifugation.

The diagram in Figure 6 below shows how centrifugation can be used to separate stem cells from other cells in bone marrow in order to treat a sports injury to the kneecap (patella).

We know that the use of human stem cells, especially embryonic stem cells, in medicine is a controversial topic. Let’s address some arguments in Table 1, both for and against stem cell use in medicine and research.

It can help to divide the arguments into ethical, scientific, and social issues.

An ethical argument in medical research is anything relating to the societal rules of conduct of using stem cells, particularly embryonic stem cells, such as debate as to when an embryo is seen as a person. While some people believe the embryo is a person from the moment of conception, others consider that this only occurs when the embryo develops more “human” characteristics, such as a beating heart or the first time it moves in the uterus.

A scientific argument refers to the opportunities or challenges that all types of stem cells present for their use in research and medicine to develop more sophisticated and efficient practices, such as curing diseases that were previously untreatable. It might also involve challenges to scientific research, such as the development of mutations in stem cells, preventing easy use in research.

Finally, social arguments refer to anything to do with society and the individuals within it that are not ethical or scientific. For example, a social argument against the use of embryonic stem cells might be that treatments using stem cells are expensive and therefore may not be accessible to everyone who needs them.

Table 1 below compares the different arguments for and against stem cell use in medicine and research, classified into ethical, scientific, and social arguments.

It is important to remember that some of these arguments may also overlap in their classification. For example, saving human lives is in the interest of ethical, scientific, and social arguments!

Let’s recap some of the key points we have covered in this explainer.