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Stem Cells

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Stem Cells

Stem cells are cells that have the potential to express any one of their genes in their genome. This means that they have the potential to differentiate into any type of cell in the body.

Stem cells are undifferentiated cells capable of proliferating (increasing in number rapidly) and turning into specialised cells.

Advantages of stem cells

In order to differentiate into a particular cell type, the stem cell needs to transcribe and translate only some specific parts of their DNA and keep the remaining parts turned off.

When becoming specialised, stem cells take on specific features and adaptations. These may include changing the cell structure, membrane proteins, as well as the type or number of the organelles within them. The process of stem cells becoming specialised cells is called cell differentiation.

Terminally differentiated cells cannot proliferate and replace themselves as they have irreversibly lost the ability to undergo mitosis while acquiring specialised adaptations.

Stem cells originate from a variety of sources in mammals. These include:

  • Embryonic stem cells: obtained from embryos in their early stages of development.

  • Umbilical cord blood stem cells: obtained from umbilical cord blood. They are similar to adult stem cells.

  • Placental stem cells: found in the placenta. They have the potential to develop into specific types of cells.

  • Adult stem cells: found in tissues of the fetus and throughout adulthood. These cells are specific to a tissue or organ and their roles are maintenance and repair.

Differentiation of the stem cells

Single-celled organisms carry out all their essential functions themselves. Although the functions they perform may be adequate, they cannot be efficient in performing all of them. This is because each function requires a different type of cellular structure and machinery.

No one cell can provide all the best conditions for all functions. In multicellular organisms, each cell becomes specialised in a variety of ways to perform a particular function, and the adaptations they acquire ensure that they are maximally efficient in their functions.

The specialised cells are the outcome of stem cells’ differentiation. In most multicellular organisms, including humans, all cells are derived from a fertilized egg called the zygote. In the early stage of development, the zygote undergoes multiple mitotic divisions forming a ball of identical cells (morula). As the organism develops and the cells mature, they change the pattern of the genes they express and hence become different from each other.

It is important to note that all cells within an organism contain the same genome, but they express different parts of it.

There are certain genes that are expressed in almost all cells. These are genes that encode essential molecules such as enzymes of respiration and enzymes involved in transcription and translation. Other genes are expressed only in specific cells and are turned off in the other cells.

For example, the gene for insulin is only expressed in beta cells of the islets of Langerhans in the pancreas. Another example would be mesophyll cells in plants that become specialised for photosynthesis. For their differentiation, the genes needed for photosynthesis are expressed while other genes that are not needed are turned off since their expression would be wasteful and would lower the mesophyll cells’ efficiency.

Let’s have a look at the specialised red blood cells and how the process of their differentiation from stem cells increases the amount of oxygen they can carry to other tissues and organs.

Erythrocytes specialisation

Erythrocytes (red blood cells) have specific adaptations to carry out a specific function (as specialised cells), unlike stem cells, which only possess the basic machine work of a cell.

During the process of erythropoiesis, the haematopoietic stem cells differentiate and adopt different shapes and features as they become erythrocytes.

Erythropoiesis (‘erythro’ meaning ‘red’ + ‘poiesis’ meaning ‘to make’) is the process that produces mature red blood cells from haemopoietic stem cells in the bone marrow.

Haematopoietic stem cells are cells that can differentiate and develop into all types of blood cells, such as white blood cells, red blood cells, and platelets.

These changes include:

  • Loss of their nucleus and mitochondria to create more space for haemoglobin, which is the main oxygen-carrying protein in erythrocytes.

  • An increase in the concentration of haemoglobin in the cell increases to ensure maximum oxygen-carrying capacity.

  • Adoption of a specific cytoskeleton that gives the erythrocytes a biconcave structure granting them a larger surface area for gas exchange and increased flexibility to travel through narrow blood vessels.

These adaptations improve the ability of the specialised cells to carry out their function. In this particular example, the adaptations in erythrocytes allow them to efficiently pick up oxygen in the lungs, travel through narrow blood vessels and give off their oxygen to tissues and organs in need.

Properties of stem cells

In mature mammals, only a few cells retain the ability to divide and differentiate into specialised cells. After differentiation, most specialised cells lose the ability to proliferate. This leads to an increased risk of permanent damage after an injury, especially in tissues such as the brain and heart. Neurons and cardiomyocytes are permanent cells with no proliferative properties and hence cannot regenerate new cells to replace the damaged cells. As result, damages caused by a stroke or a myocardial infarction are irreversible and hence can be life-threatening.

Cardiomyocytes are the cells in the cardiac wall muscle that contract in a healthy heart.

Stem cells have two main properties:

  1. They have a high capacity to differentiate into more specialised cells.

  2. They have the ability to self-renew, so they keep producing stem cells. This property is referred to as potency.

Types of stem cells

Potency is the ability of a stem cell to become differentiated into various types of specialised cells.

There are several types of stem cells, each possessing a different level of potency. These are:

  • Totipotent Stem Cells – these have the ability to become any cell in the body. This means they have the capability to form a complete organism. They are only found in the early stage of embryonic development. An example of a totipotent stem cell is the zygote itself.

  • Pluripotent Stem Cells – these cells are found in the embryo and can differentiate into almost any cell type. However, they lack the ability to form cells that make up the placenta. Examples of pluripotent stem cells are embryonic stem cells and fetal stem cells.

  • Multipotent Stem Cells – these stem cells are found in adults and can differentiate into various cells but are limited in their capacity. They usually develop into cells of a particular type. For instance, red and white blood cells can be formed from multipotent stem cells called haemopoietic stem cells found in the bone marrow

  • Unipotent Stem Cells – these cells can only differentiate into one type of cell, hence uni. They are derived from multipotent stem cells and are present in some adult tissues for regenerative purposes.

This image shows the types of stem cells and the potential parts of the body in which they may prove useful.

Stem cells in disease

Stem cell therapies have the potential to save lives and improve the quality of life for many people. This is because stem cells can divide into a large variety of differentiated cell types and so can be used to replace those cells which are damaged by injuries or diseases.

The stem cells in the bone marrow, called haematopoietic stem cells, can divide to produce other blood cells (both white and red blood cells). These stem cells are used to treat immune system diseases and illnesses affecting the blood, such as leukaemia.

In leukaemia patients, some blood cells divide uncontrollably in their bone marrow. This limits the ability of the haematopoietic stem cells to produce enough blood cells. Treatment for patients with leukaemia includes chemotherapy and radiation, both of which target leukaemia cells. After this, more haematopoietic stem cells can be placed (implanted) into the bone marrow to produce a normal, healthy amount of blood.

Severe combined immunodeficiency (SCID) is a genetic disorder that affects both B and T lymphocyte-mediated immune responses. As a result, patients suffering from SCID have a very weak immune system. Their lymphocytes are unable to create enough antibodies or lack the correct ones to fight and defend against the threat of a pathogen. Therefore, those who suffer from SCID are very vulnerable to infections. Even those infections which are not usually considered to be severe are liable to be life-threatening in SCID patients. SCID can be treated with a bone marrow transplant. This transplant replaces the faulty bone marrow with donor bone marrow that contains stem cells without the defective genes that cause SCID. The healthy bone marrow stem cells then differentiate to produce functional lymphocytes that can then produce the necessary antibodies to fight and defend against pathogens, so the immune system functions normally.

Induced pluripotent stem cells (IPS cells)

Induced pluripotent stem cells (iPS cells) are artificially produced from unipotent stem cells in a laboratory.

The process of making iPS cells involves adding specific transcription factors into already specialised cells, which are then ‘reprogrammed’ into pluripotent stem cells. The specific transcription factors used in this process cause the specialised cells to express genes that are associated with pluripotency. As a result, the iPS cells will have similar characteristics to embryonic stem cells and hence are capable of self-renewal and differentiation into almost any cell in the whole body.

IPS cells play a major role in regenerative medicine. The iPS cells derived from a patient’s own cells can be used to replace damaged cells or create new tissues or possibly even organs. For example, iPS cells can be used to regrown skin grafts for treating burn injuries or replacing damaged neurons in neurodegenerative diseases such as Parkinson’s disease. Since the cells are derived from the patient’s own cells, there will be no concern about the rejection of the cells by the patient’s immune system. Therefore, using IPS cells involves fewer complications and is surrounded by fewer ethical issues than the use of embryonic stem cells. Embryonic stem cell research raises some ethical issues because it involves obtaining stem cells from embryos created by IVF (In vitro fertilisation). This procedure results in the destruction of an embryo that has the potential to become a fetus if placed in a womb.

Treatment of other diseases using stem cells

Stem cells can be used to create specialised cells which can treat a range of human diseases and injuries. Some of the uses of stem cell-derived specialised cells are:

  • Skeletal muscle cells can be used to treat illnesses such as muscular dystrophy.

  • Beta cells of the pancreas can be used to treat Type 1 diabetes.

  • Neurons may be used to treat neurodegenerative diseases such as Parkinson’s and Alzheimer’s diseases.

  • Epidermal skin cells can be used in the treatment of burns and wounds as they form the outer layer of skin which is damaged in these injuries.

  • Retina cells of the eye can be transplanted into the eyes of patients with macular degeneration.

Stem Cells - Key takeaways

  • Stem cells are cells that can differentiate into different types of cells.
  • They have several sources: Embryonic stem cells, placenta stem cells, umbilical cord blood stem cells, and adult stem cells.
  • There are four types of stem cells:
    • Totipotent stem cells
    • Pluripotent stem cells
    • Multipotent stem cells
    • Unipotent stem cells
  • Induced Pluripotent Stem Cells (iPS cells) are produced from unipotent stem cells to which specific transcription factors are introduced to give them the ability to gain pluripotency. iPS cells can replace embryonic stem cell research and can be used to regenerate damaged tissues.
  • Stem cells can be used in regenerative medicine and can treat many illnesses and diseases e.g. SCID, leukaemia, and Parkinson’s disease.

Frequently Asked Questions about Stem Cells

The potential sources of stem cells are: adult stem cells, embryonic stem cells, umbilical cord stem cells, and induced pluripotent stem cells.

Stem cells are unspecialised cells that can develop into other types of cells.

The part of a human bone that contains stem cells is called the bone marrow. This is a sponge-like tissue found in the middle of bones

Stem cells can be used to replace cells damaged by illness or injury. They can also be used to create new tissues to replace damaged ones. This is carried out during stem cell therapies.

Stem cell therapies are treatments of diseases or injuries involving the use of stem cells e.g. hematopoietic stem cell transplantation.

Stem cell therapies can be used to treat a number of diseases for example leukaemia, SCID and muscular dystrophy.

Mutations in DNA sequences can cause genes that code for stem cells to not be expressed. 

When damage to DNA occurs via mutations, genes that control stem cell differentiation and other genes such as tumour suppressors and oncogenes, may not be expressed or their activity is deregulated. These changes can cause the development of tumours and cancers as a division of stem cells is not controlled. 



Final Stem Cells Quiz

Question

Where can totipotent stem cells be found?

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Answer

In the first few cell divisions of a zygote.

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Question

Where can totipotent stem cells be found?

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Answer

In the first few cell divisions of a zygote.

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Question

What are pluripotent stem cells?

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Answer

Stem cells found in embryos can specialise into any body cell but lose the ability to develop into cells that make up the placenta.

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Question

Why and how do stem cells become specialised?


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Answer

They become specialised because during their development they transcribe and translate only specific parts of their DNA.

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Question

How are embryonic stem cells obtained?


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Answer

They can be obtained from embryos after the first few divisions of the zygote. Embryos are created in a lab environment using IVF. After the first few cell divisions, the stem cells are removed and the embryo is destroyed.

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Question

What type of stem cells are embryonic stem cells?


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Answer

They are pluripotent stem cells

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Question

How are induced pluripotent stem cells (iPS cells) obtained?


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Answer

IPS cells are created by inserting transcription factors into already specialised cells to give them pluripotency.

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Question

Give an example of stem cell therapy and explain how this is carried out.


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Answer

Leukaemia can be treated via stem cells from bone marrow. Bone marrow transplants are used to remove the faulty bone marrow in leukaemia patients that produce abnormal blood cells. The transplanted stem cells from a donor divide and specialise into healthy blood cells.

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Question

What is the difference between cell differentiation and cell specialisation? 


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Answer

Specialisation means the cell begins to adopt specific sizes, shapes etc which enable it to carry out its function more efficiently. Differentiation means the cell starts to become a different and specific type of cell.

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Question

How do stem cells start to differentiate?


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Answer

 Both stem cells and differentiated cells possess the same genetic material. Stem cells differentiate into specialised cells by expressing specific genes and keeping the rest of the genome turned off. 

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Question

Do stem cell treatments/transplants involve a lot of risks?


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Answer

Stem cell treatments involve some risk as with any surgical treatment. However, stem therapies involve quite a lot of discomfort for both the patient and donor. The level of discomfort involved also differs depending on where the stem cells are being obtained from.

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Question

Why are iPS cells favoured over embryonic stem cell use? 


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Answer

This is because the use of embryonic stem cells is surrounded by ethical issues as embryos are destroyed which could have become a fetus. Also, iPS cells are made from the patient's own stem cells so will not trigger the same immune response as using embryonic stem cells would.

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Question

What are three adaptations of erythrocytes?

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Answer

  • Loss of their nucleus and mitochondria to create more space for haemoglobin, which is the main oxygen-carrying protein in erythrocytes.

  • An increase in the concentration of haemoglobin in the cell increases to ensure maximum oxygen-carrying capacity.

  • Adoption of a specific cytoskeleton that gives the erythrocytes a biconcave structure granting them a larger surface area for gas exchange and increased flexibility to travel through narrow blood vessels.

Show question

Question

What are the sources of stem cells?

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Answer

Adult stem cells, embryonic stem cells, umbilical cord blood stem cells, placental stem cells, and induced pluripotent stem cells.

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Question

What is a stem cell?

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Answer

A cell that can specialise to develop into specific cells in the body.

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