Stem cells
Stem cells are cells in animals that can continuously undergo cell division. There are two types of stem cell:
Embryonic stem cells - These are found in an early embryo. These cells are able to divide continuously. All 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 differentiated cells that need to be replaced. Tissue stem cells are called multipotent. This means they can only give rise to the limited range of cells of their particular tissue types.
For example, red bone marrow stem cells produce various blood cell types.
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.