Inside every brain cell, tiny drops of protein drift through the fluid like oil in water. For about a decade, scientists have known that these droplets are connected to diseases like Alzheimer's and ALS — but exactly how they turn dangerous has been a mystery. Now physicists at Leiden University in the Netherlands have found an answer that surprised even them: the dangerous fibers do not grow inside the droplets, as almost everyone assumed. They grow from the surface outward, while the droplet itself shrinks and turns glassy.
The finding, published in the journal Nature Physics on 26 October 2026, rewrites a basic picture of how protein droplets age. It also suggests a new way to think about brain disease: the droplet can age in two competing directions, and only one of them produces the fibers linked to neurodegeneration. If researchers can learn to push droplets toward the harmless path, they may one day have a new way to slow — or even prevent — the damage.
What Are Protein Droplets?
Proteins are the workhorses of the cell, but some of them do not stay dissolved in the cell's fluid. Instead, they gather into tiny liquid droplets, a process called phase separation. Scientists call these droplets "condensates." They behave a bit like drops of oil in water: they form, merge, and float freely inside the cell.
Condensates are not just a curiosity. Cells use them to organize chemical reactions, storing molecules in one place until they are needed. But the same proteins that form these helpful droplets can also form amyloid fibers — long, tough, thread-like structures that are a hallmark of Alzheimer's disease, ALS, and other neurodegenerative disorders. For years, the big open question was how one state turns into the other. Do the fibers start growing deep inside the droplet, where the protein is most concentrated? Or does something else happen?
Until now, most researchers assumed the first option. It seemed like common sense: more protein in one place should mean more fiber growth. The Leiden team set out to test that assumption directly — and found it was wrong.
Watching Droplets Age for 12 Hours
The experiments were led by doctoral candidate Liru Feng and assistant professor Louise Jawerth at Leiden University, working with theoretical physicists at the Max Planck Institute for the Physics of Complex Systems in Dresden. They used a confocal microscope to watch protein droplets in three dimensions for 12 hours at a time, tracking how each droplet changed as it aged.
The droplets they studied were only about 5 micrometers across — roughly ten times thinner than a human hair. That small scale is part of why the process was so hard to see: you need powerful microscopy and careful 3D tracking to follow a single droplet through its whole aging process.
What they saw was counter-intuitive. Instead of fibers growing from the protein-rich interior, the fibers grew from the droplet's surface to the outside. And as the fibers grew, the droplet itself shrank and became glasslike — a solid but disordered state, quite different from the ordered fibers.
"We have often thought that fibers would grow on the inside of the droplets where there is a high concentration of protein, but now we know they don't," Jawerth said. "They grow from the surface to the outside. Meanwhile, as these fibers grow, we see the droplets themselves shrink and become glasslike."
Two Ways to Age — And They Compete
The most important result is not just where the fibers grow, but what that tells us about the droplet's fate. The team showed that the glassy state and the fibrous state are two different solid forms of the same protein — two competing ways for the droplet to age. They can exist side by side, but one does not directly turn into the other.
That competition matters. In a striking experiment, the researchers first aged the droplets for two days before triggering fiber growth. The aged droplets hardly grew any fibers at all. Once a droplet had gone down the glassy path, it seemed to be locked out of the fibrous one.
This raises a hopeful possibility. The researchers suggest that the glassy state of aging may have a different, perhaps even healthy, function inside brain cells, while the fibers are possibly the unhealthy alternative. If that is right, the goal for future treatments might not be to destroy droplets or dissolve fibers, but to tip the balance — helping droplets age into the glassy state rather than the fibrous one.
It is a subtle but important shift in thinking. Much of Alzheimer's research has focused on clearing away amyloid fibers or the plaques they form. This work suggests that an earlier intervention point may exist: the moment when a droplet "chooses" which way to age. That moment happens long before any visible damage, and it may be easier to influence.
Why This Matters for Alzheimer's and ALS
The link between these proteins and brain disease has been known for about ten years. In Alzheimer's, amyloid fibers build up into plaques between brain cells; in ALS, abnormal protein clumps damage the motor neurons that control movement. But the step-by-step process that turns a healthy liquid droplet into a harmful fiber has been unclear, and that gap has made it hard to design drugs that intervene early.
The new study does not claim to have found a cure, and the researchers are careful about the limits of their work. The experiments were done on model protein systems in the laboratory, not inside living human brains. And, as Jawerth herself stresses, "we don't yet know the healthy versus unhealthy routes for aging protein droplets" — and it is not yet proven that these fibers are the main reason brain cells degenerate.
Still, the work gives drug researchers something they did not have before: a clear, measurable picture of the aging process, backed by a theoretical model developed with the Max Planck team. A process you can watch, measure, and model is a process you can eventually try to control. The next steps will be to test whether the same surface-growth pattern holds for the specific proteins involved in Alzheimer's and ALS, and whether it happens inside real neurons.
A Wider Story: The Physics of Living Cells
There is another reason this discovery is interesting beyond brain disease. It belongs to a growing field that treats living cells as physical materials — soft, squishy, ever-changing matter that follows the rules of physics as well as biology. Protein droplets are a perfect example: they are liquid-like, they can turn glassy like window glass, or they can grow fibers like crystals. Understanding them requires physicists and biologists working together, which is exactly what happened here.
That cross-disciplinary approach is becoming a pattern in modern biology. Just this month, researchers in Ottawa identified a protein called SPRIZ that acts as a rigid pillar holding the two membranes of mitochondria — the cell's powerhouses — a precise distance apart, shaping how these energy factories work. And a team in Tübingen reported a gene-targeting treatment that cut a harmful brain protein by about 90% in lab-grown human neurons. Each of these findings chips away at a different piece of the same puzzle: how tiny structures inside cells fail, and how we might fix them.
What Happens Next?
For readers, the honest message is one of cautious optimism. This is fundamental research, not a treatment. No drug based on this finding exists, and any therapy that grows out of it is years away. But fundamental research is where treatments begin, and this study answers a question that has stood open for a decade: where do the fibers actually grow?
Three questions now lead the field. First, do the same rules apply to the disease proteins in real brain cells? Second, can scientists find a way to nudge droplets toward the glassy state and away from fiber growth? And third, is the glassy state truly harmless — or could it cause its own problems over time? The Leiden team has given researchers the tools and the model to ask these questions properly.
For anyone living with Alzheimer's or ALS in the family, that may sound like small comfort. But every effective treatment in medicine started with a moment like this one: a careful experiment that showed the world works differently than everyone assumed. A droplet that shrinks as its fibers grow outward is a small detail — and it may be the detail that changes how we fight these diseases.
Sources and Further Reading
This article is based on the peer-reviewed paper "Fibres and glasses are competing material states in protein condensation" by Liru Feng, Iain Muntz, Ryota Takaki, Lionel Ndamba, Martine Ruer-Gruß, Frank Jülicher, and Louise Jawerth, published in Nature Physics on 26 October 2026 (DOI: 10.1038/s41567-026-03415-z), and on Leiden University's announcement of the same date. The study is peer-reviewed research, not a preprint or press-release-only claim; a preprint version was also posted on bioRxiv.
For related reading on The Global Spy, see our report on how the APOE4 gene damages the brain's blood vessels in Alzheimer's disease, our analysis of why dementia cases are rising fast — 11.4 million new cases a year, and our look at how a nurse-led program is helping midlife adults cut dementia risk.