Adult stem cells are crucial for physiological tissue renewal and regeneration following injury. Current models assume the existence of a single quiescent (resting) population of stem cells residing in a single niche of a given tissue.
The Linheng Li Lab and others have previously reported that primitive blood-forming stem cells can be further separated into quiescent (reserved) and active (primed) sub-populations. Emerging evidence indicates that quiescent and active stem cell sub-populations also co-exist in several tissues — including hair follicle, intestine, bone marrow, and potentially in the neural system — in separate yet adjacent microenvironments. In the review, Dr. Li proposes that quiescent and active stem cell populations have separate but cooperative functional roles.
"Both quiescent and active stem cells co-exist in separate 'zones' in the same tissue," explained Dr. Li. "Active stem cells are the 'primed' sub-population that account for the generation of corresponding tissues, whereas quiescent stem cells function as a 'back-up' or 'reserved' sub-population, which can be activated in response to the loss of active stem cells or to tissue damage."
The new model would explain how the balance can be regulated between stem cell maintenance and simultaneous support of rapid tissue regeneration, not only at the individual cell level but also at the stem cell population level. The advantage of maintaining 'zoned' sub-populations of stem cells is to increase longevity of stem cells within organisms that have long life spans and large bodies.
The existence of two sub-populations of adult stem cells offers another advantage in the rapidly regenerating tissues in mammals by reducing the risk for mutations that cause tumors.
Intriguingly, cancers may utilize this same mechanism to maintain co-existing active-quiescent pools of stem cell sub-populations that support fast tumor growth (by active stem cells) while preserving the root of malignancy (by quiescent stem cells). This may explain the basis of drug resistance to cancer treatment.
"If this hypothesis is true, the critical question will be how to target quiescent drug-resistant cancer stem cells," said Dr. Li. "We will test this model in cancers in an effort to determine how to activate quiescent (drug-resistant) cancer stem cells for further targeting."
Dr. Li also is a Professor in the Department of Pathology & Laboratory Medicine at The University of Kansas School of Medicine. Learn more about his work at www.stowers.org/labs/LiLab.asp.
Dr. Clevers is also a Professor of Molecular Genetics at the Hubrecht Institute and was the first to report the existence of active stem cells in intestine. Learn more about his work at www.hubrecht.eu/research/clevers.
The Stowers Institute for Medical Research is a non-profit, basic biomedical research organization dedicated to improving human health by studying the fundamental processes of life. Jim Stowers, founder of American Century Investments, and his wife Virginia opened the Institute in 2000. Since then, the Institute has spent over a half billion dollars in pursuit of its mission. Currently the Institute is home to nearly 500 researchers and support personnel; over 20 independent research programs; and more than a dozen technology development and core facilities.
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