Adult stem cells are undifferentiated cells that are found in most animal tissues.
Adult stem cells can divide by mitosis throughout their lifespan to form identical stem cells (self-renewal) or to form cells that can differentiate into the functioning cells of that tissue.
(a) Mitosis is important for the repair of tissues. Explain what is meant by repair of tissues.
(b) Uncontrolled cell division is a characteristic feature of tumour formation from a differentiated cell. Describe other features of tumour formation from a fully differentiated cell.
(c) Telomeres prevent loss of genes. Adult stem cells have chromosomes with long telomeres. Explain why long telomeres are an advantage to cells that carry out many cell cycles.
Haematopoietic stem cells (HSCs) are adult stem cells that are located in the bone marrow of bones. HSCs have a role in the formation of blood cells.
Fig. 4.1 is an outline summary showing the formation of some of the different types of blood cell that can be formed from HSCs. The first stage is the division of HSCs to produce progenitor cells. These cells are also able to divide by mitosis, but are not stem cells.
(d) With reference to Fig 4.1, explain why GMP cells, which are progenitor cells, cannot be described as haematopoietic stem cells (HSCs).
(e) Fig. 4.1 shows that monocytes differentiate into cell type X, which has a similar function to neutrophils. Name cell type X.
(f) Cell type Y shown in Fig. 4.1 releases molecules with antigen binding sites. Name the molecules released by cell type Y.
(g) The differentiation of T-lymphocytes begins in the bone marrow and continues in an organ known as the thymus to produce fully differentiated T-helper and T-killer cells. In the thymus, T-lymphocytes that bind to self antigens are destroyed.
Explain why T-lymphocytes that bind to self antigens need to be destroyed in the thymus.
▶️ Answer/Explanation
(a) Replacing cells that are damaged, destroyed, worn out or old.
Explanation: Tissue repair refers to the process where damaged or dead cells are replaced through mitosis. Adult stem cells play a crucial role in this process by dividing to produce new cells that can differentiate into the specific cell types needed to maintain and repair the tissue. This is essential for maintaining healthy tissues throughout an organism’s life.
(b) Any two from:
- Result of mutation (e.g., proto-oncogene to oncogene or tumour suppressor gene inactivation)
- Shorter/faster/many/continuous cell cycles/divisions
- No contact inhibition (cells continue dividing beyond available space)
- Normal cell cycle checkpoints not occurring
- Loss of original/normal function
- Cells do not undergo apoptosis (programmed cell death)
Explanation: Tumour formation involves several characteristic features beyond just uncontrolled division. Mutations in key genes disrupt normal cell cycle regulation. The cells lose their specialized functions and ignore signals that would normally stop division or trigger cell death. They may also fail to properly adhere to neighboring cells and can spread to other tissues (metastasis).
(c) Any two from:
- Telomeres shorten after each cell cycle/division/DNA replication
- Long telomeres allow more replications/divisions to occur
- Protect ends of chromosomes (prevent loss of genetic information)
- Prevent chromosome ends from joining together
Explanation: Telomeres are repetitive DNA sequences at chromosome ends that protect important genetic material. With each replication, telomeres shorten due to the “end replication problem.” Adult stem cells need to divide many times, so long telomeres provide a buffer against this shortening, preventing premature cell senescence or DNA damage that could occur if essential genes were lost.
(d) Any three from:
- Differentiation has already started (they are not undifferentiated)
- No self-renewal capability (cannot produce stem cells when they divide)
- Cannot form all blood cell types (limited differentiation potential)
- Only forms specific cell types (immature neutrophils and monocytes)
Explanation: While GMP cells (Granulocyte-Macrophage Progenitors) can divide and differentiate, they lack key stem cell properties. Unlike HSCs, they have committed to a specific lineage (myeloid cells) and cannot self-renew indefinitely or produce all blood cell types. This restricted potential is why they’re considered progenitor cells rather than true stem cells.
(e) Macrophage(s)
Explanation: Monocytes differentiate into macrophages, which like neutrophils function as phagocytes that engulf and destroy pathogens. Both cell types are important components of the innate immune system.
(f) Antibody/immunoglobulin
Explanation: Cell type Y represents mature B-lymphocytes (plasma cells) which produce and secrete antibodies. These Y-shaped proteins have antigen-binding sites that specifically recognize and bind to foreign molecules (antigens).
(g) Any three from:
- T-lymphocytes would be released into general circulation
- Could mount immune responses against body’s own cells (autoimmunity)
- Would cause harm/destruction to healthy body cells
- Prevent formation of self-reactive memory T-cells
- Prevent autoimmune diseases
Explanation: The thymus serves as a “school” for T-cells where they learn to distinguish self from non-self. Those that react strongly with self-antigens are eliminated through negative selection. This crucial process prevents autoimmunity, where the immune system would otherwise attack the body’s own tissues. Only T-cells that can recognize foreign antigens in the context of self-MHC molecules are allowed to mature and enter circulation.