STEM CELL -
Noun: An embryonic or primitive cell that gives rise to specialised cells. The stem cells that exist within the bone marrow produce about 95% of the various blood cells. Stem cells in the bone marrow produce about 3 million red blood cells and 120,000 white cells every second. Stem cells are also found in the circulating blood where they perform other functions. In a healthy human being, the number of each type of stem cell and their offspring is contained within very narrow limits. Certain proteins, such as interleukins and colony-stimulating factors, (see G-CSF ), play a key role in determining whether a stem cell will replicate itself, produce offspring that evolve into mature blood cells, do both or do neither at any given time. If this regulatory mechanism breaks down, too many or too few stem cells will be present in the bone marrow. See also diagram at Haematopoiesis .

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