Surprising force patterns and deformation waves discovered at tissue boundaries
Cells with different functions must be physically separated in many cases for an animal to develop, grow, and maintain a steady state. Such tissue boundaries also occur in diseases, for example at the intersection between healthy tissue and cancerous tumors. The separations are usually credited to local activity by cells along the boundary that cause physical tension or repulsion between populations. However, in a surprising discovery recently reported in Nature Materials, researchers from Spain and the United States found that long-lived and long-range mechanical events also play a role in keeping tissues apart.
“While studying how tissues collide and form boundaries, we unexpectedly observed mechanical waves that form after the establishment of the boundary,” says researcher Xavier Trepat from the Institute for Bioengineering of Catalonia (IBEC) and the Center for Biomedical Research Network (CIBER) in Spain. “After a collision, cells were pushed and deformed into waves that travelled at a speed of three millimeters a day. This unexpected behavior defies what we know about cellular dynamics, and could be relevant to understanding embryonic development [and] metastasis,” says Raimon Sunyer also of IBEC and CIBER, who led the study together with Trepat.
In this research, the team explored the boundary that forms between advancing tissues expressing two different ephrins; ephrins are a family of surface proteins responsible for repulsive interactions between epithelial tissues. The researchers fabricated magnetic stencils from poly(dimethylsiloxane) (PDMS). Consisting of two large compartments separated by a thin barrier, the stencils were attached to soft gels that mimicked in vivo conditions. The researchers seeded cells expressing the EphB2 receptor in one compartment and cells expressing its ligand ephrinB1 in the other. After the cells in each compartment cohered to form monolayers, the stencils were removed with a magnetic field and the tissues migrated toward one another.
On colliding, the two monolayers did not fuse due to their repulsive EphB2/ephrinB1 interactions, but formed a wavy boundary. This was followed by the formation of a cable along each edge of the boundary made from actomyosin, the protein complex that causes muscles to contract. Between the two cables was an unoccupied micrometer-sized band. As the cable formed, the boundary straightened and the cell density near the cables dramatically increased from migration and proliferation. The researchers termed this the “jamming” phase, similar to the way particles jam at high density in a granular material.
Measurements of the forces at the cell-substrate interface using soft elastic substrates showed that during the jamming phase, the monolayers showed long-lived mechanical traction patterns that oscillated in space. These oscillations acted to pull the cell-substrate adhesions away from the boundary. As the jamming phase continued, stress measurements showed that even cells several rows behind the boundary participated.
In addition, by measuring cell velocities the researchers found that as cell density increases during jamming, deformation waves are triggered that travel backwards across the monolayers. Replacing one of the two cell monolayers with a PDMS wall and repeating the experiment revealed that these waves are not specific to the EphB2/ephrinB1 interaction, but appear to be a general feature of a boundary situation.
These mechanical features are not predicted by current tissue separation theories. However, the traction patterns and deformation waves suggest that local cell activity is not exclusively responsible for tissue separation. “Our findings raise the possibility that rather than being solely controlled by genetic clocks and traveling waves of biochemical nature, tissue segregation and patterning during development and homeostasis is controlled by cell jamming,” the researchers conclude in the article.
Ulrich Schwarz, a theoretical physics professor at Heidelberg University who specializes in the adhesion and mechanics of cells and tissue, praises the careful quantitative analysis involved in this research. “Without the demonstrated high level of image processing and force measurements,” he says, “these exciting discoveries would not have been possible.” He also thinks that this work touches on many different aspects of materials science, not only because it uses microfabrication and soft elastic substrates, but also because the colliding tissues demonstrate the new physics that can be discovered in active soft matter systems.
Read the abstract in Nature Materials.