So far, much of the research into how this migration takes place has been done on individual cells. By looking at a collection of migrating cells – in this case, in cells from the surface of the kidney – the scientists discovered that their movement as a group – migration slowing as cell density rises, and the fastest cells moving in large groups whose scale grows with increasing cell density – is similar to a process called glass transition. Glass is, by definition, a non-crystalline solid that goes from brittle to molten when heated, and the other way around when cooled: this process is called a glass transition It happens to polymers, too, like plastics, and it remains one of the great unsolved problems in physics. “Our results provide a startling analogy,” says Xavier. “It had been predicted that over certain time scales, tissues might flow like fluids. What we’ve found shows that if cell density is below a certain amount, then confluent cells (cells that move together) indeed flow like a fluid. However, as density increases, the collective dynamics of the cells progressively slow down, much as the molecules of a molten glass slow down as it is cooled. “As our cells display a glass transition we could almost say that we are, in fact, made of glass!” The authors of the article, led by Harvard researchers Thomas Angelini and David Weitz, say this new knowledge provides a framework for understanding the collective dynamics of wound healing. “A glass-like behaviour of confluent cells has direct implications on collective migration and its purposes,” explains Xavier, whose research at IBEC is funded by the prestigious Catalan Institution for Research and Advanced Studies (ICREA). “In healing, for example, in which large groups of cells are removed from a cohesive tissue, the cell density is reduced to nothing at the edge of the wound and cells rapidly migrate inward to fill it. Our results suggest that this dramatic reduction of cell density is analogous to a localized reduction of particle density in colloidal glass, or analogous to a localized increase in temperature in molecular glass, creating a ‘melted’ region of the wounded cell layer. “Our next steps will be to explore the analogy between living cells and glass-forming systems in more depth. We’ll look at major diseases in which cell density plays a central role; for example, tumours in many forms of cancer display a higher cell density than the surrounding tissues. Our findings may help understanding the dynamics of tumour cells and the physical mechanisms they use to break away and metastasize.” — Source Article: Thomas E. Angelini, Edouard Hannezo, Xavier Trepat, Manuel Marquez, Jeffrey J. Fredberg, and David A. Weitz (2011). Glass-like dynamics of collective cell migration. PNAS, doi:10.1073/pnas.1010059108 Coverage of this story today in El Mundo here.