What Is a Stem Cell?
Stem cells are unspecialized cells that can self-renew (or "proliferate") for extended periods of time without differentiating. They exhibit a stable, normal chromosome complement. Stem cells cannot perform any specialized functions but have the potential to give rise to cells with specialized functions (a process known as "differentiation"), such as pulsating heart muscle cells or defensive immune cells.
A fertilized egg is said to be "totipotent" because it has the potential to generate all cell types and tissues that make up an organism. Embryonic stem cells, derived from the inner cell mass of the blastocyst stage of development (pre-implantation, ~5-6 days old) have the potential to generate cell types and tissues from all three primary germ layers of the body (pluripotent). Adult stem cells (somatic stem cells) are found in tissues such as brain, bone marrow, skin, liver, skeletal muscle and peripheral blood vessels. It is suggested that some of these cells may be able to differentiate into multiple cell types (called "plasticity" or "trans-differentiation"). For example, brain stem cells may be able to generate blood and skeletal muscle cells. However, stem cells in adult tissues do not appear to have the same capacity or potential to differentiate as embryonic stem cells do. It may be that adult stem cells in many differentiated tissues are typically unipotent (capable of only one lineage).
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