What is a stem cell?

There are three types of stem cell: embryonic, fetal and adult. Each of these three types of stem cell can turn into a different range of cell types. All have the ability to renew themselves.

Embryonic stem cells

Picture of a five day old embryo: about 100 cells

Embryonic stem cells are extracted from a five-day-old embryo.

Every organ and tissue in your body: lung, heart, nerves and so on, started out as a stem cell in the very early embryo. At around five days after conception, your embryo consisted of a bundle of about 30 stem cells with the ability to turn into any cell in your body.

We call these cells embryonic stem cells. These cells give rise to the wide variety of tissues and organs found in the human body.

Use of these cells is controversial because the embryos from which they come have the ability to form a complete human being.

Fetal stem cells

Genes have instructed this eight-week-old embryo where to put her head, arms and legs.

After eight weeks of development the embryo is called a fetus.

By this time, many of the embryonic stem cells will have become irreversibly specialised into particular cell types such as liver cells. However, the fetus still has some stem cells and these are called fetal stem cells. Their ability to turn into many different cells is less than that of embryonic stem cells, but greater than that of adult stem cells. However, scientists are still uncertain about all the differences between embryonic, fetal and adult stem cells.

Adult stem cells

Picture of adult stem cells in the jaw bone

Adult stem cells in the jaw bone.

Even when fully grown, your body contains stem cells which are a source of new tissue throughout your life. Most organs and tissues in the human body have their own supply of these tissue stem cells to use when needed. Adult stem cells can only turn into the cells that you find in the tissue in which they are found e.g. the adult liver has liver stem cells which can make more liver and bone marrow has bone marrow stem cells that can make new white blood cells.