Posted by: All About MS | September 18, 2009

Stem Cells – what they are

Stem Cells

 

 

Every tissue and organ in the human body is made up of different types of cells. Cells that make up skin, for example, are different from those that make up the heart. This makes it impossible for cells that make up one tissue or organ to be transferred to another tissue or organ.

However, all cells share one thing in common – they come from one cell source. In the early stages of human development, these cells can become any tissue or organ – that is, they have not yet become specialized. These cells are called stem cells.

Stem cells have two important characteristics that make them different from other types of cells. First, as noted above, all stem cells are unspecialized, and renew themselves for long periods of time through cell division. Second, under certain biochemical cues they can be made to differentiate (see below). This means that they can divide into cells with special functions, such as the beating cells of the heart muscle or the insulin-producing cells of the pancreas.

 

Types of stem cells

Stem cells come in three forms: embryonic stem (ES) cells, embryonic germ cells, and adult stem cells. ES cells come from embryos; embryonic germ cells come from testes, and adult stem cells can come from bone marrow. Scientists work mostly with ES cells and adult stem cells.

ES cells are found in the early beginnings of human life – at the blastocyst stage of human development. This stage is from four to five days after the union of the sperm and the egg, before the embryo implants in the uterus. The 20 or so stem cells in the blastocyst are called pluripotent, which means they are capable of forming all embryonic tissues, but they cannot form a complete organism without support from the placenta.

ES cells come from embryos, fetuses, and some can come from the blood of the umbilical cord after birth.

 

Differentiation

The process by which the stem cell can become a specific cell type is called differentiation. Stem cell differentiation begins when they are exposed to certain biochemical cues – either physiological or experimental. Biochemical cues in different parts of the body stimulate stem cells to grow into the specific cells needed in that location.

 

All stem cells have the capacity to differentiate, but to different degrees.

Totipotent stem cells can become any cell in the human body;

Pluripotent stem cells can become almost any cell in the human body, but they cannot become placental tissue needed for development in the human uterus; and

Multipotent stem cells can become only a certain type of cell, such as a blood cell.

Adult stem cells are found in the fetus, child, and adult. These “adult” stem cells are found in many human tissues, such as blood, brain, intestine, skin, and muscle. They are responsible for the repair and regeneration in the body.

It was long thought that adult stem cells had less flexibility than ES cells, and that they could normally form only cell types the same as the tissue of origin. However, recent discoveries are pointing to new sources of stem cells within the adult body. Research into adult stem cells has the potential to eliminate ethical concerns about experimentation or transplantation of ES cells. The use of adult stem cells would also reduce the chance of transplant rejection because patients could receive transplants of their own stem cells.

The possibility that adult stem cells also have a greater “plasticity” than previously believed has resulted in new experimentation. For example, scientists now believe that certain types of adult stem cells can develop into cells of another tissue (for example human blood stem cells have been shown to differentiate into liver cells if the conditions are right). However, no adult stem cell has been definitively shown to be completely pluripotent.

Some scientists are now calling adult stem cells “somatic stem cells”. Unlike ES cells, which are defined by where they originated (in the inner cell mass of the blastocyst), the origin of adult stem cells in mature tissues is unknown.

Some current research

Research into adult stem cells has caused a lot of excitement. Because adult stem cells have been found in many more tissues than originally thought possible, scientists have asked whether adult stem cells could be used for transplants. If the differentiation of adult stem cells can be controlled in the laboratory, these cells may become the basis of therapies for many serious common diseases.

Some diseases have already been targeted. For example, adult blood stem cells have been used to treat hematologic (blood) cancers. Adult blood-forming stem cells from bone marrow have been used in transplants for over 30 years. The stem cells of the matched donor are purified, and the patient’s bone marrow is then destroyed by radiation and reconstituted with the stem cell graft.

Several groups have been working on animal models in which pluripotent stem cells are grafted into damaged hearts. The stem cells were shown to “beat” with the surrounding heart cells. Several animal and early human trials are also underway to use pluripotent or adult stem cells to repair damage to the nervous system, such as in spinal cord injury, Parkinson’s disease, and Alzheimer’s disease.

While large numbers of ES cells can be grown in a laboratory, adult stem cells are rare in mature tissues and a way to increase their numbers in cell culture has not yet been developed. Much research is ongoing in this area, as large numbers of cells are needed for stem cell replacement therapies.

Potential applications of human stem cells

One of the goals of scientists is to control cell differentiation. This would allow them to create any tissue or organ in the body from a single stem cell. This research involves many biotechnology applications, such as the study of stem cell genetics, biological factors (normally occurring proteins that the body needs to function normally), receptors on the stem cells and stem cell physiology. Researchers are encouraging adult stem cells, such as those from the skin, to become other types of tissue, such as nerve or muscle. Of all the adult stem cells identified so far, hematopoeitic stem cells (a stem cell from which all red and white blood cells develop) have been the most studied.

Scientists are using genetic modification to expand the potential therapeutic applications of stem cells. Stem cells can be modified to produce enzymes or factors, such as insulin, before being transplanted into the body. Stem cells can also be modified to resist certain infections. For example, studies are underway to create stem cells resistant to HIV. Once implanted, these stem cells would repopulate the diseased immune system of AIDS patients with cells resistant to the disease.

However, the main application of stem cells is still to replace damaged, diseased or dead cells. Once implanted, a stem cell can differentiate into the correct cell type, and form natural connections with the surrounding tissue, which is very important in neurodegenerative diseases such as Parkinson’s and Alzheimer’s.

Stem cells may also be useful in tissue-engineering applications, such as the production of complete organs, including heart, liver, kidneys, eyes or even parts of the brain.

Stem cells represent a potentially unlimited source of experimental tissue, which would permit research into other areas. For example, researchers could test stem cells derived from culture in the laboratory for the effects of known environmental hazards and genetic mutations on the relevant human tissue. This would provide more applicable information than that obtained in animal models.

 

 

Stem Cell Transplant

A stem cell transplant is used to increase the chance of a cure or remission for a number of cancers and blood disorders. It usually involves intense chemotherapy followed by an infusion of stem cells. The treatment requires close nursing and medical care for a number of weeks. It can be a gruelling treatment and there are risks. Your specialist can advise when the likely benefits of this procedure can outweigh the risks.

What is a stem cell transplant?

A stem cell transplant may be used so that you can have intensive high dose chemotherapy (and sometimes radiotherapy) to kill cancerous cells. The chemotherapy is higher than conventional chemotherapy and also kills the stem cells in the bone marrow that would normally make blood cells. Therefore, following the chemotherapy, you are given back (transplanted) stem cells which can then make normal blood cells again.

A stem cell transplant is sometimes called a bone marrow transplant. However, stem cells can be obtained from blood as well as from the bone marrow. So, the term stem cell transplant is now used.

What is the bone marrow, and what are stem cells and blood cells?

Bone marrow

Blood cells are made in the bone marrow by stem cells. Bone marrow is the soft ‘spongy’ material in the centre of bones. Large flat bones such as the breast-bone (sternum) and pelvis contain the most bone marrow. To constantly make blood cells you need a healthy bone marrow. You also need nutrients from your diet including iron and some vitamins.

Stem cells

Stem cells are primitive (immature) cells. There are two main types in the bone marrow – myeloid and lymphoid stem cells. These derive from even more primitive common ‘pluripotent’ stem cells. Stem cells constantly divide and produce new cells. Some new cells remain as stem cells and others go through a series of maturing stages (‘precursor’ or ‘blast’ cells) before forming into mature blood cells.

Blood cells

Mature (fully formed) blood cells are released from the bone marrow into the bloodstream. Mature blood cells are:

  • Red cells (erythrocytes). These make blood a red colour. One drop of blood contains about five million red cells. Red cells contain a chemical called haemoglobin. This binds to oxygen, and takes oxygen from the lungs to all parts of the body.
  • White cells (leucocytes). The different types of white cells are called neutrophils (polymorphs), lymphocytes, eosinophils, monocytes, and basophils. They are part of the immune system. Their main role is to defend the body against infection.
  • Platelets. These are tiny and help the blood to clot if we cut ourselves.

Stem cells rapidly multiply to make millions of blood cells each day. Because of this they are more easily killed by chemotherapy than most other cells in the body. This is because chemotherapy drugs work by killing rapidly dividing cells (such as cancer cells).

When is a stem cell transplant used for treatment?

A stem cell transplant is an option which is considered for various cancer conditions. For example, for types of leukaemia, lymphoma and myeloma. Your specialist will advise when it may be an appropriate option. As a rule, it is not often a ‘first line’ treatment. Conventional chemotherapy or other treatments tend to be used first. However, the treatment of cancer and leukaemia is a changing and developing area of medicine. Techniques such as stem cell transplant continue to be refined and improved and may be considered in various different circumstances.

The higher doses of chemotherapy and radiotherapy that can be used in conjunction with a stem cell transplant can improve the chance of a cure for some conditions in certain circumstances.

A stem cell transplant is also used for some rare non-cancerous blood disorders.

Where are stem cells obtained from?

An autologous transplant

This means that the stem cells used for the transplant come from your own body. They are usually collected when you are free of any sign of disease (when you are ‘in remission’) following conventional chemotherapy or other treatments. The stem cells can be used soon after being collected. They can also be frozen, stored and used in the future if needed.

An autologous stem cell transplant is also called ‘stem cell support’ as the stem cells come from your own body. So, strictly speaking, it is not a transplant from a donor.

An allogenic transplant

This means the stem cells used for the transplant come from someone else – a donor. This is often a close relative such as a brother or sister where there is a good chance of a close ‘match’. Unrelated donors are sometimes matched to people needing a transplant.

Stem cells can be collected:

  • From the bone marrow. This involves a small operation to collect some marrow from the pelvic bone.
  • From the blood. Some stem cells occur in the blood (most are in the bone marrow). The stem cells in the blood can be collected (‘harvested’) by a machine called a cell separator. The bloodflow is diverted from a vein in the arm to pass through the machine which separates out the stem cells. The procedure takes about 4-6 hours. Drugs are given for a few days before this procedure to stimulate the body to make more stem cells in the bone marrow which ‘spill out’ into the blood.

How is a stem cell transplant given?

It is very similar to a blood transfusion. Following the intense course of chemotherapy (and sometimes radiotherapy), the solution containing stem cells is given into one of your veins via a drip. The stem cells travel through your bloodstream and end up your bone marrow. Here they start to make blood cells.

It can take several weeks for your bone marrow to recover, to take up the transplanted stem cells, and to make enough new blood cells. During this time you will need to be in hospital and be closely monitored. You may need several blood transfusions during this time until you are making enough blood cells. Antibiotics are given to minimise the risk of infection. Also, drugs are given to help stimulate the stem cells to multiply as quickly as possible.

What are the main risks of having a stem cell transplant?

There is a risk of serious problems with a stem cell transplant. For example:

  • Infection is the main risk. Following the intense chemotherapy, and before the time your bone marrow is working again, you have very low immunity. During this time you are at risk of serious and life-threatening infections. This is why antibiotics are given and you will be nursed away from other people until your bone marrow recovers. This can take several weeks.
  • Bleeding problems from the low level of platelets after the chemotherapy.
  • It you have a transplant from a donor, there is some risk that the ‘match’ will not be perfect, and the donor cells may react with your body’s cells. This is called ‘graft versus host disease’. This is not always serious but sometimes it can be.
  • Rarely, the transplanted stem cells fail to work.
  • There is a risk of short-term and long-term side-effects from intense chemotherapy (and/or radiotherapy).

Your specialist will discuss the risks and possible side-effects involved with a stem cell transplant.

 

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: