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 cells.

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

Tissue stem cells (also sometimes called adult stem cells)
These stem cells are derived from, or resident in, a fetal or adult tissue.  Usually they can 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 blastocyst, and is no bigger than a grain of sand. Embryonic stem cells thus obtained are said to be pluripotent – they are able to form all the different types of cell in the body, including germ cells.
Fact sheet on embryonic stem cells

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

See our fact sheet on reprogramming for more information on iPS cells.