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Stem cells are cells in animals that can continuously undergo cell division. There are two types of stem cell:

• embryonic stem cells
• tissue (adult) stem cells

Embryonic stem cells

These are found in an early embryo. These cells are able to divide continuously.

Embryonic stem cells are pluripotent. This means that all of the genes in an embryonic stem cell can be expressed so they can give rise to all types of cell. They are important for growth.

Some people find the use of embryonic stem cells ethically unacceptable because it involves the destruction of an early embryo.

Tissue (adult) stem cells

These are found in a mature organism. Their function is to replenish all of the types of (differentiated) cells found in that specific tissue. Tissue stem cells are multipotent. This means that they can differentiate into any of the types of cell of the same tissue type.

For example, blood stem cells found in bone marrow can differentiate into red blood cells, platelets, phagocytes and lymphocytes.

Stem cells have already been used therapeutically to repair damaged or diseased organs, for example bone marrow transplants, corneal transplants, and skin grafts for burns.

Stem cells have also been used as model cells to study how diseases develop or for drug testing.