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A stem cell is a cell that has the ability to self replicate for indefinite
periods of time, while at the same time, can differntiate into various
types of other cells. Stem cells can be divided into three broad categories:
(3)
- Totipotent - A totipotent stem cell is essentially a fertilized
egg (zygote). This type of cells can differentiate into any type of
cell in the body.
- Pluripotent - These stem cells come from the inner cell mass
of a blastocyst. Pluripotent stem cells can give rise to all cells
that are derived from any of the three germ layers (ectoderm, mesoderm,
and endoderm)
- Unipotent - These stem cells are found in currently differentiated
tissue in an adult organism. These cells are present for the lifetime
of the organism. These cells are derived from areas such as: bone
marrow, the bloodstream, cornea/retina of the eye, the dental pulp
of the teeth, liver, skin, the gastrointestinal tract, skeletal muscle,
and the pancreas. Unipotent stem cells are usually referred to as
adult stem cells.
(Take a look at a broad overview
of basic human tissue differentiation (3)) |