Scientists at the University of Cambridge in the UK have discovered that the nuclei of stem cells have the unusual ability to become thicker when stretched and thinner when compressed. The counterintuitive property - termed auxeticity - is already known to materials scientists, who see its application ranging from super-absorbent sponges and bulletproof vests to soundproofing.
Dr. Kevin Chalut, from the Wellcome Trust-Medical Research Council Cambridge Stem Cell Institute, says to his knowledge, auxeticity has "never been seen before at a cellular level and is highly unusual in the natural world."
"This is a pretty bizarre finding and very unexpected," he adds. "When the stem cell is in the process of transforming into a particular type of cell, its nucleus takes on an auxetic property, allowing it to 'sponge up' essential materials from its surrounding."
Dr. Chalut and colleagues, who include biologists, engineers and physicists, report their findings in the journal Nature Materials.
Most materials get thinner when you stretch them and fatter when you squeeze them. However, scientists have begun to explore materials that do the opposite - they get thinner when you squeeze them and get fatter when you stretch them. They demonstrate the unusual property of auxeticity, a term that comes from the Greek "auxesis," which means "increase" or "growth."
Because of this unusual property, auxetic materials make excellent shock absorbers and sponges, and engineers are already developing applications in industries ranging from aerospace to construction, health care, automotive, defense and textiles.
The video below demonstrates honeycomb auxeticity - an example of ordered auxeticity, the form predominant in man-made materials:
Auxeticity rarely seen in nature
Until this new study, where Chalut and colleagues describe observing the property in embryonic stem cells, auxeticity has been seen very rarely in nature - only in some species of sponges, they note.
Stem cells are "master cells" that have the potential to become virtually any of the 200 or so different cells in the body. Embryonic stem cells are derived from early stage embryos and considered the "gold standard" for research on stem cells.
In this study, the team observed how auxeticity appears in the nuclei of the embryonic stem cells. And the nuclei only show this property when in the transition stage, as the stem cells change from embryonic, non-specific cells into differentiated, tissue-specific cells, such as heart cells.
To clearly demonstrate the property, the team put a colored dye in the fluid surrounding the nucleus - the cytoplasm. They saw how the nucleus absorbed the dye when they stretched it, suggesting it had expanded and become porous.
They suggest such a property may help the nucleus absorb molecules from the cytoplasm that help the stem cell differentiate.
Good example of 'disordered auxeticity'
The discovery is a good example of what materials scientists and engineers call "disordered auxeticity," such as that demonstrated by a scrunched up sheet of paper. If you pull on both ends of the ball of paper, the circumference around the ball expands.
The vast majority of man-made auxetic materials are highly ordered, for instance as shown in this video of the "auxetic honeycomb."
Dr. Chalut says there "is clearly a lot we can learn from nature," and adds:
"We are already seeing auxeticity explored for its super-absorption properties, but despite great technological effort, auxetic materials are still rare and there is still much to discover about them in order to manufacture them better."
Studying how this unusual property has evolved in nature could lead to "new ways to produce auxetic materials, which might have many diverse applications in our everyday life," he explains.
The Royal Society, the Wellcome Trust and the Medical Research Council helped finance the study.
Meanwhile, Medical News Today learned how a key ingredient in the children's play material Silly Putty may help treat neurological disorders. Researchers at the University of Michigan in the US suggest the ingredient can turn embryonic stem cells into working spinal cord cells more efficiently. The finding could lead to a new way of generating high-yield and high-purity motor neurons from stem cells and improve treatments for diseases like Alzheimer's disease, amyotrophic lateral sclerosis (Lou Gehrig's disease), and Huntington's disease.
Written by Catharine Paddock PhD