Mar. 17, 2000 When it comes to stem cells, it appears the eyes have it. Researchers at the University of Toronto and the Hospital for Sick Children (HSC) have identified retinal stem cells in the adult mammalian eye, opening the door for retinal regeneration as a possible cure for damaged or diseased eyes.
"Before our study, it wasn't known whether retinal regeneration was possible in adult mammals, especially humans," says Vincent Tropepe, a PhD student in U of T's developmental biology program and lead author of the study that appears in the March 17 edition of the journal Science. "We've shown that by removing these cells from the eye, we can encourage the production of new neurons even after the retinas have fully matured and cell division has stopped."
Stem cells give rise to a lineage of other cells by simultaneously dividing and self-renewing, beginning in the embryo and continuing throughout post-natal life and into adulthood. When this cell division occurs, one of the two new cells is identical to the original while the other is slightly different. These new cells continue to divide and can become specialized and replace others that die or are lost.
In their study, researchers discovered retinal stem cells in the tissue of adult mice, cows and humans. Previously, only amphibians and fish were thought to have retinal stem cells capable of regenerating and making new neurons. "The stem cells we discovered appear to be under inhibitory control while still in the eye, but proliferate once they are removed," says Roderick McInnes, holder of the Anne and Max Tannenbaum Chair in Molecular Medicine at HSC and U of T.
The research team now hopes to be able to stimulate the stem cells in their natural region inside the eye in order to generate new neurons to help return the eyes to their proper function. "Our next goal is to find those factors that inhibit them from proliferating in their natural region inside the eye and release that inhibition so as to give the cells the ability to regenerate and ultimately produce the different types of cells needed to make a new retina," says Derek van der Kooy, professor of anatomy and cell biology in U of T's Faculty of Medicine.
While finding that elusive inhibitory factor would be ideal, researchers say other methods can be explored. "If we can't find a way to relieve the inhibitory factors in the real eye then an alternative would be to remove and culture the cells, make the right tissue type that's missing and then put them back," adds Tropepe, who conducted the research as part of his thesis on characterizing neural stem cells during development of the brain. "We now need to determine if these cells are completely committed to producing their own tissue or if they can be convinced to make other tissue types."
Tropepe says this finding contributes to the rapidly growing body of evidence that the adult brain has more potential to regenerate and grow new neurons than people previously thought. "It's a matter of trying to figure out how we can generate new neurons from these stem cells in vivo."
This research was partly funded by the Medical Research Council of Canada, University Medical Discoveries Inc. and two members of the federal Networks of Centres of Excellence program - the Canadian Genetic Diseases Network and the NeuroScience Network.
Steven de Sousa
U of T Public Affairs
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