Morbidity due to severe wounds is a common medical problem. Yet our understanding of wound healing processes is still limited.
To close wounds, epithelial cells migrate from the edges of the wound. Depending on the wound’s contours, the cells that lead the migration and the cells that follow, sometimes jam together. The leadership positions change dynamically and unjam elsewhere to close the entire wound. How is this dynamic heterogeneity related to cell density, another factor that is known to be involved in epithelial wound closure?
Scientists from TIFR, India collaborated with researchers from the UK and Germany, to find out. To understand the dynamics of the development of heterogeneity in one layer of cells, they imaged a Madin–Darby canine kidney epithelial monolayer – a model mammalian cell line used in biomedical research. To visualise and measure cell migration, they used particle image velocimetry – an optical method of flow visualization.
While migrating, the cells change shape. The researchers traced the shapes of the cells. They continued doing this, starting from a low-density culture until the cells started dying due to senescence.
They found that, when cell density is low, the shapes of cells vary considerably and the movements of the cells are not correlated. But, as density increases, the shape indices reduce and coordinated cell migration becomes apparent.
“Thus, there seems to be a linear relationship between cell density and dynamic heterogeneity during cell migration for wound closure”, says PhD scholar, Basil Turakkal, TIFR Hyderabad.
“Even at the highest density, the epithelial monolayer had a heterogeneous population consisting of both jammed and dynamic, unjammed cells”, says Medhavi Vishwakarma, University of Bristol.
Collective migration depends crucially on cellular ability to exert traction forces on underlying substrates. To understand the forces, the researchers grew cells with different densities over a soft polyacrylamide substrate containing fluorescent beads. Using traction force spectroscopy, they measured the traction forces between the cells and the substrate.
There was a stress peak in the densely packed monolayer. The distance between neighbouring peaks in the bulk stress landscape correlated with distance between two neighbouring leader cells at the wound margin.
The researchers then used reflection interference contrast microscopy, a non-invasive optical technique, to observe cell-substrate contacts, including focal adhesions. At all densities, the leader cells were oriented to the direction of wound closure. But the movements of the followers were not coordinated. At higher cell densities, however, the movement of the cells became coordinated.
“Wound closure is a complex process. In this series of experiments, we could show that increase in cell density, reduction in the diversity of cell shapes and dynamic changes in leadership to coordinate cell migration for wound closure, are all closely related processes”, says Tamal Das, Collective Cellular Dynamics Lab, TIFR Hyderabad.
These findings, the researchers hope, will lead to further research on the molecular processes involved in cell migration for wound closure.
Phil. Trans. R. Soc. B 375: 20190391 (2020)
Reported by Sileesh Mullasseri