In 1955, Rudolf Virchow, a German physiologist, discovered that cells come from preexisting cells in an organism. These types of cells were later to be called stem cells. Stem cells are cells that keep the ability to divide in the human body. These cells divide repeatedly, which allow for continual growth, renewal, and repair of damaged tissue. During division, stem cells divide into either two daughter stem cells or one daughter stem cell and a progenerator cell. A progenerator cell is a semi-specialized cell that is in-between a stem cell and a fully differentiated cell. Growth factors and other biochemicals around the cell help influence the full differentiation of the cell. Cells in the early embryo are referred to as totipotent, which means that the embryonic cells can differentiate into any cell type. Stem cells in the late embryo, progenerator cells, and stem cells in the adult are referred to as pluripotent, which means that the cell can become several cell types, but not all cell types. There are two types of stem cells being researched presently. These cell types are referred to as embryonic stem cells and adult stem cells.
Embryonic Stem Cells
Stem cells can be found in embryos. It is preferred that the stem cell be harvested from the inner cell mass of the five day embryo. This is preferred because there is less risk of rejection from the immune system of the recipient. There are two sources of embryonic stem cells. There are extra blastocysts frozen in in-vitro clinics. There is also a procedure called stem cell nuclear transfer. In stem cell nuclear transfer, scientists take the nucleus from a patient that needs new cells and inject it into a donor egg without its nucleus. The scientists let the embryo divide for five days and give it growth factors and other biochemicals to help the cells differentiate into cell that the patient needs. There are many ethical dilemmas with using human embryos for this type of therapy. In the summer of 2009, the first clinical trial will take place using embryonic stem cells for treatment of spinal cord injuries.
Adult Stem Cells
Adult stem cells are referred to as somatic stem cells. These stem cells are undifferentiated cells found after birth. They have the characteristics of self-renewal and multipotency. Multipotency means that they are able to differentiate into several cell types. Some examples of adult stem cells are hematopoietic stem cells, mammary stem cells, olfactory adult stem cells, and testicular cells. Adult stem cells are used in stem cell treatments and do not have the ethical dilemmas associated with embryonic stem cells.
"Adult Stem Cells." Wikipedia. 10 Oct. 2009. < http://en.wikipedia.org/wiki/Adult_stem_cell>.
Sheir, David, Jackie Butler, and Ricki Lewis. Hole's Human Anatomy and Physiology. 11th. Boston: McGraw-Hill, 2007. Print.