Stem cells are unique because they have the ability to generate new cells of any kind during early life and growth.
There are about 200 different types of cells in the body; such as muscle, skin and brain cells. Most of these cells have to be repaired or replaced once in a while. For example our taste buds are replaced every 10 days, skin cells every couple of weeks and liver cells every 300-500 days. Stem cells serve as an internal repair system dividing essentially without limit to replenish other cells. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.
Stem cells are distinguished from other cell types by two important characteristics. First, they are unspecialized cells capable of renewing themselves through cell division. Second, under experimental conditions, they can be induced to become tissue or organ specific cells with special functions. Unlike muscle cells, blood cells or nerve cells which do not normally replicate themselves; stem cells may replicate many times. After 6 months in a laboratory a starting population of stem cells can yeild millions of cells.
Stem cells are categorized by their potential to differentiate into other types of cells. The most potent is embryonic as they must become every type of cell in the body. Totipotent stem cells have the ability to differentiate into all possible cell typles, for example the zygote (fertilized egg) and the first few cells that result from the division. Pluripotent stem cells the ability to differentiate into almost all cell types, for example, embryonic stem cells. Multipotent stem cells the ability to differentiate into a closley related family of cells, for example, hematopoietic stem cells that can become red and white blood cells or platelets.
There are two main types of stem cells found in humans and animals, embryonic...
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