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.