Stem Cells (AQA A-Level Biology): Revision Notes
Stem Cells
What are stem cells?
Stem cells are unspecialised cells that can develop into different types of specialised cells. All multicellular organisms contain stem cells, which are essential for growth, development, and tissue repair.
During development, stem cells divide to produce new cells that then become specialised for specific functions. This process allows complex organisms to develop from a single fertilised egg into organisms with many different cell types.
The ability of stem cells to remain unspecialised while maintaining the potential to become any type of cell makes them unique and valuable for both natural biological processes and medical applications.
Types of stem cells
Stem cells are classified based on their potency - their ability to differentiate into different cell types:
Understanding stem cell potency is crucial: the more potent a stem cell, the greater variety of cell types it can become. This classification system helps scientists choose the right type of stem cell for specific research or therapeutic applications.
Totipotent stem cells
- Can mature into any type of body cell in an organism
- Found only in mammals during the first few cell divisions of an embryo
- Can develop into all the specialised cells needed to form a complete organism
Pluripotent stem cells
- Can differentiate into most types of body cell
- Embryonic stem cells become pluripotent after the totipotent stage
- Can still specialise into any cell in the body, but lose the ability to form placental cells
Multipotent stem cells
- Can differentiate into a few different types of cell within a related group
- Examples include stem cells in bone marrow that can form different types of blood cells (both red and white blood cells)
Unipotent stem cells
- Can only differentiate into one type of cell
- Example: stem cells that produce epidermal skin cells in the outer layer of skin
How stem cells become specialised
Stem cells become specialised through differential gene expression. This process involves several key steps:
- Same genes, different expression: All stem cells contain the same genes, but during development, only certain genes are transcribed and translated
- Selective gene activation: Under specific conditions, some genes are switched on while others remain switched off
- Protein production: Only mRNA from active genes is transcribed and then translated into proteins
- Cell modification: These proteins modify the cell structure and control cellular processes, determining what type of cell it becomes
- Irreversible changes: Once a cell has specialised, these changes are difficult to reverse, so the cell typically remains specialised throughout its life
Worked Example: Red Blood Cell Development
Red blood cells develop from stem cells in the bone marrow through selective gene expression:
Step 1: Stem cells in bone marrow receive signals to become red blood cells
Step 2: Genes for haemoglobin production are switched on
Step 3: Genes involved in removing the nucleus are activated
Step 4: Many other genes are switched off
Result: A specialised cell with lots of haemoglobin and no nucleus
Sources of human stem cells
Scientists can obtain stem cells from three main sources, each with distinct characteristics and applications:
Adult stem cells
- Obtained from body tissues of adults (e.g., bone marrow)
- Relatively simple to obtain with little risk but some discomfort
- Multipotent - can only specialise into a limited range of cells
- Already being used successfully in treatments like bone marrow transplants
Adult stem cells are currently the most widely used in clinical treatments because they can often be obtained from the patient's own body, eliminating the risk of immune rejection.
Embryonic stem cells
- Obtained from embryos at an early stage of development (4-5 days old)
- Created through in vitro fertilisation (IVF) in laboratories
- Pluripotent - can develop into all types of body cells
- The embryo is destroyed during the extraction process
Induced pluripotent stem cells (iPSCs)
- Created by reprogramming adult body cells in the laboratory
- Scientists introduce transcription factors (often using specially modified viruses) that make adult cells express genes associated with pluripotency
- The adult cell's DNA is modified so it can produce pluripotency transcription factors
- These cells become pluripotent like embryonic stem cells but are derived from adult tissue
Medical applications of stem cells
Current therapies
Several stem cell therapies are already in use and showing positive results:
- Bone marrow transplants: Used to treat leukaemia (blood cancer) and lymphoma (cancer of the lymphatic system)
- Treatment of genetic disorders: Such as sickle-cell anaemia and severe combined immunodeficiency (SCID)
- Blood disorders: Stem cells can replace faulty bone marrow in patients who produce abnormal blood cells
Future potential treatments
Scientists are researching stem cell therapies for many conditions:
- Spinal cord injuries: Replacing damaged nerve tissue
- Heart disease: Generating new cardiomyocytes (heart muscle cells) to replace damaged tissue
- Bladder conditions: Growing new bladders for transplantation
- Respiratory diseases: Repairing damaged windpipe tissue
- Organ transplants: Growing organs from stem cells to reduce donor waiting lists
The potential applications of stem cell therapy continue to expand as our understanding of stem cell biology improves. Many treatments currently in clinical trials may become standard therapies in the coming decades.
Benefits of stem cell medicine
- Could save lives by providing organs for transplant without waiting for donors
- Could improve quality of life by replacing damaged cells (e.g., restoring sight by replacing damaged eye cells)
- May provide treatments for previously untreatable conditions
Ethical considerations
The use of stem cells, particularly embryonic stem cells, raises important ethical questions that society must carefully consider:
The ethical debate surrounding stem cell research, especially involving embryonic stem cells, involves fundamental questions about the nature of life, medical advancement, and moral responsibility.
Arguments against embryonic stem cell use
- Destruction of embryos: The process destroys embryos that could potentially develop into humans
- Right to life: Some believe individual life begins at fertilisation, making embryo destruction morally wrong
- Alternative sources: Adult stem cells can be used without destroying embryos
Arguments supporting embryonic stem cell use
- Unfertilised eggs: Some stem cells come from eggs that haven't been fertilised and cannot survive
- Limited viability: Many research embryos cannot survive beyond a few days anyway
- Medical benefits: The potential to save lives and reduce suffering
- Informed consent: Donors understand and consent to the research use
Induced pluripotent stem cells as a solution
iPSCs may resolve many ethical concerns because:
- They can be made from a patient's own cells, avoiding immune rejection
- No embryos are destroyed in their production
- They have similar flexibility to embryonic stem cells
- More research is needed to ensure they function exactly like natural pluripotent cells
The development of iPSCs represents a significant breakthrough that may provide a way to harness the benefits of pluripotent stem cells while addressing many ethical concerns about embryonic stem cell research.
Remember!
Key Points to Remember:
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Stem cells are unspecialised cells that can differentiate into specialised cell types through selective gene expression
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The four types of stem cells differ in potency: totipotent (any cell type) > pluripotent (most cell types) > multipotent (few related types) > unipotent (one type)
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Human stem cells can be obtained from adult tissue, embryos, or created through reprogramming (iPSCs)
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Stem cell therapies are already treating blood disorders and genetic conditions, with many more applications being researched
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Embryonic stem cell research raises ethical questions about embryo destruction, while iPSCs may provide an alternative solution