The identification of stem cells as "embryonic" and "adult" refer to their origin.
Embryonic Stem Cells are found within the human embryo at the blastocyst stage (7-14 days after fertilization). They are pluripotent stem cells. These embryonic stem cells are amazing because they will multiply and differentiate to make up every cell of the human body, from those in the blood to those in the bone. It is truly a testament to the beauty of life that these simple looking cells can create the entire structure of the human body.
Adult stem cells can be found in many places within the human body. It can be found in bone marrow, umbilical cord blood, placenta, brain, skeletal muscle, hair follicales, fat, and many others places. Up until recently, adult stem cells were always referred to as multipotent stem cells, that is, they could only differentiate into the tissue cells of their origin tissue. Because of this, proponents of embryonic stem cell research look down upon adult stem cells because of their "limited" characteristics. However, as recent studies have confirmed, scientists have begun to unlocked the pluripotency of adult stem cells.
These adult stem cells are isolated from the patient, multiplied in the lab, directed towards the cell needed, and inserted at the place where tissue needs to be repaired or replaced. Because these stem cells are original to the host, there is no rejection by the immune system. Because they are retrieved from a certain type of tissue, there is no threat of multiplication in teratomas.
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