Specification Reference 3.17(ii) · Distance Learning Resource · Lessons 4 & 5
Before reading anything, activate your prior knowledge. This makes new learning stick better.
Every cell in your body contains the same DNA — the same complete instruction manual. Yet a neurone looks and behaves completely differently from a red blood cell. How?
The answer is differentiation — the process by which cells become specialised by switching certain genes on and off. Once a cell differentiates, it typically cannot go back.
Stem cells are different. They are undifferentiated cells that retain the ability to divide and develop into different cell types. Their potential depends on how many types of cell they can become.
Not all stem cells are equal. Scientists classify them by their potency — how many different cell types they can produce:
| Type | Meaning | Example | Can form… |
|---|---|---|---|
| Totipotent | Can form any cell type including extraembryonic tissue (placenta) | Zygote; cells up to ~4-day embryo | Entire new organism |
| Pluripotent | Can form most cell types (all 3 germ layers) but not placenta | Embryonic stem cells from inner cell mass of blastocyst | Most cell types |
| Multipotent | Can form a limited range of related cell types | Adult bone marrow stem cells; brain stem cells | Several related types |
| Unipotent | Can form only one cell type | Skin stem cells; muscle satellite cells | One specific cell type |
Harvested from the blastocyst — an early embryo ~5 days after fertilisation. The inner cell mass contains pluripotent stem cells. Extraction destroys the embryo, raising significant ethical debate.
Found in bone marrow, brain, and skin. Typically multipotent — bone marrow stem cells form all blood cell types but not neurones. In the 1990s, scientists also found they can generate bone, fat, cartilage, and tissue.
Contains stem cells more versatile than typical adult stem cells. Some parents bank this blood at birth.
In 2006, Japanese researchers took adult mouse cells and, using genetic engineering, reprogrammed them to become pluripotent again — without using an embryo.
The process uses modified viruses to carry four genes for specific transcription factors into adult skin cells. These induced pluripotent stem cells (iPS cells) renew themselves and behave very similarly to embryonic stem cells.
iPS cells overcome the major ethical objection — no embryo is destroyed. A patient's own cells can be used, eliminating rejection risk. However, the reprogramming genes are strongly associated with cancer development, and controlling differentiation remains difficult.
Therapeutic cloning is an experimental technique aimed at producing large quantities of healthy tissue. The process:
This is meant to be challenging. Spend at least 3 minutes thinking before using the hints.
Reflect on how your understanding has changed during this lesson.
Before starting, connect to what you already know from Lesson 4.
When stem cells were first cultured, scientists hoped they could produce replacement tissues. Progress has been promising but challenging — controlling differentiation remains difficult, and some early treatments caused unexpected cancers.
The most established success is bone marrow transplantation. About 30 years ago, scientists discovered that bone marrow stem cells form all types of blood cell. These transplants now regularly treat certain cancers and immune diseases. Transplants require matched donors to avoid rejection.
Adult stem cells seeded onto collagen-based frameworks have successfully grown new tracheas, and research into repairing hearts damaged by heart attacks using adult stem cells has shown significant improvement in some cases.
Parkinson's is the second most common age-related brain disorder. Nerve cells producing dopamine stop working and are lost. As dopamine falls, people develop uncontrollable tremors, rigidity, and eventually cannot move normally.
Scientists created dopamine neurones from mouse stem cells and transplanted them into rats with Parkinson's — the cells grew, released dopamine, and improved movement. The hope: pluripotent stem cells could replace damaged brain cells.
Insulin-secreting cells in the islets of Langerhans are destroyed or stop producing insulin. Stem cell therapy could provide working pancreas cells to restore insulin production.
Mouse stem cells formed insulin-producing cells that improved glucose control when transplanted. In 2014, Harvard researchers developed mature human beta cells from embryonic stem cells in large quantities — human trials are underway.
Spinal cord injuries cause permanent paralysis because nervous tissue doesn't regrow naturally. Stem cells transplanted into mice/rats with spinal damage grew into working nerve cells, restoring some movement. In 2013, Australian researchers produced mini-kidneys from skin-derived stem cells.
Many people die waiting for transplants. Pluripotent stem cells could provide unlimited organs grown from a patient's own cells, eliminating rejection. This remains experimental but is advancing rapidly.
Immune rejection: Cell membrane glycoproteins trigger immune responses. The immune system recognises "self" vs "non-self." Unless stem cells come from the patient (iPS/therapeutic cloning), lifelong immunosuppressants are needed, increasing infection risk.
Cancer risk: Stem cells may trigger cancer. Bone marrow transplant recipients are at higher risk of later cancers.
Differentiation control: We cannot yet fully control which cell types stem cells become.
Limited success: Few successful pluripotent stem cell therapies in humans so far, though results with macular degeneration are positive and scientists expect dramatic increases within 10 years.
The debate is framed by four ethical principles:
Respect for autonomy — not performing procedures without consent.
Beneficence — doing good, relieving suffering.
Non-maleficence — doing no harm.
Justice — treating everyone equally, sharing resources fairly.
Plan your answer. Include: scientific challenges, ethical principles, different stem cell sources, and a balanced conclusion.
1. Define totipotent, pluripotent, and multipotent.
2. Explain why therapeutic cloning avoids immune rejection.
3. State two concerns about using iPS cells.
Construct an evidence-based argument about stem cell research.
Reflect on how your understanding has evolved across both lessons.