Applied Physicists Discover That Migrating Cells Flow Like Glass

By studying cellular movements at the level of both the individual cell and the collective group, applied physicists have discovered that migrating tissues flow very much like colloidal glass. The research, led by investigators at Harvard's School of Engineering and Applied Sciences (SEAS) and the University of Florida, advances scientists' understanding of wound healing, cancer metastasis, and embryonic development.

The finding was published online February 14 in Proceedings of the National Academy of Sciences.

Cells often move from one part of the body to another. In a developing embryo, for example, cells in the three germ layers have to arrange themselves spatially so that the cells that will become skin are all on the outside. Similarly, as a cancerous tumor expands, the cells proliferate and push others aside. In wound healing, too, new cells have to move in to replace damaged tissue.

It is well known that cells accomplish these movements through internal cytoskeletal rearrangements that allow them to extend, retract, and divide. At some point during the migration, though, the new tissue settles into place and stops.

"We're trying to understand it from a fundamental point of view," says principal investigator David Weitz, Mallinckrodt Professor of Physics and Applied Physics at SEAS. "What we're really trying to get at is, why do things stop moving?"

The glass under discussion here is not the kind used in windows—though that is part of the larger category. Glasses include any amorphous materials that are viscous enough to remain solid for a reasonable period of time (often considered to be 24 hours) but which flow over longer periods (see sidebar).

Cream that is churned into butter goes through a sort of glass transition, as the increasing density of particles in the fatty emulsion forces it to become solid. Like any glass, butter will lose its form if the temperature rises.

As supercooled fluids and colloids (like cream) become more dense and approach the glass transition, the particles exhibit certain characteristic motions.