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	Cultured Stem Cells and Bone Marrow Transplantation - As stem cells are undifferentiated cells, which have the potential of differentiating into several kinds of cells. Stem cells can also be of different kinds. For the purpose of bone marrow transplantation, the stem cells used are called 'hematopoietic stem cells'. These stem cells first appear in human embryo in the yolk sac and then migrate to the liver as the fetus develops. In the fetus, the blood cells are created in the liver, but soon after birth they are produced exclusively in bone marrow. These stem cells can be isolated, stored, and are also cultured rarely. They are the, main ingredient of bone marrow transplants.
If marrow is transplanted to the same individual from whom it was taken for culture, it is described as autotransplantation and if donated to someone else, it is called allogeneic transplantation. The hematopoietic stem cells produce, in one hour, 3-10 billion platelets, red cells, neutrophils, lymphocytes and even macrophages of the immune system. These cells are used for transplantation in patients, whose immune and blood forming systems have been devastated by leukemia, cancer, chemotherapy, or other unknown causes. Isolation of stem cells from the mass of cells in the bone marrow has been achieved through the use of monoclonal antibodies which react to a cell marker or antigen called CD34, which is specific for primitive bone marrow stem cells.
However, all CD34 positive cells may not be the pluripotent stem cells required for bone marrow transplantation. Therefore, more specialized technique may be used for further screening of CD34 positive cells. Culture systems are also available, which allow multiplication of these cells. These cultures may also be used in future for transplant therapy. Several hormones have also been identified, which stimulate replication of precursor blood cells. These include granulocyte colony-stimulating factor (G- CSF), interleukin-6, interleukill-11 and a stem cell factor.
Bone marrow donors are classified by a method called HLA typing, which is used for matching the recipient However, for autologous transplantation, a patient's own stem cells are collected, frozen and stored outside the body, while chemotherapy or radiation therapy is administered to remove the malignant cells. The bone marrow thus stored is returned directly into a vein. However sometimes bone marrow may be invaded by cover cells, so that healthy cells will have to be separated from malignant stem cells. Monoclonal antibodies linked with toxins can be used for killing the malignant cells.
	Chemotherapy with extracted bone marrow can also be used for destroying malignant stem cells. It has also been shown that whereas the malignant cells have an advantage over normal cells for growth within the body, they have a relative disadvantage outside the body. In view of this, long term culture systems have been used to remove com so us cells from bone marrow. In general 200 X 106 bone marrow cells are needed for 15 kilogram of body weight. This requires multiple extractions involving 500 - 1000 milliliters of bone marrow, for one patient. (One extraction gives 10-15 ml of bone marrow). Administering hormones like CSF and G - CSF, helps, since it stimulates blood production. Adding stem cells from peripheral blood to bone marrow may further help engraftment. In future, it may be possible for a healthy person to store his own stem cells permanently and use them whenever required.

This will become simpler, if semi cells can be grown and multiplied in culture. Umbilical cord blood (which is normally discarded after birth) may become an important source of stem cells and may be stored for subsequent use during a medical crisis.