Textbooks Were Wrong: Human Hair Doesn't Grow the Way Scientists Thought

By Curie

Human hair doesn't grow by being pushed out from the root. It gets pulled up — by a microscopic cellular motor inside the follicle that scientists never saw until now.

That's the finding from a collaboration between L'Oréal Research & Innovation and Queen Mary University of London, published in Nature Communications. Using advanced 3D time-lapse microscopy on living human hair follicles in culture, researchers discovered that cells in the outer root sheath — the layer encircling the hair shaft — move in a coordinated spiral pattern, generating an upward pulling force.

"For decades, it was assumed that hair was pushed out by the dividing cells in the hair bulb," said Dr. Inês Sequeira, a lead author and reader in oral and skin biology at Queen Mary. "We found that instead it's actively being pulled upwards by surrounding tissue acting almost like a tiny motor."

The team tested the prevailing model directly: they blocked cell division inside the follicle, expecting growth to halt if division-driven pushing was the mechanism. Instead, hair kept growing at nearly the same rate. But when they interfered with actin — the protein that allows cells to contract and move — growth slowed by more than 80%.

Computer simulations backed the observations. Only models incorporating a pulling force from the outer root sheath could reproduce the measured speed of hair movement.

The shift matters because it reframes how we think about hair loss. Current treatments like minoxidil work largely by stimulating cell division. If the pulling mechanism is key, future therapies might instead target the mechanical environment of the follicle — its contractile machinery, tissue stiffness, and force-generating capacity.

The catch: this work was done on follicles maintained in laboratory culture, not inside a living human scalp. Whether the same pulling dynamics occur in vivo isn't yet established. But the researchers argue the ex vivo system is physiologically relevant, and the imaging method itself could become a tool for testing hair-loss drugs on living follicles.