2.2 Cell differentiation, growth and stem cells

Why this matters

Every cell in your body started as an unspecialised cell. A fertilised egg divided by mitosis to make a ball of identical cells, and then — gradually — those cells began to specialise. Some became muscle, some became nerve, some became gut lining. That process is called differentiation, and in most animal cells it is a one-way trip: once a cell is specialised, it can't go back. Stem cells are the exception. They stay unspecialised, dividing by mitosis to top up the body's supply, and only differentiating when needed. Plants do things slightly differently — they keep pools of unspecialised cells (meristems) at the tips of their roots and shoots throughout their lives, which is why a plant can grow a new branch from anywhere or be propagated from a cutting. Stem cell research is one of the most promising — and ethically contested — areas of modern medicine: it offers possible cures for leukaemia, diabetes, Parkinson's, paralysis, and burns, but obtaining embryonic stem cells requires destroying an embryo.

How to learn this topic

Build on what you already know

• KS3: cells in the body are specialised for different jobs (red blood cells, nerve cells, muscle cells).
• Edexcel 1.x: examples of specialised cells (sperm, egg, ciliated epithelial, red blood, root hair, palisade).
• Edexcel 2.1: mitosis produces two genetically identical, diploid daughter cells — used for growth and tissue repair.

1. Differentiation first — what it means, why it usually can't be reversed in animals, and how it produces the specialised cells from 1.x.
2. Stem cells defined — two key features: divide by mitosis AND can differentiate.
3. Three sources: embryonic (pluripotent / any cell type), adult (limited range, e.g. bone marrow), plant meristem (any plant cell type, lifelong).
4. Medical uses scenario-by-scenario: bone marrow transplants for leukaemia, skin grafts for burns, paralysis trials, diabetes/Parkinson's research.
5. Risks: rejection, infection, uncontrolled growth/tumour.
6. Therapeutic cloning as a workaround for rejection.
7. Ethical debate: embryo destruction vs potential benefit.

Key terms

differentiation

The process by which an unspecialised cell becomes specialised for a particular function (e.g. becoming a red blood cell or a nerve cell). In most animal cells this is permanent. (Edexcel marking phrases: 'becomes specialised', 'develops into a specific cell type', 'allows specialisation'.)

specialised cell

A cell that has differentiated and now has a specific structure and function (e.g. red blood cell, nerve cell, muscle cell, root hair cell). (Edexcel often asks you to name a specialised cell type that a stem cell could become — pick one that fits the context (blood disorder → blood cell).)

stem cell

An undifferentiated cell that can divide by mitosis to produce more cells AND can differentiate into specialised cell types. (Always state BOTH features — 'undifferentiated / unspecialised' AND 'can differentiate' AND 'divide by mitosis'.)

embryonic stem cell

A stem cell from an early embryo. Can differentiate into any cell type in the body — pluripotent. ('Any cell type' is the marking phrase that distinguishes embryonic from adult. 'Pluripotent' is accepted.)

adult stem cell

A stem cell found in some adult tissues (e.g. bone marrow). Can only differentiate into a limited range of cell types. (Edexcel wants both 'found in adult tissue (e.g. bone marrow)' AND 'limited range of cell types'.)

pluripotent

Describes a stem cell that can differentiate into any cell type in the body. Used of embryonic stem cells. (Accepted as an alternative to 'any cell type'.)

meristem

A region of undifferentiated, actively dividing cells in a plant — found at the tips of roots and shoots. Plant equivalent of a stem cell pool. (Edexcel marking phrase chain: 'meristem contains undifferentiated cells | differentiate into specialised cells | divide by mitosis | allows plant growth'.)

bone marrow

Soft tissue inside bones containing adult stem cells. The source of all blood cells (red, white, platelets) in the body. (Canonical Edexcel example of where adult stem cells are found.)

therapeutic cloning

Producing an embryo with the same genes as a patient (by transferring the patient's nucleus into an empty egg cell), then extracting stem cells from that embryo to grow new cells/tissue for treatment — without rejection. (The 'no rejection' point is the marking phrase, because the cells share the patient's own genes.)

rejection

When the patient's immune system recognises transplanted cells or tissue as foreign and attacks them. Avoided if the cells share the patient's own genes. (Marking phrase: 'own cells not recognised as foreign / will not be rejected'.)

embryo destruction

The unavoidable consequence of obtaining embryonic stem cells: the embryo they come from is destroyed in the process. The central ethical objection to embryonic stem cell research. (Edexcel marking phrase: 'embryonic stem cells involve destroying an embryo (ethical issues)'.)

Notes

Differentiation — becoming specialised

Differentiation is the process by which an unspecialised cell becomes specialised for a particular function. It is how a single fertilised egg (one unspecialised cell) eventually produces all the different cell types in your body — red blood cells, nerve cells, muscle cells, gut lining cells, and so on.

During differentiation, a cell:

• Changes its shape to suit its function (a nerve cell grows long extensions; a red blood cell loses its nucleus and becomes a biconcave disc).
• Develops the sub-cellular structures it needs (muscle cells gain many mitochondria; root hair cells develop a long projection).
• Switches on a specific set of genes — every cell has the same DNA, but specialised cells only use the genes relevant to their job.

In most animal cells, differentiation is permanent — once a cell has become a nerve cell, it stays a nerve cell. In plants, by contrast, cells stay capable of differentiating throughout the plant's life (which is why plants can regrow from cuttings).

(Mitosis and the cell cycle are covered separately in 2.1 — here you only need to remember that stem cells divide by mitosis to top themselves up before differentiating.)

Stem cells — the two-feature definition

A stem cell is an undifferentiated cell that has two key features:

1. It can divide by mitosis to produce more cells.
2. It can differentiate into specialised cell types.

Edexcel mark schemes almost always want both features. Saying just 'a cell that can become other cells' is not enough.

Three sources of stem cells

### 1. Embryonic stem cells — pluripotent

Found in early embryos (just a few days old, before any cells have specialised). Embryonic stem cells can differentiate into ANY cell type in the body — they are described as pluripotent.

Because they can become any cell type, embryonic stem cells could in theory be used to replace a greater variety of damaged cells than adult stem cells. They are particularly attractive for diseases where many different tissues are involved.

### 2. Adult stem cells — limited range

Found in some adult tissues, the most important example being bone marrow. Adult stem cells can only differentiate into a limited range of cell types. Bone marrow stem cells, for example, can produce all the different types of blood cells (red blood cells, white blood cells, platelets) — but they can't become muscle, nerve, or liver cells.

This makes them well-suited to blood disorders like leukaemia, but less useful for diseases that affect other tissues.

### 3. Plant meristems — undifferentiated, lifelong

In plants, the equivalent of stem cells are found in meristem tissue at the tips of roots and shoots. Meristem cells are undifferentiated and can differentiate into any plant cell type — root cells, stem cells, leaf cells, flower cells. Meristems are active throughout the plant's life, which is why a plant can grow new branches and why growers can clone valuable plants from cuttings: a piece of meristem tissue can grow a complete new plant.

Medical uses of stem cells

### Bone marrow transplants for blood disorders

In leukaemia (cancer of the white blood cells) and other blood disorders, the patient's own bone marrow produces faulty blood cells. Adult stem cells from a donor's bone marrow can be transplanted into the patient. The donor stem cells then divide by mitosis and differentiate into healthy blood cells. Adult stem cells are perfect for this because the limited range they can become (blood cells) is exactly what's needed.

### Skin grafts for burns patients

Patients with severe burns lose large areas of skin. Stem cells can be encouraged to differentiate into new skin cells, which can be grown in the lab and grafted onto the burn. Embryonic stem cells are particularly attractive here because they can become any cell type — so they can produce not just skin cells but also the blood vessels, nerves, and connective tissue that real skin needs.

### Paralysis (spinal cord injury)

Spinal cord injuries damage nerve cells that cannot normally be replaced. Trials are exploring whether stem cells can differentiate into new nerve cells to repair the spinal cord and restore movement. This is an area where embryonic stem cells' ability to become any cell type is critical.

### Diabetes and Parkinson's

In type 1 diabetes the pancreas can no longer produce insulin. Stem cells could be used to make new insulin-producing cells to replace the lost ones. In Parkinson's disease the brain loses cells that produce the chemical dopamine. Stem cells could be used to grow new dopamine-producing nerve cells.

Risks

Stem cell therapy is promising but not risk-free:

• Rejection — donor stem cells (especially embryonic ones) have different genes from the patient, so the patient's immune system may recognise them as foreign and attack them.
• Infection — stem cells grown in the lab can pick up viruses or other pathogens and transmit infection to the patient.
• Uncontrolled growth — stem cells divide rapidly, and there is a risk that transplanted cells may keep dividing uncontrollably and form a tumour.

Therapeutic cloning — a way around rejection

Therapeutic cloning is a way of producing stem cells that are genetically matched to the patient. The cell nucleus from one of the patient's own cells is transferred into an empty egg cell, which is then grown into an early embryo. Stem cells taken from that embryo carry the patient's own genes, so any tissue grown from them will not be rejected by the immune system.

For the same reason, simply using the patient's own adult stem cells (where available) avoids rejection — for example, taking a patient's stem cells to grow new cartilage for their own damaged knee.

The ethical debate

Embryonic stem cells are powerful (any cell type) but obtaining them destroys an embryo, which some people consider unacceptable on religious or ethical grounds — they argue an embryo is a potential human life. Others argue that the potential benefit to patients with currently-untreatable conditions (leukaemia, paralysis, Parkinson's) outweighs the cost.

Adult stem cells are less powerful (limited range) but don't involve embryos, so they raise fewer ethical concerns. Plant meristems raise no ethical objections at all, which is why meristem-based cloning of crops and ornamentals is uncontroversial.

Therapeutic cloning solves the rejection problem but raises the further ethical question of whether it is acceptable to create embryos specifically for research and treatment.

Exam phrases to memorise

Five Edexcel-style scenarios you will see again and again:

• Adult stem cells for blood disorder: adult stem cells differentiate into blood cells; divide by mitosis to produce new healthy cells; adult limited range of cell types; embryonic become any cell type — more useful for other diseases.
• Why embryonic > adult: embryonic differentiate into any cell type; adult limited range; embryonic could replace greater variety of damaged cells; differentiation = specialisation, embryonic have not lost this.
• Meristem tissue: meristem contains undifferentiated cells; differentiate into specialised cells; divide by mitosis; allows plant growth.
• Patient's own stem cells: own cells not recognised as foreign / not rejected; embryonic involve destroying an embryo (ethical); undergo differentiation; divide by mitosis.
• Embryonic for skin burns: embryonic differentiate into any cell type; adult limited range; stem cells must differentiate; divide by mitosis.

Exam tips

• On every stem cell scenario, name BOTH defining features: 'divide by mitosis' AND 'differentiate into specialised cells'. Edexcel usually awards a mark for each separately.
• When a scenario specifies a blood disorder (leukaemia, sickle-cell), pick ADULT stem cells from bone marrow — the limited range is exactly what's needed.
• When a scenario specifies skin, nerve, paralysis, or 'many different tissues', pick EMBRYONIC stem cells — only they can become any cell type.
• For meristem questions use the exact phrases: 'meristem contains undifferentiated cells', 'divide by mitosis', 'differentiate into specialised cells', 'this allows plant growth'.
• For 'patient's own stem cells' questions name 'not recognised as foreign / not rejected' — that's where the mark sits.
• Ethical questions: if a question has 'ethical issues' in the stem, you need to write the word 'embryo' (and ideally 'destroyed' or 'human life') for the mark.
• Don't bring in cell cycle / mitosis detail from 2.1 — Edexcel splits the topics. Stick to differentiation, sources, uses, ethics.

Mark-scheme phrasing

Common misconceptions

• —
• —
• —
• —

Worked example

Question:

Answer:

Frequently asked questions

Related topics in Edexcel GCSE Biology (1BI0)

• 2.1 Mitosis and the cell cycle
• 2.3 The nervous system