Stem cells are the body’s natural reservoir – replenishing stocks of specialized cells that have been used up or damaged. We all have stem cells at work inside us. Right now, inside your bone marrow, stem cells are busy making the 100,000 million new blood cells you need every single day!

We need to make new cells all the time, just to keep our body functioning. Some specialized cells, such as blood and muscle cells, are unable to make copies of themselves through cell division. Instead they are replenished from populations of stem cells.

Stem cells have the unique ability to produce both copies of themselves and other more specialized cell types every time they divide. Stem cells, therefore, are essential to the maintenance of tissues such as blood, skin, and gut that undergo continuous turnover (cell replacement), and muscle, which can be built up according to the body's needs and is often damaged during physical exertion.

Stem cells are unspecialized. Unlike a red blood cell, which carries oxygen through the blood stream, or a muscle cell that works with other cells to produce movement, a stem cell does not have any specialized physiological properties.

Stem cells can divide and produce identical copies of themselves over and over again. This process is called self-renewal and continues throughout the life of the organism. Self-renewal is the defining property of stem cells. Specialized cells such as blood and muscle do not normally replicate themselves, which means that when they are seriously damaged by disease or injury, they cannot replace themselves.

Stem cells can also divide and produce more specialized cell types. This process is called differentiationⁱ. Stem cells from different tissues, and from different stages of development, vary in the number and types of cells that they can give rise to. According to the classical view, as an organism develops, the potential of a stem cell to produce any cell type in the body is gradually restricted.

Stem cells are found in the early embryo, the fetus, placenta, umbilical cord, and in many different tissues of the body. Recently, stem cells have also been engineered from somatic cellsⁱ.

Stem cells are often divided into two groups: adult or tissue stem cells and embryonic stem cellsⁱ.

Tissue stem cells (also sometimes called adult stem cells)
These stem cells are derived from, or resident in, a fetal or adult tissue, and can usually only give rise to the cells of that tissue. In some tissues, these cells sustain turnover and repair throughout life. For example, stem cells that are found in the skin will give rise to new skin cells, ensuring that old or damaged skin cells are replenished.

Embryonic stem cells
Cells derived from a small group of cells (called the inner cell mass) within the very early embryo. Human embryonic stem cells are obtained from embryos that are 5-6 days old. At the stage that embryonic stem cells are derived, the embryo is called a blastocystⁱ, and is no bigger than a grain of sand. Embryonic stem cells thus obtained are said to be pluripotentⁱ – they are able to form all the different types of cell in the body, including germ cellsⁱ.

Recently, a third type of stem cell, with properties similar to embryonic stem cells, has emerged. Scientists have engineered these induced pluripotent stem cellsⁱ (iPS cells) by manipulating the expression of certain genes - 'reprogramming' somatic cells back to a pluripotent state.

Stem cells can be used to study development
Stem cells may help us understand how a complex organism develops from a fertilised egg. In the laboratory, scientists can follow stem cells as they divide and become increasingly specialized, making skin, bone, brain, and other cell types. Identifying the signals and mechanisms that determine whether a stem cell chooses to carry on replicating itself or differentiate into a specialized cell type, and into which cell type, will help us understand what controls normal development.

Some of the most serious medical conditions, such as cancer and birth defects, are due to abnormal cell division and differentiationⁱ. A better understanding of the genetic and molecular controls of these processes may yield information about how such diseases arise and suggest new strategies for therapy. This is an important goal of stem cell research - this poster, by EuroStemCell scientist Fiona Watt, gives an overview of what scientists have discovered so far (December 2006).

Stem cells have the ability to replace damaged cells and treat disease
This property is already used in the treatment of extensive burns, and to restore the blood system in patients with leukaemia and other blood disorders.

Stem cells may also hold the key to replacing cells lost in many other devastating diseases for which there are currently no sustainable cures. Today, donated tissues and organs are often used to replace damaged tissue, but the need for transplantable tissues and organs far outweighs the available supply. Stem cells, if they can be directed to differentiate into specific cell types, offer the possibility of a renewable source of replacement cells and tissues to treat diseases including Parkinson's, stroke, heart disease and diabetes. This prospect is an exciting one, but significant technical hurdles remain that will only be overcome through years of intensive research.

Stem cells could be used to study disease
In many cases it is difficult to obtain the cells that are damaged in a disease, and to study them in detail. Stem cells, either carrying the disease gene or engineered to contain disease genes, offer a viable alternative. Scientists could use stem cells to model disease processes in the laboratory, and better understand what goes wrong.

Stem cells could provide a resource for testing new medical treatments
New medications could be tested for safety on specialized cells generated in large numbers from stem cell lines – reducing the need for animal testing. Other kinds of cell lines are already used in this way. Cancer cell lines, for example, are used to screen potential anti-tumour drugs.