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(This message was added in version 6.7.0.) in /home4/scienrds/scienceandnerds/wp-includes/functions.php on line 6114Source:https:\/\/www.quantamagazine.org\/biophysicists-uncover-powerful-symmetries-in-living-tissue-20231025\/#comments<\/a><\/br> \u201cIt\u2019s a great paper,\u201d said Linda Hirst<\/a>, a physicist at the University of California, Merced, who was not involved in the work. \u201cThey are really describing the symmetry of the cell sheets in more detail than has been done before.\u201d<\/p>\n Liquid crystals flow like fluids, but they still have a degree of crystalline order \u2014 a sort of inherent symmetry or directionality that\u2019s a bit like the grain of wood. And just as a wood plank is strongest along its grain, a liquid crystal\u2019s response to stimuli depends on its symmetry and orientation. This directionality, called anisotropy, is the optical magic behind modern liquid crystal displays, which refract light differently depending on their orientation.<\/p>\n 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 \u2014 organized groups of cells that act together \u2014 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.<\/p>\n However, these efforts hit a geometric roadblock. Experimentalists and theorists couldn\u2019t agree on tissue\u2019s symmetry \u2014 a liquid crystal\u2019s most defining characteristic, and the key to predicting its behavior using fluid dynamics. In simulations of small groups of cells, theorists could describe tissues as liquid crystals with sixfold \u201chexatic\u201d symmetry, a bit like tilings of hexagons. But in experiments, tissues instead acted like fluids made of bar-shaped particles with twofold \u201cnematic\u201d symmetry \u2014 a bit like what you\u2019d see if you poured a barrel of toothpicks into a tube and watched them flow.<\/p>\n \u201cThere was a contradiction: Experiment says nematic; numerical experiments and models in general say hexatic,\u201d said Livio Carenza<\/a>, a computational physicist at Ko\u00e7 University in Istanbul. \u201cHow do these two things speak with each other?\u201d<\/p>\n Preliminary simulations by Carenza \u2014 a former researcher in Giomi\u2019s group \u2014 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\u2019d find smaller-scale hexatic symmetry.<\/p>\n \u201cBut you cannot verify theory with theory,\u201d Giomi said. \u201cSo we did the experiments.\u201d<\/p>\n To do that, Giomi recruited Julia Eckert<\/a>, then a doctoral student at Leiden University,\u00a0to gather data from living tissue cultures.<\/p>\n \u201cI pulled them to the microscope and showed them real cells, not only the cells they can see in the literature,\u201d said Eckert, who is now a biophysicist at the University of Queensland. \u201cI say, \u2018Have you ever seen cells, you know, in real life?\u2019 And it was like \u2018No.\u2019 No? OK, let\u2019s go!\u201d<\/p>\n Eckert started by growing thin layers of epithelial tissue in the lab. Then she carefully marked out the boundaries of each individual cell in microscope images. Now Giomi and his team could get to work. They wanted to see whether the tissue\u2019s symmetry differed between small scales \u2014 when they considered just a few cells and their neighbors \u2014 and zoomed-out, larger scales.<\/p>\n But to disentangle the nested symmetries in Eckert\u2019s sheets of cells, the team needed a reliable way to distinguish nematic and hexatic orders in messy biological data.<\/p>\n The Leiden biophysicists devised a mathematical object called a shape tensor to capture information about the cells\u2019 shapes and directions. Using it, Eckert measured the symmetries in the tissues at different scales, first treating individual cells as the crystal\u2019s basic units and then doing the same for groups of cells.<\/p>\n 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.\u00a0The experimental results neatly agreed with Carenza\u2019s simulations.<\/p>\n \u201cIt was pretty amazing how well the experimental data and numerical simulation matched,\u201d Eckert said. In fact, it matched so closely that Carenza\u2019s first response was that it must be wrong. The team jokingly worried that a peer reviewer might think they\u2019d cheated. \u201cIt really was that beautiful,\u201d Carenza said.<\/p>\n The observations answer a \u201clong-standing question about the type of order present in tissues,\u201d said Joshua Shaevitz<\/a>, a physicist at Princeton University who reviewed the paper (and did not think they\u2019d cheated). Science often \u201cgets murky,\u201d he said, when data points to seemingly conflicting truths \u2014 in this case, the nested symmetries. \u201cThen someone points out or shows that, well, those things aren\u2019t so distinct. They\u2019re both right.\u201d<\/p>\n Accurately defining a liquid crystal\u2019s symmetry isn\u2019t just a mathematical exercise. Depending on its symmetry, a crystal\u2019s stress tensor \u2014 a matrix that captures how a material deforms under stress \u2014 looks different. This tensor is the mathematical link to the fluid dynamics equations Giomi wanted to use to connect physical forces and biological functions.<\/p>\n Bringing the physics of liquid crystals to bear on tissues is a new way to understand the messy, complicated world of biology, Hirst said.<\/p>\n The precise implications of the handoff from hexatic to nematic order aren\u2019t yet clear, but the team suspects that cells may exert a degree of control over that transition. There\u2019s even evidence<\/a> 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 \u2014 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\u2019s multiscale liquid crystallinity could be related to embryogenesis \u2014 the process by which embryos mold themselves into organisms.<\/p>\n If there\u2019s one central idea in tissue biophysics, Giomi said, it\u2019s 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.<\/p>\n There\u2019s \u201ca triangle of form, force and function,\u201d Giomi said. \u201cCells use their shape to regulate forces, and these in turn serve as the running engine of mechanical functionality.\u201d<\/p>\n Quanta\u00a0is conducting a series of surveys to better serve our audience. Take our\u00a0<\/i>physics reader survey<\/i><\/a>\u00a0and you will be entered to win free\u00a0<\/i>Quanta\u00a0merchandise.<\/i><\/p>\n<\/div>\n <\/br><\/br><\/br><\/p>\n
\nBiophysicists Uncover Powerful Symmetries in Living Tissue<\/br>
\n2023-10-26 21:58:42<\/br><\/p>\nFlow and Symmetry<\/strong><\/h2>\n
A New Fluid Order<\/strong><\/h2>\n
Form, Force and Function<\/strong><\/h2>\n