What are stem cells and where do they come from?
Stem cells are powerful cells found in both humans and other animals. They are the precursors of all cells in the human body, possessing the ability to replicate themselves and to repair and replace other tissues in the human body. Most cells in the human body have a specific identity and function that cannot be changed; they might be skin cells, muscle cells or some other kind of cell. Stem cells are undifferentiated, which means they do not yet have a fixed identity and function. They possess an ability to be manipulated in the laboratory such that they can turn into a number of different types of cells or tissues.
Stem cells differ according to their source and their malleability. 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 make blood cells and can be found in bone marrow or in cord blood from umbilical cords. Fetal stem cells are taken from discarded fetal tissue, and embryonic stem cells (ES cells) are derived from 5 day-old blastocysts – precursors to embryos.
There is a continuing debate over the properties possessed by adult human and ES cells. Research into adult stem cells continues to evaluate their ability to differentiate which may be more limited than ES cells. Another difference between specialized (adult) stem cells and ES cells is their behaviour in culture. It is easier to get ES cells to replicate in culture and to keep those cells alive for a very long time, an attribute that is very useful in creating cell lines to study diseases or cell functioning. Regardless of their differences, both avenues of research will likely yield valuable and different information for the development of clinical therapies.
Stem cells can be derived by reprogramming certain cells. Induced pluripotent stem cells (iPS) are created from adult (non-pluripotent) cells – often skin cells. A cloning technique called somatic cell nuclear transfer (SCNT) can be used to create embryos as a source of stem cells. SCNT or research cloning is controversial as it may require a large supply of human eggs but particularly as the technique can be used for reproductive cloning as well as research cloning. Considerable disagreement about the permissibility of SCNT exists in the international community as evidenced by attempts to pass a convention on human cloning at the United Nations.