1. Stem Cells

2. What are stem cells?
The first cells are stem cells. These are unspecialized cells capable of developing into many different types of cell.
Stem cells found in embryos are called embryonic stem cells and develop into all the different types of cell in the body.
Photo credit: Steve Gschmeissner / Science Photo Library
Coloured scanning electron micrograph (SEM) of groups of embryonic stem cells (ESCs). ESCs are pluripotent, that is they are able to differentiate into any cell type. The type of cell they mature into depends upon the biochemical signals received by the immature cells. This ability makes ESCs a potential source of cells to repair damaged tissue in diseases such as Parkinson’s and insulin-dependent diabetes. However, research using ESCs is controversial as it requires the destruction of an embryo. Magnification: x3000 when printed at 10 centimetres wide.
If stem cells continued to divide as they were, humans would end up as a large jelly-like blob!

3. Changing cells
When the embryo contains about 500 cells, the cells stop being the same and they stop getting smaller with each division. They start to differentiate into different types of cell.
At this point, stem cells no longer form two new stem cells when they divide. Instead, one of the two daughter cells becomes a progeny or tissue cell.
stem cell
stem cell
tissue cell

4. Becoming specialized
Tissue cells continue to divide and differentiate, each time becoming more and more specialized.
stem cell
tissue cells
Some will become nerve cells, others will become blood cells, muscle cells, bone cells, etc.
nerve cells
cardiac muscle
cells
red blood cells

5. A controversial source?
Most stem cell research has used cells obtained from embryos left over from fertility treatment.
These are called embryonic stem cells (ESCs) and are the most powerful type because they can turn into any type of cell.
Their use in research is strongly criticized by people who believe it is unethical to kill embryos for their cells.
Teacher notes
In 2001, US President George W Bush banned the use of Government money for research on newly-derived embryonic stem cells, despite widespread opposition from scientists, patients with diseases that could potentially be treated by stem cells, and Democrat politicians. In the UK, human embryonic stem cells research was made legal in 2001, but is strictly controlled by the Human Fertilization and Embryology Authority (
International guidelines on the conduct of human embryonic stem cell research are available from the Society for Stem Cell Research (ISSCR), available at:
There are different potencies of stem cells:
totipotent = capable of becoming any type of cell including extra-embryonic tissues such as the placenta (e.g. the earliest embryonic stem cells)
pluripotent = capable of becoming most types of cell (e.g. embryonic stem cells after about 4 days, fetal stem cells)
multipotent = capable of becoming some types of cell (e.g. umbilical cord stem cells, adult bone marrow stem cells, which are capable of becoming several different types of blood cell)
unipotent = capable of becoming only one type of cell (e.g. adult skin stem cells, which are only capable of becoming epithelial cells)
Stem cells from fetuses are almost as powerful, and these have also been found in the blood of umbilical cords and amniotic fluid of pregnant women.

6. If you ran the country… Teacher notes
This voting activity enables the individual opinions of the class to be represented graphically. It could be used at the start of a topic, then repeated as a plenary exercise to see if the students’ opinions have changed after learning the relevant material.

7. Therapeutic cloning Teacher notes
The process used in therapeutic cloning is called ‘somatic cell nuclear transfer’. Some scientists feel it is better to use this term rather than ‘therapeutic cloning’ to distinguish it from the more controversial ‘reproductive cloning’ and make it more acceptable to the public.
Scientists are still not able to clone human stem cells. In 2005, Korean cloning researcher Woo Suk Hwang claimed to have created 11 lines of cloned human stem cells tailored to individuals. His results were later shown to be fake.

8. What are adult stem cells?
Adults also contain stem cells, where they are found in small numbers in many organs, including bone marrow, brain, skin and muscle.
Adult stem cells normally maintain and repair damaged tissue, and can usually only make a small number of cell types. For example, adult stem cells in the skin only normally form skin cells.
Teacher notes
To date, adult stem cell populations have been found in bone marrow, blood, skin, muscle, heart, liver, brain, the cornea and retina of the eye, the lining of the gastrointestinal tract, and the pancreas.
More information about stem cells is available at:
One of the main difficulties facing researchers is accurately identifying true stem cells, and determining that the stem cells originate in that specific organ and haven’t migrated there from the bone marrow or other locations. Students should be aware that stem cell research is at the cutting-edge of science and that initial findings may prove to be inaccurate as other researchers try and replicate the earlier results. This replicability is at the very heart of the scientific process.
Research has shown that some adult stem cells can be manipulated to produce many different cell types (e.g. brain stem cells can make muscle cells). This is called plasticity.

9. What are bone marrow transplants?
Stem cells found in bone marrow have been used for many years to treat diseases that affect blood cells, such as leukaemia, by a bone marrow transplant.
Bone marrow is harvested from a suitable donor under general anaesthetic.
The patient’s abnormal blood cells are destroyed by chemotherapy and/or radiotherapy.
The donated bone marrow is injected into the patient.
Teacher notes
The aim of a bone marrow transplant is to replace the patient’s haematopoietic (‘blood forming’) stem cells (HSCs), which naturally form all the different types of blood cell in the body. It is important that the donated bone marrow is as similar as possible to that of the patient (a good match) to minimise the risk of complications and immune rejection. A close relative is more likely to be a good match than a non-relative.
HSCs are increasingly being taken from the peripheral blood rather than the bone marrow. This avoids surgery and the need for general anaesthetic, is less painful for donors and speeds up recovery time in patients. In a peripheral blood stem cell transplant (PBSCT), the donor is injected with growth factors to increase the number of circulating stem cells, which are then removed by apheresis. In this technique, blood is withdrawn through a needle, the white blood cells are removed for donation and the red blood cells are returned to the donor.
Blood from umbilical cords is also a source of HSCs, where they are found in a higher concentration than adult blood.
It is possible for parents of a sick child to take stem cells from the umbilical cord of a new sibling (which has been screened for the disease) to treat certain diseases. For more information on ‘saviour siblings’ and pre-implantation genetic diagnosis, see the GCSE Science (Biology) ‘Genes and Genetic Engineering’ and ‘Hormones’ presentations.
Stem cells in the injected bone marrow circulate in the blood and end up in the patient’s bone marrow, where they start to divide and produce new blood cells.

10. Advantages of using adult stem cells
They come from volunteers so they are more ethically acceptable.
A patient’s own stem cells could be used to treat their own disease, avoiding the problem of immune rejection.
It might be easier to guide their development into specific cell types.
Teacher notes
Studying adult stem cells and how they differentiate into different cell types could also provide useful information on how cancers arise and develop, and could therefore lead to improved cancer treatments.
They are less likely to become cancerous.
There is a great deal of research into using adult stem cells to treat diseases. They could reduce the need for organ transplants, which have long waiting lists.

11. How could adult stem cells be used?
Teacher notes
Bone marrow contains at least two types of stem cell population:
haematopoietic stem cells (HSCs): these naturally form all the types of blood cell in the body, and have shown to also differentiate into neurons and other brain cells, skeletal and cardiac muscle cells, and liver cells
stromal cells: these naturally form bone, cartilage, fat and connective tissue, and have been shown to also differentiate into skeletal and cardiac muscle cells.
An ICT extension activity to this guide would be for students to find out the latest developments in adult stem cell research and present their findings to the class or in small groups.