How do cells become specialised, and why are stem cells important?
Cellular differentiation as the process by which cells express only the genes needed for their function, the difference between embryonic and tissue (adult) stem cells, the meaning of potency, and the therapeutic and research uses of stem cells along with cancer cells.
An SQA Higher Biology answer on cellular differentiation, covering how cells express only the genes for their function, the difference between embryonic and tissue stem cells, the meaning of potency, the therapeutic and research uses of stem cells, and how cancer cells differ from normal cells.
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What this key area is asking
The SQA wants you to define cellular differentiation in terms of gene expression, distinguish embryonic from tissue stem cells, explain the meaning of potency, describe the therapeutic and research uses of stem cells, and explain how cancer cells differ from normal cells.
What differentiation is
All cells in an organism carry the same genome, but a specialised cell only switches on the genes it needs; the rest are switched off. For example, a red blood cell expresses the genes for haemoglobin, while a pancreatic cell expresses the genes for insulin, even though both cells contain both sets of genes. In plants, the cells able to divide and differentiate are found in regions called meristems, which is why a small cutting can grow into a whole new plant.
Stem cells and potency
Potency describes the range of cell types a stem cell can become. Pluripotent embryonic stem cells, found in the early embryo, have the widest range. Multipotent tissue stem cells, found in tissues such as bone marrow, have a narrower range and are used in the body for natural repair and replacement, such as making new blood cells throughout life.
Uses of stem cells
Stem cells have two broad uses:
- Therapeutic uses - repairing damaged or diseased tissue, for example bone marrow transplants to treat leukaemia, corneal repair to restore sight, and skin grafts for burns patients.
- Research uses - acting as model cells to study how diseases develop, to test the effects of new drugs before trialling them in people, and to investigate how cells differentiate.
The use of embryonic stem cells raises ethical issues, because the embryo is destroyed to obtain the cells. These issues are managed by regulation, and one way to avoid them is to use tissue stem cells or to reprogram adult cells back into a stem-cell-like state.
Cancer cells
Unlike normal cells, cancer cells fail to attach to one another, so cells can break off, spread through the body and form secondary tumours, a process called metastasis. Cancer cells are linked to differentiation because they are often poorly differentiated, dividing rapidly instead of carrying out a specialised function. This is why understanding how cells normally differentiate and control their division is central to understanding cancer.
Examples in context
Example 1. Bone marrow transplants for leukaemia. Leukaemia is a cancer of the blood. In treatment, the patient's faulty bone marrow is destroyed and replaced with healthy multipotent stem cells from a donor. These transplanted stem cells differentiate into all the normal blood cell types, restoring the patient's blood and immune system. This is the most established therapeutic use of stem cells and relies directly on their ability to self-renew and differentiate.
Example 2. Induced pluripotent stem cells in research. Scientists can now take ordinary adult skin cells and reprogram them into induced pluripotent stem cells, which behave like embryonic stem cells but avoid destroying an embryo. These cells are used to grow patient-specific tissue in the laboratory, to model diseases such as Parkinson's, and to test new drugs, showing how research uses of stem cells can sidestep some of the ethical issues.
Try this
Q1. Explain what cellular differentiation is in terms of genes. [2 marks]
- Cue. A cell expresses only the genes that make the proteins for its specialised function; the other genes are switched off.
Q2. State one difference between embryonic and tissue stem cells. [1 mark]
- Cue. Embryonic stem cells are pluripotent; tissue stem cells are multipotent.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20193 marksExplain what is meant by cellular differentiation, and describe the difference between pluripotent and multipotent stem cells.Show worked answer →
A 3-mark answer needs the gene-expression definition plus the two potency terms.
Cellular differentiation is the process by which a cell expresses only the genes that make the proteins characteristic of that cell type, so it becomes specialised for a particular function. All cells keep the full genome, but the unneeded genes are switched off.
Pluripotent stem cells (embryonic stem cells) can differentiate into all the cell types of the organism. Multipotent stem cells (tissue or adult stem cells) can differentiate only into the limited range of cell types found in their particular tissue.
Markers reward (1) differentiation as selective gene expression, (2) pluripotent forms all cell types and (3) multipotent forms a limited range.
SQA Higher 20213 marksDescribe two therapeutic uses of stem cells and explain one ethical issue raised by the use of embryonic stem cells.Show worked answer →
A 3-mark answer needs two genuine therapeutic uses and one ethical point.
Therapeutic uses include repairing or replacing damaged tissue, such as bone marrow transplants to treat leukaemia, corneal repair to restore sight, and skin grafts for burns patients. Any two of these gain the marks.
An ethical issue is that obtaining embryonic stem cells destroys an embryo, which raises questions about the moral status of the embryo. This is why their use is tightly regulated.
Markers reward two valid therapeutic uses and a clearly stated ethical issue.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)