Stem cells demonstrate rare absorption property

Scientists at the University of Cambridge have found that stem cells demonstrate a bizarre property, known as auxeticity, never before seen at a cellular level. Their research has been published in the journal Nature Materials.

Most materials will contract when stretched: if one pulls on an elastic band, the elastic will get thinner. They will also typically expand when squeezed: if one squeezes a tennis ball between both hands, the circumference around the ball gets larger. However, in auxetic materials the opposite effect occurs - squeeze it and it will contract, stretch it and it will expand.

Until now, auxeticity has only been demonstrated in man-made materials and very rarely in nature, such as some species of sponge. But the University of Cambridge researchers report having observed auxeticity in the nuclei of embryonic stem cells - master cells within the body which can turn into any other type of cell.

“Embryonic stem cells (ESCs) self-renew in a state of naïve pluripotency in which they are competent to generate all somatic cells,” the researchers stated. “It has been hypothesised that, before irreversibly committing, ESCs pass through at least one metastable transition state … we show that during the transition, the nuclei of ESCs are auxetic: they exhibit a cross-sectional expansion when stretched and a cross-sectional contraction when compressed, and their stiffness increases under compression.”

Study leader Dr Kevin Chalut and colleagues treated a transitioning cell’s cytoplasm, the fluid surrounding the nucleus, with a coloured dye and found that when they stretched the nucleus, it absorbed the dye, suggesting that it had expanded to become porous. It is possible that it does so to absorb molecules from the cytoplasm or environment which would help the cell differentiate.

“[The cell’s] nucleus takes on an auxetic property, allowing it to ‘sponge up’ essential materials from its surrounding,” said Dr Chalut. “This property has not, to my knowledge, been seen before at a cellular level and is highly unusual in the natural world.”

The discovery may provide clues to different ways to manufacture auxetic materials. The vast majority of the materials are highly ordered, such as the auxetic honeycomb, but some are disordered - for example, if one pulls both ends of a scrunched up ball of paper, the circumference around the ball expands. The nucleus of the transitional stem cell is likewise disordered.

“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,” said Dr Chalut.

“Studying how auxeticity has evolved in nature will guide research into new ways to produce auxetic materials, which might have many diverse applications in our everyday life,” he said. Such applications could range from soundproofing and super-absorbent sponges to bulletproof vests.

This work is licensed under a Creative Commons Licence (CC BY-NC-SA 3.0).

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