Ever wondered how cells self-organize into the beautifully ordered tissues of our bodies? Our latest study shows that tiny “topological defects” in endothelial monolayers play a surprisingly critical role in shaping living tissue structure.
But living cells aren't simple rods, and biological tissues aren't clean physics experiments. Cells have complex shapes, they stick to their neighbors, they respond to chemical signals, and they're constantly remodeling themselves. The big question was whether the beautiful mathematical theories of active nematics could actually explain anything about how real tissues organize themselves during development, wound healing, or disease.
The answer, it turns out, is a resounding yes—but with a twist that makes the biology even more fascinating than the physics. When the researchers tracked the number and positions of topological defects in their endothelial monolayers over time, they didn't see the smooth, monotonic decrease that simple theories would predict. Instead, the defects followed a more complex choreography, with their numbers and arrangements evolving through distinct phases as the tissue found its way to order.
How Topological Defects and Strings Drive Tissue Organization in Endothelial Cell Layers
The authors Ruider, Thijssen, and Vannier of the Nature paper "Strings and topological defects govern ordering kinetics in endothelial cell layers" reveal a hidden middle act in tissue ordering: as endothelial sheets align, they pass through an intermediate phase ruled by strings that connect defect pairs, changing how order spreads across living matter. Instead of a simple, steady clean‑up of disorder, the pattern of defects evolves non‑monotonically, with strings acting like dynamic zippers that pull regions into register. This bridges active‑matter physics and vascular biology by showing that tissue‑scale order emerges from local excitations in a living nematic material.
The team cultivated confluent endothelial monolayers and followed their collective motion over time with large‑field imaging to track how cell orientations and defects evolve during ordering . They paired these measurements with physical models to test how string excitations bind defects and control annihilation rates across multicellular distances. This approach lets them quantify when and where alignment takes hold, and why the journey isn’t a straight line from disorder to perfect order.
Strings that tie it all together
The key insight came from recognizing that defects in living tissues don't just wander around independently until they bump into each other and disappear. Instead, they become connected by "string excitations"—coherent multicellular structures that can span dozens of cell lengths and effectively tether defect pairs together. These strings aren't physical objects you could grab with tweezers; they're more like organized waves of cellular orientation that propagate through the tissue, creating temporary but crucial connections between distant parts of the monolayer.
Think of it like this: imagine you're trying to organize a massive crowd of people, and instead of everyone just milling about randomly until they happen to line up, special "organizer" lines form spontaneously that connect different trouble spots in the crowd. These organizer lines help coordinate the movement of distant groups, making the overall organization process much more efficient and predictable. That's essentially what the string excitations do in endothelial tissues.
Intermediate Phase Reveals How Strings and Defects Control Endothelial Tissue Organization
Perhaps the most surprising discovery was the existence of what the researchers call an "intermediate phase"—a distinct stage in tissue organization where strings and defects coexist in a delicate balance that's neither fully disordered nor completely aligned. This phase emerges because the strings don't just help defects find each other faster; they also create "defect domains," spatial regions where local rules either accelerate or inhibit the ordering process.
In some domains, strings efficiently shepherd defects toward annihilation, rapidly clearing patches of tissue and allowing perfect alignment to take hold. But in other domains, the same string mechanisms can actually trap defects or create new ones, temporarily stalling the ordering process and maintaining pockets of controlled disorder. The overall kinetics of tissue organization emerge from the competition between these accelerating and decelerating domains, producing the non-monotonic evolution that distinguishes living active nematics from their simpler physical cousins.
This discovery has profound implications for understanding how blood vessels form during development and how they repair themselves after injury. The intermediate phase suggests that tissues have access to a much richer palette of organizational states than previously thought. Instead of being stuck with a binary choice between disordered and ordered, endothelial tissues can tune their degree of organization by modulating the balance between string-mediated acceleration and domain-mediated inhibition.
Real-world implications for vascular biology
The practical applications of this research extend far beyond academic curiosity. Blood vessel formation—both during normal development and in pathological conditions like cancer—depends critically on the ability of endothelial cells to organize themselves into properly aligned, functional tubes. Understanding the physics of this organization could lead to new strategies for promoting healthy blood vessel growth or for disrupting the aberrant vessel formation that feeds tumors.
The researchers' use of HAOECs is particularly relevant here, since these cells come from the aorta—the body's main highway for blood flow. Aortic endothelial cells experience some of the highest mechanical stresses in the cardiovascular system, and their alignment is crucial for maintaining the smooth, laminar flow patterns that prevent turbulence and reduce the risk of atherosclerosis. The string-and-defect mechanisms revealed in this study could help explain why some people are more prone to vascular disease and could suggest new therapeutic targets for prevention.
PELOBiotech spotlight
The research team used Human Aortic Endothelial Cells (HAOECs) cultured in PELOBiotech's specialized cell culture medium to create the perfect experimental playground for watching this cellular ballet unfold. These aren't just any cells—aortic endothelial cells line the walls of our largest blood vessels, where they face the most intense mechanical forces and must maintain perfect coordination to keep blood flowing smoothly. By growing them into confluent sheets and tracking their movements with sophisticated time-lapse imaging, the scientists could watch in real-time as thousands of cells negotiated their way from randomness to synchronized alignment.
Ruider, I., Thijssen, K., Vannier, D.R. et al.Strings and topological defects govern ordering kinetics in endothelial cell layers. Nat. Phys.21, 1629–1637 (2025). https://doi.org/10.1038/s41567-025-03014-4
