The cells of animals, plants, and fungi start their lives by being torn apart. Cells are born by division, and just before a parent cell becomes two daughters, it doubles its nuclear DNA and carefully condenses it into X-shaped chromosomes. The nucleus disassembles, letting these crucial genetic instructions float free in the cell’s soupy interior. Then the cell performs an astounding, microscopic feat of strength.
Proteinaceous cables extend from the cell’s poles toward the equator and latch onto the chromosomes. They drag, tilt, and nudge the precious cargo until every chromosome has been ushered into a tidy line around the cell’s middle. Then this spindle apparatus, as it’s known — a sinewy, dynamic rib cage made of bundles of microtubules — shortens itself at both poles. This wrenches the chromosomes apart into two sets and reels them to opposite ends of the cytoplasm sea. With its genetic material segregated at either pole, one cell can safely become two, born from a microscopic tug-of-war.
The spindle strains against itself as it shortens and pulls; how it does this without ripping itself apart has been a scientific mystery since biophysicists first observed cell division with microscopes 150 years ago. “They saw them [the chromosomes] moving, which led to this idea that there’s probably forces that are pulling or pushing things around,” said Colleen Caldwell, a biophysicist at the University of Groningen.
If absorbing those forces caused the spindle’s integrity to fail, it could spell the end for both daughter cells or cause diseases that arise from errors in cell division and chromosome arrangement. In this way, all eukaryotic life, including human life, rides on the spindle’s success with each cell division across an organism’s lifetime.
Until recently, researchers didn’t have the tools to physically manipulate the mammalian spindle structure at the subcellular scale to toy with it and find out how it works. Recently a team of researchers led by Sophie Dumont, a biophysicist at the University of California, San Francisco, used microneedles to physically manipulate and stress the structure in mammal cells for the first time — and then observe how the spindle holds together through intense strain as it wrenches the chromosomes apart.
The experiments have shown how a self-repair mechanism enables the spindle to stabilize itself under force and avoid disintegrating. These findings, which were published in February 2026 in Current Biology, provide a window into the physics of the cellular world, where complex living machines endure physical forces and stresses like machines in a factory. The spindle’s mechanical quirks show just how weird materials science can get at the finest scales of life.
By virtue of being biological, the cell spindle presents massive complexity for materials physicists. Most human-made materials contain just a few different types of molecules, said Colm Kelleher, a biophysicist at Syracuse University who was not involved with the new research. Meanwhile, the spindle is made of hundreds of different types of individual protein molecules, and any one of them is “an extremely complex object,” he said.
That puts the spindle in an unusual size class that complicates experiments. “There’s quite a bit that scientists know about the mechanics of individual molecules, and there’s quite a bit that scientists know about the mechanics of tissues and organisms, like how muscles generate force,” Dumont said. “But mechanics at this scale of many molecules together forming this macromolecular structure is harder to probe. So we know less about it, but it’s just as important.”
One last wrinkle is that, by being part of a living organism, these biomolecular structures are constantly consuming energy from within the materials themselves — very unlike how human-made materials and machines work. Kelleher gave the example of a car: It has a fuel tank and an engine, which power components that transfer torque to the wheels, which then push against the ground. A system made of biological materials works very differently.