When egg meets sperm and fertilisation occurs, an amazing pattern of development starts in order to create a new person. That fertilised cell goes on to divide many, many times to create the billions of cells that make up you and I. They can also become any one of the many different tissues that make up the body. These early cells can become nerve cells, muscle cells, bone cells, liver cells, kidney cells and so on. These multi-talented cells are called stem cells.

In the normal bone marrow there are very similar stem cells. There are only a few of them, maybe 1 in every 10,000 marrow cells, and they look very ordinary, but they are very important. They can divide many, many times to make all the millions of new blood cells we need each day. They can also replace themselves so that we never run out of them. These stem cells can develop into any of the different sorts of blood cell that we have - red cells, the different types of white cells, or platelets.

Stem cells are very resistant to chemotherapy, so when other cells in the bone marrow have been killed off, including leukaemic ones, they will survive to replace all the normal blood cells which have been lost.

During a bone marrow transplant, a very small amount of bone marrow or blood is given back to the patient. This blood or bone marrow is packed with these stem cells and they will home in on the bone marrow and quickly multiply. Here they develop into all the normal blood cells that are needed so that, over a couple of weeks, the blood count slowly returns back to normal.

Stem cells are causing a great deal of excitement to doctors and scientists because they may offer a cure for a whole range of diseases such as Alzheimer's, heart disease, diabetes, or stroke, by developing and replacing the damaged tissue. So they are the subject of a great deal of research at the present time.