Human umbilical cord blood stem cells present an exciting field of study. They were first used in 1988 to treat a genetic form of anemia, and have since been used in the treatment of various types of blood cancers such as leukemia.
The issue of using stem cells in medical research has been a controversial one, mostly because of religious, moral and ethical objections to the destruction of human embryos that was once thought necessary to obtain them. Researchers have long known that the cells of an embryo at the blastocyst stage--three to five days after fertilization--can develop into any other cell type. Embryonic stem cells are "harvested" at that stage and then grown in laboratory cultures. The process of harvesting the cells destroys the embryo, which is the source of the controversy. The first human embryonic stem cell lines were created in the early 1980s. In theory, stem cells can be made to differentiate into any cell type by controlling the conditions of their culture. In reality, only a few cell types have so far been created from embryonic cells. Not enough is yet known about the mechanisms that regulate differentiation to allow precise control of the growing cells.
Researchers have also long known that adult human bone marrow contains undifferentiated cells which develop into the various types of blood cells. These are called hematopoietic stem cells, and they have been used since the early 1970s in bone marrow transplants. In response to the objections to embryonic stem cells, researchers have begun investigating other sources, and have found that hematopoietic stem cells can be made to become other cell types than just blood. They have also found that stem cells exist in most body tissues of all adult mammals. These are called mesenchymal or somatic stem cells, and are what the body uses to regenerate and repair damaged tissue after an injury or illness. Mesenchymal stem cells are slightly more differentiated than embryonic stem cells, but can be made to differentiate into various cell types. Adult stem cells, however, carry the same tissue-typing restrictions as organ transplantation, and can be rejected by the recipient.
An in-between solution has been developed: stem cells from umbilical cord blood. The hematopoietic stem cells in cord blood are very similar to embryonic cells, do not yet carry tissue antigen markers as adult cells do, and are derived from a limitless source that would otherwise be medical waste. As long as they are donated with informed consent, there is no moral or ethical objection because no life is harmed. In many research hospitals, new parents are now routinely asked to donate their child's umbilical cord blood cells, and there are cord blood cell storage banks in most developed countries.
All stem cells, regardless of source, share three essential characteristics:
1) They are able to reproduce themselves indefinitely. 2) They are undifferentiated. 3) They are capable of differentiating into multiple cell types.
Research on cord blood has shown that in addition to hematopoietic stem cells, mesenchymal stem cells are also present. So far, nerve cells, endocrine cells, and bone and connective tissue cells have been successfully differentiated from cord blood stem cells. This work offers great promise for treatment of diabetes, neurological disorders, and wound healing. Because cord blood cells are already somewhat differentiated, their cultures can be controlled more predictably than embryonic cells. Because they do not yet have tissue antigen markers, they do not carry the risk of rejection posed by adult stem cells.
Although there are still more questions than answers with any type of stem cell, researchers hope that further work will lead to more effective treatments for many different diseases.