by Dan Roberts
(Links updated January 2007)
Background On The Research
Stem cells are undeveloped structures which are able develop into any of the nearly 220 cell types that make up the human body, and which can theoretically reproduce themselves infinitely. Recent discovery of adult stem cells, or progenitor cells, in the eyes of adult rodents has generated research in the area of retinal cell transplantation.
This discovery was reported at a meeting in 2000 of the Association for Research in Vision and Ophthalmology (ARVO) by Dr. Derek van der Kooy (University of Toronto) and Dr. Iqbal Ahmad (University of Nebraska). These researcher found that stem cells have certain characteristics of photoreceptor cells, but that further study is needed to determine if they can be fully functioning as such.
Dr. Michael Young (Schepens Eye Research Institute in Boston) added to this knowledge, reporting on his discovery that transplanted cells from a mouse retina were able to reproduce, and that some of them contained the photoreceptor-specific protein, rhodopsin, which initiates phototransduction (the process which turns light into an electrical signal in the brain). In addition, it was found that the transplanted neural progenitor cells are capable of responding to injury cues in the mature central nervous system by differentiating into cells that could take on the job of retinal neurons. The research was reported in the journal Molecular and Cellular Neuroscience (Academic Press, September 2000). In addition to Young, co-authors are Jasodhara Ray, Ph.D., of the Salk Institute for Biological Studies; Simon J.O. Whiteley, Ph.D., of The Schepens; Henry Klassen, M.D., Ph.D., of Children's Hospital of Orange County, Calif.; and Fred H. Gage, Ph.D., of the Salk Institute. Other encouraging research from Kyoto University has also shown that transplanted stem cells from rodents seem to be able to form nerve synapse connections.
Stem cell transplants may also be able to restore lost vision, as was suggested by unpublished findings of researchers at the University of Cologne in Germany. In their experiments, treated rats had developed new photoreceptor cells after two months, while the retinas of the untreated control group had completely degenerated. Studies have also shown that transplanted embryonic cells in rats can revive lost photoreceptor function.
The Ethical Issue
The use of embryonic stem cells raises serious moral issues. There are four potential sources for embryonic stem cells: 1) leftover, unwanted embryos from fertility clinics, 2) donated fetuses from abortions, 3) cloning, and 4) creating new embryos expressly for the purpose of harvesting the cells. All of these methods raise ethical issues, which are under debate. In August 2001, President Bush approved government funding for research using embryonic stem cell lines (self-replenishing colonies), but only those which are already in existence. The basis for this compromise is that there is a large enough supply (about 65 lines) to continue the research indefinitely. Some scientists argue, however, that hundreds of lines may be necessary in order to really understand the capabilities of stem cells.
Another method of acquiring stem cells is to harvest them from bone marrow and brain tissue. These would be adult cells, which would sidestep the ethical problems encountered by using embryos. In some studies, adult stem cells have shown more promise than embryonic cells for developing into retinal photoreceptors, but some researchers argue that adult cells are more difficult to produce in large quantities and that they may eventually lose their potency.
Biologists have also discovered stem cells in umbilical cord blood. These cells have been shown to be effective in combatting blood disorders such as leukemia, but more research is needed to test the viability of this approach.
All of the research on stem cells has been promising, and the moral issue seems to have been defused for now by President Bush's announcement. Now the work continues in hopes of verifying the findings and determining the effectiveness of stem cell transplantation in humans.
For more information on latest stem cell research, follow these links (each will open a separate window):