Lesson Explainer: Stem Cells Biology

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.

Definition: Specialized

A specialized cell is one that has differentiated to have a particular structure to serve its specific function.

Definition: Unspecialized

An unspecialized cell is one that has not yet differentiated originating from mitosis or meiosis.

Definition: Differentiation

Differentiation is the process of a cell becoming specialized by selective expression of certain genes.

Definition: Stem Cell

A stem cell is an unspecialized cell that can differentiate into any type of specialized cell.

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.

Example 1: Describing the Difference between Stem Cells and Normal Body Cells

What is the key difference between a stem cell and a normal body cell?

  1. Normal body cells contain genetic material, whereas stem cells do not.
  2. Stem cells can divide and replicate, whereas normal body cells cannot.
  3. Stem cells are only found in embryos, whereas normal body cells are only found in adults.
  4. Normal body cells are specialized to carry out a particular function, whereas stem cells are unspecialized.
  5. Stem cells are specialized to carry out a particular function, whereas normal body cells are unspecialized.

Answer

All stem cells are unspecialized cells that can differentiate into different types of specialized cells. A normal body cell usually differentiates into a specialized cell, whether by changing its shape or by changing the proportion of different organelles it contains.

Both stem cells and normal body cells contain genetic material, eliminating option A. Both body cells and stem cells can divide and replicate, though stem cells may do this more readily. This means that option B is also incorrect. Not only do embryos contain stem cells, but depending on their stage of development, they are likely to also contain normal body cells. Adults also contain normal body cells, and though their stem cells are located in fewer places and in smaller numbers than in embryos, they still have stem cells too, so option C is incorrect.

Our correct answer is therefore D: Normal body cells are specialized to carry out a particular function, whereas stem cells are unspecialized.

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.

Definition: Meristem

A meristem is a type of tissue found in regions of growth such as the root and shoot tip in plants containing meristematic stem cells.

Example 2: Identifying the Location of Stem Cells in Plants

Where are stem cells found in plants?

  1. Root hair cells
  2. Palisade cells
  3. Pollen
  4. Meristem
  5. Chloroplasts

Answer

All stem cells are unspecialized cells that can differentiate into different types of specialized cells. In plants, stem cells are found in the constantly growing regions of the plant such as the shoot and root tips. These areas are called meristematic regions and contain meristem tissues, which are made up of meristem cells.

Root hair cells, palisade cells, and pollen are all specialized cells. Root hair cells are specialized to absorb water from soil and palisade cells to absorb maximum light energy from the Sun, and pollen is a haploid sex cell known as a gamete, specially adapted to fertilize the female gamete. A chloroplast is not a cell itself but an organelle found within cells, specialized to absorb light from the Sun.

The correct location of stem cells in plants is D: in the meristem.

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.

Key Term: Embryo

An embryo is the early developmental stage of an animal while it is in the egg or uterus of its mother.

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.

Key Term: In Vitro Fertilization

In vitro fertilization (IVF) is when an egg is fertilized by a sperm in a petri dish in a laboratory, usually to allow parents to reproduce where they would otherwise be unable to.

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.

Example 3: Describing the Difference between Embryonic and Adult Stem Cells

What is the key difference between embryonic and adult stem cells?

  1. Adult stem cells can differentiate to become any type of specialized cell, but embryonic stem cells can only become certain types of cells.
  2. Large numbers of adult stem cells can be easily grown in culture, but embryonic stem cells can only divide inside the body of the embryo.
  3. Embryonic stem cells can differentiate to become any type of specialized cell, but adult stem cells can only become certain types of cells.
  4. Adult stem cells can divide rapidly to make new cells, but embryonic stem cells take a long time to divide and generate new cells.
  5. They do not differ in any of their properties.

Answer

All stem cells are unspecialized cells that can differentiate into different types of specialized cells.

Embryonic stem cells are “less” specialized than adult stem cells, however, as there are such few cells at this stage of development that none of them have started to differentiate at all. Embryonic stem cells are said to be pluripotent as they have the capacity to become any cell of the body.

Adult stem cells are said to be multipotent as they can become a more limited number of cell types compared to pluripotent embryonic stem cells. For example, a hematopoietic stem cell in bone marrow can become any type of blood cell, but it cannot become a brain cell. This means that embryonic stem cells could be stimulated to become any specialized cell, while adult stem cells are limited to certain groups of cells.

Embryonic stem cells are also capable of almost unlimited divisions, while adult stem cells may not divide quite as many times. There is not a huge difference in the time they take to physically divide, however, and it is not the key difference between the two types of stem cell, eliminating options B and D.

Our correct answer is therefore C: embryonic stem cells can differentiate to become any type of specialized cell, but adult stem cells can only become certain types of cells.

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.

Example 4: Describing the Uses of Stem Cells

Macular degeneration is when the light-sensitive cells in the center of your eye stop working. Which of the following best explains how stem cells could be used to treat someone with macular degeneration?

  1. Stem cells can be inserted into the eye to fix the damaged cells.
  2. Stem cells can be used to create an entire eyeball and replace the eye of someone with this condition.
  3. Stem cells can stimulate the immune response to repair the damaged cells.
  4. Stem cells cannot be used to treat this condition.
  5. Stem cells can be stimulated to differentiate into light-sensitive cells to replace the damaged ones.

Answer

Take care with questions asking for the “best” explanation, as though they are multiple-choice, they are not easy as more than one answer may technically be correct.

Stem cells are unspecialized cells, with the potential to differentiate into any type of specialized cell. Their use in medicine is usually to target a specific cell type that has been damaged or destroyed by a disease. The stem cells will be stimulated to differentiate into this particular cell and be inserted into the damaged area to replace the nonfunctional cells.

In this example, the damaged cells are the light-sensitive cells in the eye. The cells will not be fixed themselves by stem cells, nor will the entire eyeball be replaced as this would be a lengthy and challenging process involving a risky eye transplant surgery. Stem cells can, however, be stimulated to differentiate into the damaged cell type to replace the cells via a safer procedure.

Our correct answer is therefore E: stem cells can be stimulated to differentiate into light-sensitive cells to replace the damaged ones.

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. 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 (yellow), scientific (blue), and social arguments (green).

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!

Example 5: Evaluating the Use of Embryonic Stem Cells

Stem cells are unspecialized cells that can differentiate into a variety of different cell types found in a human body; this makes them incredibly useful in medicine to treat disease. However, the use of stem cells faces a lot of opposition. Which of the following would be a correct scientific objection to using stem cells in medicine?

  1. Stem cells could mutate in the host body and lead to diseases like cancer.
  2. Stem cells cannot be taken from consenting adult volunteers.
  3. Stem cells can only be used to treat genetic disorders, not injuries or acquired diseases.
  4. Stem cells can be used to create large amounts of tissue or even new organs.
  5. Embryos are created solely for the purpose of taking embryonic stem cells.

Answer

The question is asking us to identify a scientific objection to using stem cells in medicine.

A scientific argument refers to the opportunities for stem cells to be used 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. A scientific objection might be that these mutations could lead to diseases like cancer.

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

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.

Let’s eliminate some of the options in the question that are incorrect before deciding which is a scientific objection.

Stem cells can be taken from consenting adult volunteers, so the option saying they cannot is incorrect, and it would also be an ethical, not a scientific, objection.

Stem cells can be used in treating some injuries and acquired diseases, by replacing dead, damaged, or malfunctioning cells, so the option saying they can only treat genetic disorders is also incorrect.

The option referring to the use of stem cells to produce large amounts of tissue or new organs is actually an advantage of stem cells, rather than a scientific objection.

Often, “spare” or “discarded” embryos result from IVF treatments to assist someone in having a child. Therefore, the embryos are not created just for the purpose of use in stem cell treatments, making the option that refers to this incorrect.

Therefore, the option that is a scientific objection to the use of stem cells is A: stem cells could mutate in the host body and lead to diseases like cancer.

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

Key Points

  • Stem cells are unspecialized cells that can be used in medicine to replace damaged cells or even regrow whole organs.
  • Stem cells in humans are either embryonic or adult stem cells from bone marrow and other organs.
  • Stem cells in plants are called meristem cells and are found in the areas of the plant that are constantly growing.
  • Stem cells, particularly embryonic stem cells, in research and medicine is a contentious topic, with ethical, scientific, and social arguments both supporting and opposing their use.
  • Induced pluripotent stem cells are a new revolutionary source of human stem cells that enable us to bypass many issues posed by embryonic and adult stem cells.

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