Interpreting stem-cell research

In general terms, a stem cell is any cell that can multiply indefinitely and has the capacity to differentiate into more than one cell type. A stem-cell line is a group of stem cells in laboratory conditions (i.e., in vitro), continually producing new cells of the same type. Stem cells can be derived from embryos, from adult cells, from umbilical cords that are discarded after babies are born, and from human placentas. Under special laboratory conditions, stem cells can be encouraged to produce a variety of human cell types, including nerve cells, liver cells, and heart cells.

To more accurately interpret media references to stem-cell research ask the following questions:

What animal is involved? Research conducted with mice often provides insight into human systems and the research has fewer moral and regulatory issues attached. Because of current limitations in the U.S., federally-funded studies on embryonic stem cells can only be conducted with non-human animals or existing stem-cell lines. There are no limitations on privately-funded studies other than the restriction that cloning cannot be used as a source of stem cells. Research is also being conducted with tissues cultured from humans or other organisms.

Where is the work being done? Much of the current human embryonic stem-cell research is being conducted outside of the U.S. Inside the U.S., work on non-adult stem cells is limited to the few institutions who have, or can obtain, existing embryonic stem-cell lines. There are currently no U.S. limitations on non-embryonic stem-cell research, other than those already in place for any human clinical study.

What was the source of the cells?

Non-embryonic, or adult, stem cells are relatively undifferentiated cells removed from an adult and allowed to multiply in vitro. Bone marrow cells, grown in vitro before being injected into a recipient, are one type of stem cell. Non-embryonic stem cells have been tested in the treatment of cancer, Crohn's disease, and multiple sclerosis.

Embryonic stem cells are cells harvested from a special part of a late blastocyst --a layered ball of cells resulting from multiplication of a fertilized egg. Under special conditions, embryonic stem cells can develop in vitro into any tissue of the body. Muscle, nerve, and heart (cardiac) tissues have been grown this way. While the entire blastocyst, under proper conditions, can develop into a human being, embryonic stem cells cannot.

Umbilical cord blood contains neonatal stem cells. These cells are capable of differentiating into various cell types; such as hematopoietic cells (blood and immune system-forming cells), mesenchymal cells (muscle, cartilage, bone and fat cells), and neural cells (brain and central nervous system cells). These properties of cord blood stem cells are similar to those shown by embryonic stem cells. However, unlike embryonic stem cells, umbilical cord blood stem cells are abundant, easily collected and are non-controversial.

Who was the source of the genetic material? Non-embryonic stem cells can come from the same person who later receives the injected stem cells (autologous transplant), a closely related person (allogenic transplant), or an immunologically matched but unrelated person (Matched Unrelated Donor or MUD transplant). Syngenic bone marrow transplants are performed from one identical twin to the other. Prior to receiving the newly grown cells, the recipient is treated to decrease their own immune system's cells to as few as possible. After receiving transplanted cells, the recipient may be given drugs to prevent rejection of the new cells and possible graft-versus-host disease. Once begun, anti-rejection drugs must be continued throughout the recipients life. Embryonic stem cells can be harvested from an embryo that has developed to the blastocyst stage (14 days and about 150 cells) from a naturally-fertilized egg, an artificially-fertilized egg, or an egg whose genetic material was replaced by genetic material from another cell. Genetic material from the intended stem-cell recipient can be used to replace genetic material of a fertilized egg.

Stem cells can also be identified by the terms pluripotent, totipotent, and multipotent:

Totipotent stem cells form when a fertilized egg first divides. Totipotent stem cells can develop into a complete individual.

Pluripotent stem cells are also called embryonic stem cells. A few days after a fertilized egg divides, the totipotent stem cells form a blastocyst, or a ball of cells. The inner layer of this blastocyst contains pluripotent stem cells, which are capable of developing into any tissue in the body. Pluripotent stem cells cannot, however, become a complete individual.

Multipotent stem cells are sometimes called somatic or adult stem cells. These stem cells are found in mature tissue and are maintained by the body to replace worn out cells in tissues and organs. Stem cells from bone marrow, called hematopoietic stem cells, form the various kinds of blood cells. Neural stem cells form the brain and central nervous system. Mesenchymal stem cells form fat, bone, muscle and cartilage.

Marie Godfrey, PhD

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