Stem Cell Sources

Stem cells differ according to their source and their malleability. Just as there are many different types of specialized or differentiated  cells in the body, there are many different types of stem cells in the body. Within the human body, adult stem cells continue to replenish cells that need replacement from normal wear and tear. Adult stem cells can be found in specific tissues in the body and include neural stem cells, skin stem cells, and blood (hematopoietic) stem cells. Hematopoietic stem cells can be found in adult bone marrow and blood and umbilical cord blood. These stem cells are regularly used in standard therapies, as they make new blood cells. Adult bone marrow stem cells are used in repopulating the bone marrow and white blood cells (leukocytes) of patients suffering from leukemia. Peripheral blood stem cells can be collected from circulating blood and used to treat leukemia, other cancers and blood disorders.  A particularly promising source of stem cells for treating the same type of disorder is from umbilical cord blood. After a baby is born the
umbilical cord is routinely discarded. In recent years, the blood in the cord has been found to be a rich source of stem cells that are less prone to rejection from a transplant recipient’s body than cells or tissues transplanted from another individual.

Additional sources of stem cells include fetal stem cells that are derived from discarded fetal tissue, and human embryonic stem cells (hES cells) that are derived from 5 day-old blastocysts – precursors to embryos. A blastocyst is a sphere of cells with an inner cell mass of about 30-34  undifferentiated cells that have the potential to form all the tissues in the human body. hES cells are found in the inner cell mass of the blastocyst. The isolation and removal of those hES cells from the blastocyst necessarily makes it unsuitable for transfer into a woman. In other words, the removal of the hES cells from a blastocyst compromises the ability of that blastocyst to ever become an embryo, and hence its potential to develop into a baby were it implanted in a woman and born alive. It is primarily this destruction of the human blastocyst that causes hES research to be controversial.