5.06 Stem Cells
Definition and Function
- Stem Cells: Undifferentiated cells capable of unlimited division (via mitosis).
- Potential to Differentiate: New cells from stem cell division can either:
- Remain as stem cells.
- Differentiate into specialized cells (e.g., blood cells, muscle cells).
Types of Stem Cells and Potency
- Potency: The ability of stem cells to differentiate into various cell types.
- Totipotent:
- Can differentiate into any cell type, including placenta and embryo cells.
- Present in zygote and up to the 16-cell stage in human development.
- Pluripotent:
- Can form all cell types within the embryo (but not placenta).
- Found in embryonic stem cells.
- Multipotent:
- Can only differentiate into a limited range of cell types.
- Adult stem cells (e.g., bone marrow stem cells) are multipotent.
- Unipotent Cells
- Unipotent cells are stem cells that can only differentiate into one specific cell type.Key Features: Self-renewal: Can divide to produce more of the same cell type.
- Examples:
- Muscle stem cells: Can only form muscle cells.
- Skin stem cells: Generate only skin cells for repair and renewal.
- Function:
- Provide specialized cells for routine turnover or healing.
- Unipotent cells are stem cells that can only differentiate into one specific cell type.Key Features: Self-renewal: Can divide to produce more of the same cell type.
- Totipotent:
Comparison of Stem Cell Types
| Feature | Totipotent | Pluripotent | Multipotent | Unipotent |
|---|---|---|---|---|
| Definition | Can form all cell types, including placenta and embryo. | Can form almost all body cell types, but not the placenta. | Can form a limited range of related cell types. | Can form only one specific cell type. |
| Differentiation Ability | All cell types (body + supporting structures). | Almost all body cells (e.g., muscle, nerve). | Specific cell types within a tissue (e.g., blood). | One cell type (e.g., muscle cells). |
| Location | Zygote and first few divisions. | Inner cell mass of blastocyst or iPSCs. | Adult tissues (e.g., bone marrow, brain). | Specific tissues (e.g., skin, muscle). |
| Function | Forms a whole organism, including placenta. | Forms body tissues and organs. | Maintains and repairs tissues. | Routine repair and renewal of specific tissues. |
| Key Points | Present in early development; essential for organism formation. | Versatile; used in research and therapy, but raises ethical concerns. | Found in adults; used in therapies like bone marrow transplants. | Least versatile; limited to single tissue type repair. |
Early Embryo:
- All cells are totipotent (can form any type of specialised cell).
Adult Organism:
- Most cells are differentiated (specialised, cannot divide).
- Some stem cells remain in tissues:
- Capable of division.
- Functions: Growth, tissue repair, replacement.
Limitations in Adults:
- Adult Stem Cells:
- Limited differentiation potential.
- Typically multipotent, meaning they can differentiate into multiple related cell types (e.g., blood cells from bone marrow stem cells).
- Example:
- Bone Marrow Stem Cells:
- Form blood cells.
- Cannot form nerve cells or muscle cells.
- Bone Marrow Stem Cells:
Stem Cells in Growth and Repair
- Differentiation and Commitment:
- As cells specialize, they lose their ability to divide.
- Small populations of adult stem cells remain in the body to aid in growth and repair.
- Adult Stem Cells:
- Found in tissues like bone marrow, skin, gut, heart, and brain.
- Maintain specific tissues by replacing cells lost through normal cell turnover.
- Example: Bone Marrow Stem Cells:
- Multipotent: Produce various blood cells (e.g., red and white blood cells).
- Essential due to the high turnover rate of blood cells:
- Approx. 250 billion red blood cells and 20 billion white blood cells are replaced daily.
Medical Applications and Research
- Stem Cell Therapy:
- Introducing new adult stem cells into damaged tissues to promote healing.
- Bone Marrow Transplantation:
- Routine treatment for diseases like leukemia and bone marrow disorders.
- Future Potential:
- Potential treatments for conditions like diabetes, muscle damage, nerve damage, Parkinson’s (a progressive neurological disorder that affects movement, balance, and coordination due to the degeneration of neurons in the brain), and Huntington’s disease (a genetic neurodegenerative disorder caused by a mutation leading to the progressive breakdown of brain cells).
- Bone Marrow Transplantation:
- Introducing new adult stem cells into damaged tissues to promote healing.
- Laboratory Research:
- Experiments in growing new tissues and organs from stem cells, advancing possibilities in regenerative medicine.
iPSC (Induced Pluripotent Stem Cell)
Definition:
- iPSCs are adult cells (e.g., skin cells) reprogrammed into a pluripotent state, meaning they can differentiate into almost any cell type.
How iPSCs are Made:
- Scientists introduce specific genes or factors into adult cells.
- These factors “reset” the cells, making them behave like embryonic stem cells.
Significance:
- iPSCs avoid ethical concerns associated with embryonic stem cells since they are not derived from embryos.
- They offer a renewable source of cells for research and therapy.
Applications:
- Disease Modelling: Creating cell models of diseases for study.
- Drug Testing: Testing new drugs on human-like cells without animal models.
- Regenerative Medicine: Potential to replace damaged tissues (e.g., nerve, heart, liver).
Advantages:
- No need for embryonic cells, reducing ethical debates.
- Can be patient-specific, lowering the risk of immune rejection.
Challenges:
- Risk of mutations during reprogramming.
- Difficult to control the differentiation process.
Key Point:
- iPSCs are a breakthrough in stem cell research, offering promising alternatives for both science and medicine.
Key Concepts and Question
- Genetic Uniformity and Differentiation:
- All specialized cells in the body retain the same genes as the zygote.
- Question: What does this suggest about how cells become different?
- Answer: Specialization likely involves selective gene expression, where specific genes are activated or repressed based on cell type.
Practise Questions
Question 1
Define stem cells and explain their potential for differentiation. (5 marks)
Mark Scheme:
- Stem cells are undifferentiated cells capable of unlimited division by mitosis. (1 mark)
- They can either remain as stem cells or differentiate into specialized cells. (1 mark)
- Totipotent stem cells can differentiate into any cell type, including the placenta. (1 mark)
- Pluripotent stem cells can form all body cell types but not supporting structures like the placenta. (1 mark)
- Multipotent stem cells can differentiate into a limited range of related cell types, while unipotent cells can form only one specific type. (1 mark)
Question 2
Compare the characteristics of totipotent, pluripotent, and multipotent stem cells. (6 marks)
Mark Scheme:
- Totipotent cells can form all cell types, including both embryo and supporting structures like the placenta. (1 mark)
- They are present in the zygote and up to the 16-cell stage in human development. (1 mark)
- Pluripotent cells can form almost all body cells but not supporting structures like the placenta. (1 mark)
- They are found in the inner cell mass of the blastocyst or induced pluripotent stem cells (iPSCs). (1 mark)
- Multipotent cells can differentiate into a limited range of cell types within a specific tissue (e.g., blood cells from bone marrow stem cells). (1 mark)
- They are found in adult tissues like bone marrow, brain, and skin. (1 mark)
Question 3
What are induced pluripotent stem cells (iPSCs), and why are they significant? (5 marks)
Mark Scheme:
- iPSCs are adult cells reprogrammed into a pluripotent state, allowing them to differentiate into almost any cell type. (1 mark)
- They are created by introducing specific genes or factors into adult cells. (1 mark)
- iPSCs bypass ethical concerns related to embryonic stem cells, as they are not derived from embryos. (1 mark)
- They offer a renewable source of cells for research, disease modeling, drug testing, and regenerative medicine. (1 mark)
- iPSCs reduce the risk of immune rejection when derived from a patient’s own cells but face challenges like mutation risks during reprogramming. (1 mark)
Question 4
Explain the role of bone marrow stem cells in maintaining blood cell populations. (5 marks)
Mark Scheme:
- Bone marrow stem cells are multipotent, capable of differentiating into various blood cells (e.g., red and white blood cells). (1 mark)
- These stem cells continuously divide to replenish blood cells lost due to turnover. (1 mark)
- Approximately 250 billion red blood cells and 20 billion white blood cells are replaced daily. (1 mark)
- Bone marrow stem cells are essential for maintaining immune function and oxygen transport. (1 mark)
- They are used in treatments like bone marrow transplants for diseases such as leukemia. (1 mark)
Question 5
Discuss the ethical and practical challenges associated with embryonic stem cells and how iPSCs address these challenges. (6 marks)
Mark Scheme:
- Embryonic stem cells are pluripotent and can differentiate into any body cell type. (1 mark)
- They raise ethical concerns as their extraction involves destroying an embryo, which some view as potential human life. (1 mark)
- iPSCs are derived from adult cells (e.g., skin cells) and avoid the destruction of embryos. (1 mark)
- iPSCs are patient-specific, reducing the risk of immune rejection in therapies. (1 mark)
- However, iPSCs face challenges such as mutations during reprogramming and difficulty controlling differentiation. (1 mark)
- iPSCs represent a promising, ethical alternative for regenerative medicine and research. (1 mark)
Question 6
Explain how selective gene expression leads to cell specialization. (5 marks)
Mark Scheme:
- All cells in the body retain the same genes as the zygote. (1 mark)
- Cell specialization occurs through selective gene expression, where specific genes are activated or repressed. (1 mark)
- For example, genes for hemoglobin production are active in red blood cells but inactive in nerve cells. (1 mark)
- Gene expression is regulated by transcription factors and environmental signals, ensuring cells develop appropriate functions. (1 mark)
- This process allows stem cells to differentiate into specialized cells like muscle, nerve, or skin cells. (1 mark)
Question 7
What is the role of unipotent stem cells, and where are they found? (4 marks)
Mark Scheme:
- Unipotent stem cells can only differentiate into one specific cell type. (1 mark)
- They have self-renewal capability, dividing to produce more of the same cell type. (1 mark)
- Examples include muscle stem cells, which form muscle cells, and skin stem cells, which regenerate skin. (1 mark)
- They are found in tissues requiring routine repair and renewal, like skin and muscle. (1 mark)
Question 8
What are the potential medical applications of stem cells? (6 marks)
Mark Scheme:
- Stem cell therapy: Introducing new stem cells to repair damaged tissues, e.g., in spinal injuries or heart disease. (1 mark)
- Bone marrow transplants: Treating leukemia and bone marrow disorders by replacing diseased stem cells with healthy ones. (1 mark)
- Potential treatments for conditions like Parkinson’s disease, diabetes, and nerve damage. (1 mark)
- Regenerative medicine: Growing new tissues and organs in the laboratory using stem cells. (1 mark)
- Drug testing: Testing new drugs on stem cell-derived tissues to reduce reliance on animal models. (1 mark)
- Disease modeling: Studying diseases like Huntington’s using stem cell models to understand pathology and test treatments. (1 mark)
Question 9
Compare adult stem cells and embryonic stem cells in terms of potency and application. (6 marks)
Mark Scheme:
- Adult stem cells are multipotent, meaning they can differentiate into a limited range of related cell types. (1 mark)
- They are found in tissues like bone marrow and skin, maintaining and repairing specific tissues. (1 mark)
- Embryonic stem cells are pluripotent, capable of forming almost all body cell types. (1 mark)
- They are derived from the inner cell mass of the blastocyst and are more versatile in differentiation potential. (1 mark)
- Adult stem cells are used in therapies like bone marrow transplants, while embryonic stem cells are explored in regenerative medicine. (1 mark)
- Embryonic stem cells face ethical concerns, while adult stem cells are more accepted but less versatile. (1 mark)
Question 10
Discuss the challenges of using stem cells in medicine. (6 marks)
Mark Scheme:
- Ethical concerns surround the use of embryonic stem cells due to embryo destruction. (1 mark)
- Difficulty controlling stem cell differentiation into desired cell types. (1 mark)
- Risk of tumor formation if stem cells divide uncontrollably after transplantation. (1 mark)
- Limited availability and potency of adult stem cells compared to embryonic stem cells. (1 mark)
- Reprogramming iPSCs may introduce mutations, affecting their safety and functionality. (1 mark)
- Immune rejection remains a challenge for non-patient-specific stem cells. (1 mark)
Quizzes
Test 1
1. What is a defining characteristic of stem cells?
2. Which type of stem cell has the potential to differentiate into any cell type, including placenta cells?
3. What is the main limitation of adult stem cells?
4. Which type of stem cell is found in bone marrow?
5. Why are stem cells important in growth and repair?
6. Which phase of human development contains only totipotent stem cells?
7. What is the role of bone marrow stem cells in the body?
8. Why is bone marrow transplantation used in medical treatments?
9. What concept explains how all specialized cells retain the same genes as the zygote?
10. What future potential does stem cell research hold?
Correct Answers: 0%
Test 2
1. What does the term “potency” refer to in stem cells?
2. Which type of stem cell is found in adult tissues?
3. Why are totipotent cells only present in the early stages of development?
4. What is the key feature of pluripotent stem cells?
5. How do stem cells assist in tissue repair?
6. What distinguishes multipotent stem cells from pluripotent stem cells?
7. Which of the following is an example of a disease treated by bone marrow transplants?
8. Which stem cell type is used in laboratory research for regenerative medicine?
9. Why do most adult cells lose their ability to divide?
10. What role does selective gene expression play in differentiation?
Correct Answers: 0%
Test 3
1. Which type of stem cell can differentiate into all cell types in the body except the placenta?
2. In which tissue are multipotent stem cells responsible for producing blood cells?
3. Which type of cells are produced at a high turnover rate in the human body?
4. What is the function of adult stem cells in the skin?
5. What limits the differentiation potential of adult stem cells?
6. What makes embryonic stem cells valuable in regenerative medicine?
7. Which of the following is an example of selective gene expression?
8. Which of the following conditions is NOT currently treated using stem cells?
9. What is one major challenge of using stem cells in therapy?
10. How do adult stem cells differ from embryonic stem cells?
Correct Answers: 0%