...Stem cells were first identified and characterized in bone marrow in the 1950s and ’60s... These blood stem cells were rare, slow to divide, and capable of both self-renewal and differentiation into any of the blood’s more specialized cell types...Because of their relevance to healing and recovery, stem cells in other tissues became coveted prizes among researchers and physicians searching for ways to treat all sorts of conditions and diseases.
Then the story got more complicated. Stem cells were identified in other adult tissues throughout the body: in the skin, in hair follicles, in the gut, and in bone. They, too, could self-renew and give rise to their tissue’s various cell lineages. But otherwise, they looked very different from the blood stem cells. They expressed different genes, exhibited different protein and surface markers, and divided in different ways and at different rates.
In the 1990s, scientists isolated embryonic stem cells, which were even more powerful than those in adult tissues, with the ability to become any cell type in the body.... And in 2006, researchers succeeded in transforming differentiated connective tissue cells into induced
pluripotent stem cells (iPSCs), which had the versatility of embryonic stem cells. That result showed that stemness could be induced.
Yet looming over these findings, according to the molecular geneticist Hans Clevers, an author on the new PNAS cardiac stem cell paper, is the assumption that stem cells throughout the body are “a precious, hard-wired, magical entity” like the ones in bone marrow. In fact, he said, those first insights gained from blood stem cells have coloured how scientists think about stem cells in other tissues – sometimes in ways that have been profoundly limiting.
One often-overlooked point is that many tissues can repair themselves in very ingenious ways. In the blood, the small stem cell population is the only means of regeneration, but in solid tissues, that’s not always the case. The stem cells themselves are different: They tend to divide more rapidly, for instance, and because they exhibit unique molecular profiles, they have to be identified by methods specific to them. Reliance on tissue-specific markers (which aren’t always stringent) is one of the reasons there’s been so much debate surrounding whether cardiac stem cells exist – and why it remains so difficult to determine other types of stem cells.
Moreover, when the stem cells in solid tissues are destroyed, more specialized cells in those tissues can often revert to a stemlike state to take over repair functions on their behalf. Cells are therefore much more plastic than previously thought possible, with less fixed identities. There’s more and more evidence saying that our bodies can respond to damage independently of what we would consider a classic stem cell population...
That’s been shown in a slew of organs, including the kidney, lung, stomach, and intestine. Perhaps most striking, some tissues don’t seem to have a stem cell population. The adult liver – the epitome of efficient organ regeneration – has no stem cells; instead, its differentiated cells can act like stem cells when needed. “In essence,” Clevers said, “every cell in the liver has the potential to behave like a stem cell.”
And so, “it’s more useful to find out how a particular tissue performs its stem cell function than to identify individual stem cells,” he said. The way various cells all contribute to maintaining a tissue constitutes stemness – not any one cell type or entity. Sticking to the more dogmatic definition of what a “true” stem cell should be, instead of considering that they fall along a more nebulous spectrum, has hindered progress...
1. Stem cells identified in other tissues throughout the body are different from blood stem cells in all the following ways EXCEPT: a) they have different protein and surface markers.
b) they have different rates and methods of division.
c) they can repair themselves and give rise to the tissues’ various cell lineages.
d) they exhibit unique molecular profiles
2. The main argument of the passage can be weakened if it is proven that a) only a specific type of cells has the genetic content that can initiate response to tissue damage.
b) stem cells have various other functions apart from self-repair and renewal.
c) it is not just designated stem cells which are responsible for maintaining a tissue.
d) not all tissues in the body are capable of maintaining and repairing themselves.
3. The ‘assumption’ pointed out by Hans Clevers in the sentence (‘Yet looming over these findings…is the assumption…’) is that a) all stem cells throughout the body are precious.
b) the function performed by stem cells is akin to magic because of lack of comprehension of their functions.
c) stem cells are hard-wired to perform only repair and renewal functions.
d) specific cells are designated to perform the role of a stem cell.
4. Based on the information provided in the passage, which of the following statements is the author most likely to approve of? a) The imposing of our understanding from blood stem cells has led to false conclusions.
b) Tissue-specific markers are the only way to identify stem cells.
c) We do not entirely understand how a tissue performs its stem cell function.
d) Regeneration of tissues is dependent on the stem cell population.
5. The author uses the example of the liver to demonstrate that a) the body doesn’t need stem cells.
b) plasticity of cell function drives tissue regeneration.
c) livers regenerate themselves efficiently.
d) specialized cells can perform repair functions when stem cells are damaged.