Types and Uses of Stem Cells
Stem cells are undifferentiated cells that can develop into specialised cell types. Their varying potential (potency) underpins their use in regenerative medicine, but also creates significant ethical and practical challenges.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Totipotent stem cells, found in the earliest embryonic stages (e.g., zygote), can differentiate into every cell type, including those forming the placenta, allowing for the development of a complete organism.
- Pluripotent stem cells are derived from the inner cell mass of an embryo's blastocyst stage. They can become any cell type in the body but cannot form an entire organism on their own as they don't form extra-embryonic tissues.
- Multipotent stem cells are found in adult tissues (e.g., bone marrow, skin) and are responsible for maintenance and repair. Their differentiation is limited to the cell types of their specific tissue of origin.
- Medical benefits include repairing damaged tissues (e.g., spinal cord) and treating diseases (e.g., diabetes, Parkinson's). Key risks include immune system rejection, potential for uncontrolled growth leading to tumours, and ethical objections to using embryonic sources.
Definitions
- Totipotent
- The ability of a single cell to differentiate and develop into a complete organism, including all extra-embryonic tissues like the placenta.
- Pluripotent
- The ability of a stem cell to differentiate into any of the three primary germ layers of the embryo (ectoderm, endoderm, mesoderm), meaning it can form any cell type of the body, but not extra-embryonic tissues.
- Multipotent
- The ability of a stem cell to differentiate into a limited range of cell types, typically within a specific tissue or organ.
Worked example
A lab wants to grow new cartilage tissue to repair a damaged knee joint. They consider using hematopoietic stem cells from bone marrow and embryonic stem cells from a blastocyst. Evaluate the suitability of each source for this specific application.
- 1
Identify the target tissue:
Cartilage is a specialised connective tissue.
- 2
Evaluate the first source:
Hematopoietic stem cells are found in bone marrow and are multipotent.
Their differentiation pathway is restricted to forming blood cells (red cells, white cells, platelets).
- 3
Conclude on the first source:
Hematopoietic stem cells are unsuitable as they cannot differentiate into cartilage cells.
- 4
Evaluate the second source:
Embryonic stem cells from a blastocyst are pluripotent.
This means they have the potential to differentiate into any cell type in the body, including cartilage cells.
- 5
Conclude on the second source:
Embryonic stem cells are biologically suitable for generating cartilage tissue.
Answer: The hematopoietic stem cells are unsuitable because they are multipotent and restricted to forming blood cells. The embryonic stem cells are suitable because they are pluripotent and can differentiate into cartilage cells.
Common mistakes
- ×Confusing the different levels of potency. Remember the hierarchy: Totipotent (total organism) > Pluripotent (all body cells) > Multipotent (limited cell types).
- ×Assuming all stem cells are the same. Adult stem cells are multipotent and much more restricted than embryonic (pluripotent) ones.
- ×Discussing the benefits of stem cell therapy without acknowledging the major constraints, particularly the risk of tumour formation from uncontrolled cell division and the ethical issues surrounding embryonic sources.
No-calculator tips
- ✓Focus on the source of the cells to determine their potency. If it's from an adult tissue (bone marrow, skin), it's multipotent. If it's from a blastocyst, it's pluripotent.
- ✓For benefit/risk questions, create a quick mental table of pros and cons. Pros: treat disease, repair tissue. Cons: rejection, tumours, ethics. This ensures a balanced answer.