Cell Division and Growth
4. Stem Cells
Learning outcomes
- I can define stem cells.
- I can explain how stem cells differ from specialized cells.
- I can describe the potential uses of stem cells in medicine.
- I can compare embryonic and adult stem cells.
- I can evaluate the benefits and challenges of stem cell research.
Stem Cells
Stem cells are unspecialized cells that have two important abilities:
- they can divide to produce more stem cells
- they can develop into specialized cell types
This makes stem cells different from most specialized cells, which already have a particular structure and function.
Stem cells are important for:
- growth and development
- replacing damaged or worn-out cells
- maintaining tissues
- medical research
- developing possible treatments for disease and injury
What Is a Stem Cell?
A stem cell is an unspecialized cell that can divide and can develop into one or more types of specialized cells.
Two key properties define stem cells.
Self-renewal
Stem cells can divide and produce more stem cells.
Differentiation
Stem cells can develop into cells with specialized structures and functions.
For example, depending on the type of stem cell, daughter cells may develop into:
- blood cells
- muscle cells
- nerve cells
- skin cells
- bone cells
The process by which an unspecialized cell becomes specialized is called differentiation.
Stem Cells and Specialized Cells
A specialized cell has adaptations that allow it to perform a particular function.
Examples include:
- red blood cells carrying oxygen
- nerve cells transmitting electrical signals
- muscle cells contracting
- root hair cells absorbing water and mineral ions
Stem cells have not yet developed all the features required for one particular function.
A simple comparison is:
| Stem Cells | Specialized Cells |
|---|---|
| Unspecialized | Adapted for a particular function |
| Can often divide repeatedly | Some divide only rarely |
| Can differentiate | Usually already differentiated |
| Can produce other cell types | Usually perform one main role |
Cell Differentiation
Differentiation occurs when cells develop different structures and functions.
Most cells in a multicellular organism contain essentially the same genetic information, but they do not use all genes in the same way.
Different genes can be switched on or off in different cells.
As a result, cells produce different proteins and develop different structures.
For example:
A stem cell developing into a nerve cell may form:
- a long axon
- branching dendrites
- specialized membrane proteins
A stem cell developing into a muscle cell develops structures needed for contraction.
Therefore:
same basic DNA → different gene activity → different specialized cells
Why Stem Cells Are Important During Development
Early in development, an embryo begins with only a small number of cells.
These cells divide repeatedly.
Some remain relatively unspecialized, while others begin to differentiate.
This eventually produces the many cell types needed to form:
- tissues
- organs
- organ systems
Without differentiation, all cells would remain similar and could not form complex tissues.
Stem cells therefore play an essential role in building a multicellular organism.
Types of Stem Cells
Two major categories commonly discussed are:
- embryonic stem cells
- adult stem cells
These differ in where they come from and in the range of cell types they can usually produce.
Embryonic Stem Cells
Embryonic stem cells are obtained from very early-stage embryos.
They are pluripotent.
This means they can develop into almost any type of body cell.
Possible cell types include:
- nerve cells
- muscle cells
- liver cells
- skin cells
- pancreatic cells
Because they can form such a wide variety of cell types, embryonic stem cells are valuable for studying development and disease.
However, their use also raises ethical questions because obtaining them may involve the destruction of an early embryo.
Adult Stem Cells
Adult stem cells are found in some tissues of children and adults.
They help maintain and repair tissues.
Examples are found in:
- bone marrow
- skin
- intestine
- some other tissues
Adult stem cells are usually more limited in the types of cells they can produce.
For example, blood-forming stem cells in bone marrow can produce several types of blood cells.
These are often described as multipotent because they can produce several related cell types, but not usually every type of body cell.
Embryonic vs Adult Stem Cells
| Feature | Embryonic Stem Cells | Adult Stem Cells |
|---|---|---|
| Source | Early embryo | Certain body tissues |
| Potential | Can form almost any body cell | Usually form a more limited range |
| Term | Pluripotent | Often multipotent |
| Medical value | Broad differentiation potential | Important for tissue maintenance and treatment |
| Ethical concerns | More significant | Generally fewer |
| Risk | Can form unwanted tissues if poorly controlled | Usually more restricted |
Neither type is simply "better."
Each has different advantages and limitations.
Potency
Scientists classify stem cells partly according to the range of cells they can produce.
Totipotent
Can produce all cell types needed to form an entire organism, including supporting tissues in early development.
Pluripotent
Can produce almost any type of body cell.
Multipotent
Can produce several related cell types.
For example:
Embryonic stem cells → pluripotent
Many adult stem cells → multipotent
The greater the potency, the wider the range of possible cell types.
Induced Pluripotent Stem Cells
Scientists can also create induced pluripotent stem cells, or iPS cells.
These begin as specialized adult cells, such as skin cells.
Scientists reprogram them so they behave more like pluripotent stem cells.
iPS cells are important because they:
- can form many different cell types
- can be made from a patient's own cells
- reduce some ethical concerns associated with embryonic stem cells
- are useful for studying diseases in the laboratory
However, they still require careful control and testing before many medical uses are possible.
Stem Cells in Bone Marrow
One of the longest-established medical uses of stem cells involves blood-forming stem cells.
These are found in bone marrow and blood.
They can produce:
- red blood cells
- white blood cells
- platelets through their precursor cells
Stem cell transplantation can be used in the treatment of some diseases affecting the blood and immune system.
Examples include certain:
- leukemias
- lymphomas
- bone marrow disorders
This is an important reminder that stem-cell medicine is not only theoretical. Some stem-cell treatments are already well established.
Stem Cells and Tissue Repair
Scientists hope stem cells may help replace cells damaged by disease or injury.
Possible targets include:
- damaged heart muscle
- nerve cells
- insulin-producing pancreatic cells
- damaged cartilage
- retinal cells
The basic idea is:
stem cell → controlled differentiation → replacement cell → tissue repair
However, producing the correct cell type safely and integrating it into functioning tissue can be very difficult.
Potential Use: Diabetes
In some forms of diabetes, insulin-producing cells in the pancreas are damaged or destroyed.
Scientists have investigated whether stem cells can be directed to form insulin-producing cells.
If successful and safe, these cells could potentially help restore insulin production.
This illustrates the potential of cell replacement therapy.
Potential Use: Nervous System Damage
Nerve cells in the brain and spinal cord are often difficult to replace naturally.
Researchers have therefore investigated whether stem cells might help replace or support damaged nerve cells.
Possible areas of research include:
- spinal cord injury
- Parkinson's disease
- some forms of neurodegeneration
However, the nervous system is extremely complex, so simply producing new nerve cells does not guarantee that they will form the correct connections.
Potential Use: Heart Damage
A heart attack can damage heart muscle cells.
Because adult heart tissue has limited ability to replace large numbers of these cells, researchers have investigated whether stem-cell-derived cells could help repair damaged heart tissue.
Challenges include ensuring that the new cells:
- survive
- integrate with existing tissue
- contract correctly
- do not cause abnormal heart rhythms
Stem Cells in Medical Research
Stem cells are useful even when they are not transplanted into patients.
Scientists can grow stem cells in laboratories and use them to study:
- how cells develop
- how diseases begin
- how genes influence differentiation
- how new medicines affect cells
- how tissues respond to damage
Stem-cell-derived tissues can also be used to test potential drugs before they are tested more widely.
Organoids
Scientists can sometimes grow stem cells into small, simplified three-dimensional structures called organoids.
Organoids can resemble some features of organs such as:
- intestine
- brain
- liver
- kidney
Researchers use organoids to study:
- development
- genetic diseases
- infections
- responses to drugs
An organoid is not a complete functioning organ, but it can provide a useful model for research.
Benefits of Stem Cell Research
Stem cell research has several important potential benefits.
Replacing damaged cells
Stem cells may provide new cells for damaged tissues.
Understanding disease
Scientists can study how diseased cells develop.
Testing medicines
Stem-cell-derived cells can be used to investigate how drugs affect particular tissues.
Understanding development
Stem cells help scientists study how cells differentiate.
Personalized research
Cells from a particular patient can sometimes be reprogrammed into iPS cells and used to study that person's disease.
Challenges of Stem Cell Research
Stem cell research also presents significant scientific and practical challenges.
Controlling differentiation
Scientists must ensure stem cells develop into exactly the desired cell type.
Uncontrolled growth
Some stem cells can divide rapidly.
If growth is not controlled, unwanted cell masses or tumours may form.
Immune rejection
Transplanted cells from another person may be recognized as foreign by the immune system.
Integration
New cells must connect and function correctly within existing tissue.
Cost and complexity
Growing and preparing cells safely can require highly specialized facilities.
Long-term safety
Researchers must determine whether treatments remain safe and effective over many years.
Ethical Questions
Embryonic stem-cell research can raise ethical concerns because obtaining embryonic stem cells may involve destroying an early human embryo.
Different people and societies may have different views about the moral status of embryos.
Some argue that:
- embryos deserve significant moral protection
- destroying embryos for research is unacceptable
Others argue that:
- early embryos do not yet have the characteristics of a developed human being
- research may help treat serious diseases
- unused embryos from fertility treatments may otherwise be discarded
Stem-cell research therefore involves both scientific questions and ethical judgments.
A balanced evaluation should recognize that reasonable people may reach different conclusions.
How iPS Cells Affect the Ethical Debate
Induced pluripotent stem cells can be produced without using embryos.
This can reduce some ethical concerns.
However, iPS cells do not eliminate every challenge.
Researchers must still consider:
- genetic changes during reprogramming
- tumour risk
- reliability of differentiation
- long-term safety
Therefore, iPS cells provide another valuable research tool, but they do not make all other stem-cell research unnecessary.
Evaluating Stem Cell Research
When evaluating stem-cell research, consider both benefits and risks.
A strong evaluation might include:
Potential benefits
- treating serious diseases
- replacing damaged cells
- understanding development
- testing medicines
- reducing suffering
Potential challenges
- ethical concerns
- tumour formation
- immune rejection
- high costs
- difficulty controlling cell development
- uncertain long-term outcomes
A balanced conclusion should consider whether the potential benefits justify the scientific, medical, and ethical challenges.
Worked Example: Stem Cell or Specialized Cell?
A cell can divide repeatedly and can develop into several types of blood cells.
Is it more likely to be a stem cell or a specialized cell?
It is most likely a:
stem cell
because it can both self-renew and produce different specialized cells.
Worked Example: Embryonic vs Adult Stem Cells
A researcher needs cells that could potentially develop into many different types of body cells.
Which type would generally provide greater developmental potential?
Embryonic stem cells
because they are pluripotent.
Adult stem cells are generally more restricted in the types of cells they can form.
Worked Example: Bone Marrow
A patient receives blood-forming stem cells after treatment for a blood cancer.
Why are stem cells useful?
The transplanted stem cells can divide and produce new blood-cell populations.
This helps rebuild the patient's blood-forming system.
Worked Example: Medical Evaluation
A new stem-cell therapy could potentially replace damaged nerve cells.
What questions should scientists ask before using it widely?
They should consider:
- Does it actually improve function?
- Do the cells survive?
- Do they become the correct cell type?
- Could they form tumours?
- Could the immune system reject them?
- Are there harmful long-term effects?
A treatment should not be considered successful simply because stem cells can be grown.
It must also be shown to be safe and effective.
Unproven Stem Cell Treatments
Because stem cells have attracted considerable attention, some clinics have marketed treatments that have not been shown to be safe or effective.
Claims about stem-cell therapies should therefore be evaluated carefully.
Strong medical evidence should come from:
- controlled research
- clinical trials
- appropriate regulatory review
- published scientific evidence
The phrase "stem cell treatment" does not automatically mean that a therapy is scientifically proven.
Common Misconceptions
Stem cells can automatically turn into any cell the body needs.
Incorrect. Differentiation must be controlled by particular signals, and different stem cells have different levels of potency.
All stem cells come from embryos.
Incorrect. Stem cells are also found in adult tissues, and pluripotent cells can be created by reprogramming adult cells.
Adult stem cells can become every type of body cell.
Most adult stem cells are more limited than embryonic stem cells.
Stem cells already provide cures for almost every disease.
Incorrect. Some stem-cell treatments are established, but many proposed applications are still being researched.
Stem cells and specialized cells are the same.
Stem cells are relatively unspecialized and can produce other cell types. Specialized cells are adapted for particular functions.
Stem-cell research has only scientific challenges.
It can also involve medical, economic, regulatory, and ethical questions.
Did You Know?
Blood-forming stem-cell transplantation has been used medically for decades.
These stem cells can rebuild blood and immune-cell populations after certain diseases or treatments damage the bone marrow.
At the same time, researchers continue investigating newer stem-cell approaches for conditions affecting tissues such as the nervous system, pancreas, heart, and eyes.
Key Terms
Stem cell – An unspecialized cell capable of self-renewal and differentiation.
Self-renewal – The ability of a stem cell to divide and produce more stem cells.
Differentiation – The process by which a cell becomes specialized.
Specialized cell – A cell adapted to perform a particular function.
Embryonic stem cell – A pluripotent stem cell obtained from an early-stage embryo.
Adult stem cell – A stem cell found in developed tissues that usually produces a more limited range of cells.
Pluripotent – Able to develop into almost any type of body cell.
Multipotent – Able to develop into several related cell types.
Induced pluripotent stem cell (iPS cell) – A specialized adult cell that has been reprogrammed to behave more like a pluripotent stem cell.
Stem-cell transplant – Transfer of stem cells into a patient to restore or replace particular cell populations.
Organoid – A small three-dimensional tissue model grown from cells that reproduces some features of an organ.
Key Takeaways
- Stem cells are unspecialized cells capable of dividing and differentiating.
- Stem cells can produce specialized cell types.
- Specialized cells have particular structures and functions.
- Embryonic stem cells are generally pluripotent and can form a wide range of body cells.
- Many adult stem cells are multipotent and produce a more limited range of related cells.
- Blood-forming adult stem cells are already used in established medical treatments.
- Induced pluripotent stem cells are made by reprogramming specialized adult cells.
- Stem cells may help researchers study disease, test medicines, and develop cell-replacement therapies.
- Possible future applications include repairing damaged nerve, heart, pancreatic, retinal, and other tissues.
- Important challenges include controlling differentiation, preventing uncontrolled growth, avoiding immune rejection, and proving long-term safety.
- Embryonic stem-cell research raises ethical questions because embryos may be destroyed to obtain the cells.
- iPS cells reduce some ethical concerns but still have scientific and safety limitations.
- A balanced evaluation of stem-cell research should consider both its potential medical benefits and its scientific, ethical, and safety challenges.