The most primitive of these is the pluripotent stem cell that is believed to be the origin of all blood cells. Pluripotent stem cells differ from other blood cells in that they are capable both of unlimited self-renewal and differentiation. Self-renewal means the cell is able to reproduce another cell identical to itself, thus maintaining a steady number of these types of cells in the body. Differentiation means the cell is capable of generating one or more subsets of more mature cells that eventually evolve into either erythrocytes, neutrophils, eosinophils, basophils, lymphocytes, monocytes or platelets. When physicians harvest bone marrow for use in a transplant, it is the stem cells they are seeking.
Stem cells resemble medium sized white blood cells.
It has been estimated that less than one in 100,000 cells in the bone marrow are stem cells. These committed progenitors produce colonies of cells that eventually mature into red blood cells, white blood cells, or platelets. The process used to extract stem cells from the bloodstream is similar to the process used to collect platelets from platelet donors. A needle is inserted in each arm and blood is withdrawn from one arm and circulated through the machine to extract the stem cells. The remaining cells are returned to the patient through a needle in the opposite arm.