Biophysical controls (Introduction)

by David Turell , Wednesday, October 25, 2023, 21:36 (393 days ago) @ David Turell

A new approach:

https://www.quantamagazine.org/biophysicists-uncover-powerful-symmetries-in-living-tiss...

"Giomi and his colleagues just took an important step toward that goal. In a study published in Nature Physics, they conclude that sheets of epithelial tissue, which make up skin and sheathe internal organs, act like liquid crystals — materials that are ordered like solids but flow like liquids. To make that connection, the team demonstrated that two distinct symmetries coexist in epithelial tissue. These different symmetries, which determine how liquid crystals respond to physical forces, simply appear at different scales.

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"Though we might be more familiar with the liquid crystals in TV screens, they are also common in cell biology, found inside cells and in cell membranes. Over the past few years, researchers have tried to show that tissues — organized groups of cells that act together — could be considered liquid crystals, too. If tissue could be accurately described as a liquid crystal, then the set of tools that physicists use to predict how crystals respond to forces could be put to work in biology, Hirst said.

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"In simulations of small groups of cells, theorists could describe tissues as liquid crystals with sixfold “hexatic” symmetry, a bit like tilings of hexagons. But in experiments, tissues instead acted like fluids made of bar-shaped particles with twofold “nematic” symmetry — a bit like what you’d see if you poured a barrel of toothpicks into a tube and watched them flow.

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"Preliminary simulations by Carenza — a former researcher in Giomi’s group — suggested that the disagreement could be resolved if both symmetries, sixfold and twofold, existed simultaneously in tissues. The idea was that if you zoomed in on a tissue with nematic symmetry, you’d find smaller-scale hexatic symmetry.

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"The Leiden biophysicists devised a mathematical object called a shape tensor to capture information about the cells’ shapes and directions. Using it, Eckert measured the symmetries in the tissues at different scales, first treating individual cells as the crystal’s basic units and then doing the same for groups of cells.

"At small scales, they found that the tissue had sixfold rotational symmetry and looked a bit like a tiling of smooshed hexagons. But when they examined groups larger than about 10 cells, twofold rotational symmetry emerged. The experimental results neatly agreed with Carenza’s simulations.

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"Accurately defining a liquid crystal’s symmetry isn’t just a mathematical exercise. Depending on its symmetry, a crystal’s stress tensor — a matrix that captures how a material deforms under stress — looks different. This tensor is the mathematical link to the fluid dynamics equations Giomi wanted to use to connect physical forces and biological functions.

"Bringing the physics of liquid crystals to bear on tissues is a new way to understand the messy, complicated world of biology, Hirst said. (my bold)

"The precise implications of the handoff from hexatic to nematic order aren’t yet clear, but the team suspects that cells may exert a degree of control over that transition. There’s even evidence that the emergence of nematic order has something to do with cell adhesion, they said. Figuring out how and why tissues manifest these two interlaced symmetries is a project for the future — although Giomi is already working on using the results to understand how cancer cells flow through the body when they metastasize. And Shaevitz noted that a tissue’s multiscale liquid crystallinity could be related to embryogenesis — the process by which embryos mold themselves into organisms.

"If there’s one central idea in tissue biophysics, Giomi said, it’s that structure gives rise to forces, and forces give rise to functions. In other words, controlling multiscale symmetry could be part of how tissues add up to more than the sum of their cells.

"There’s “a triangle of form, force and function,” Giomi said. “Cells use their shape to
regulate forces, and these in turn serve as the running engine of mechanical functionality.'”

Comment: a new wonderful way to look at living biochemistry, which is a "messy, complicated world of biology." God didn't make it easy to understand how cells work, but they appear to use God's intelligent instructions.