Active and passive stem cells
In a review of the Coherence of Quiescent and Active Adult Stem Cells in Mammals, published in the January issue of the journal Science, Linheng Li, a professor at the Stovers Institute for Medical Research in Kansas City, co-authored with Hans Clevers, director of the Hubrecht Institute (Holland), proposed a new model for regulating the state of adult mammalian stem cells, which explains how the coexistence of two different states of stem cells ensures their preservation and, at the same time, rapid tissue regeneration.
Recently, the study of stem cells has enjoyed great popularity (and is well funded, in particular, in the USA), because stem cells have a powerful therapeutic potential.
Unlike embryonic stem cells, which can differentiate into absolutely any cells, adult stem cells present in the already formed tissues of an adult organism can only turn into cells of a limited number of types. Under certain conditions, adult stem cells of one tissue can differentiate into cells of another tissue.
Adult stem cells are necessary for physiological regeneration – replacement of specialized cells that have served their time, as well as for the restoration of damaged ones (for example, as a result of injury) fabrics. According to modern concepts, adult stem cells that are in the resting phase (not dividing) are localized in certain areas of tissues, the so-called niches.
Recently, a group of scientists led by Lee and other researchers have shown that primitive blood stem cells (hematopoietic cells) can be divided into two subpopulations: dormant (conserved) and active (primary) stem cells. The coexistence of dormant and active stem cell subpopulations has also been found in several other tissues, including hair follicles, intestinal tissues, bone marrow, and the nervous system. Both subpopulations exist in adjacent but different microenvironments – "zones" of the same tissue. Active stem cells are "primary" subpopulations in the sense that they provide the formation of the corresponding tissue, and the subpopulation of dormant ones is like a backup in case of loss of an actively used resource: they can "wake up" in case of loss of active stem cells or tissue destruction.
The proposed model allows us to explain how the preservation of the reserve of stem cells and at the same time rapid tissue regeneration is ensured. The obvious advantage of preserving localized subpopulations of stem cells is an increase in their lifetime in large and long-lived organisms.
The coexistence of two subpopulations of stem cells has another advantage: in rapidly regenerating mammalian tissues, the risk of mutations leading to the development of cancer is significantly reduced, since there is a reserve of intact stem cells at rest.
Lee suggested that cancer can develop by a similar mechanism, in which active and dormant subpopulations of cancer stem cells are maintained, one of which ensures rapid tumor growth (due to active stem cells), and the other preserves the focus of tumor origin in the form of dormant stem cells. Such a model can explain the causes of drug resistance in the treatment of oncological diseases.
If Lee's hypothesis is experimentally confirmed, the following important question will arise: how can cancer stem cells that are in the resting phase be destroyed?
In the near future, Lee plans to test his model in relation to oncological diseases in an attempt to find a way to activate resting cancer stem cells and further study them.
Daria Chervyakova
Portal "Eternal Youth" based on the materials of the Stowers Institute: Linheng Li Proposes Novel Theory for Mammalian Stem Cell Regulation04.02.2010