several of which have specialized roles in the blood.

In the early 1960s it was shown that if bone marrow was taken from one mouse and transplanted into another,

rare and unspecialized cells could travel to the spleen where they would divide and form small colonies

These colonies contained a mixture of different blood cell types.

Incredibly when these colonies were removed and retransplanted,

these rare cells formed new colonies in the spleen of a second mouse.

These reveal two defining properties of a stem cell.

First a stem cell must be able to self renew. That is, it must be able to divide and give rise to more cells like itself.

Second, it must be able to differentiate. In other words it must be able to divide and give rise to specialized cells.

Since these early experiments, many different types of stem cells have been discovered

And this general definition applies to all of them.

Following fertilization, the developing embryo contains a group of cells

known as the "inner cell mass" that will eventually go on to form all tissues in the body.

However at the early stage of development these cells have not yet committed to become a specific type of cell

and are said to be in an undifferentiated state.

If the cells of the "inner cell mass" are isolated

they can grow indefinitely in a plastic dish and maintain this undifferentiated state.

These cells are known as "embryonic stem cells" or ES cells

Like cells in the developing embryo

ES cells retain the ability to form any type of cell in the adult body

Thus ES cells are said to be "pluripotent"

By studying the signals required to control how ES cells develop into specific cell types

scientists may be able to develop therapies capable of restoring damaged tissues

Most of the cells in our bodies are not stem cells.

For example blood and skin cells have become specialized to perform a specific function in the body.

By a process known as differentiation, that is normally not reversible.

However, if some adults cells are taken grown in plastic dishes and given specific genetic instructions over time,

a small number of these cells will reverse from their differentiated state and develop the ability to redifferentiate into any cell type in the body.

This newfound ability is known as "pluripotentcy", and therefore these cells are known as "induced pluripotent stem cells" or IPS cells.

The creation of IPS cells from individual patients may one day allows us to develop replacement tissues that are a perfect genetic match to the patient.

In addition, the creation of IPS cells from patients with specific diseases will allow scientists to develop new drugs to treat a wide variety of disorders.