Textbooks Were Wrong: Human Hair Grows by Being Pulled Upward, Not Pushed From the Root

For generations, biology textbooks have taught that hair grows the way toothpaste leaves a tube: cells divide at the base of the follicle and shove the hair shaft steadily upward and out of the scalp. A new study says that picture is fundamentally wrong. Human hair, the researchers found, is not pushed from below — it is actively pulled upward by coordinated cell movements higher up in the follicle, working almost like a tiny biological motor.

The work, a collaboration between L'Oréal Research & Innovation and Queen Mary University of London, was published in the journal Nature Communications. Using advanced 3D live imaging, the team tracked individual cells in real time inside human hair follicles kept alive in laboratory culture — a technically demanding feat that let them watch the growth process unfold rather than inferring it from static snapshots.

What they saw upended the conventional model. Cells in the outer root sheath, the layer that encases the hair shaft, move in a coordinated spiral, and it is this collective motion that generates an upward pulling force on the hair. "Our results reveal a fascinating choreography inside the hair follicle," said Dr. Inês Sequeira of Queen Mary University of London. The hair, she explained, "is actively being pulled upwards by surrounding tissue acting almost like a tiny motor."

To test whether cell division really was the engine of growth, the scientists blocked it — and found that hair kept growing at nearly normal rates. When they instead disrupted actin, the protein machinery that cells use to move, growth plummeted by more than 80 percent. That contrast pinned the mechanism squarely on mechanical movement, not the proliferation of new cells at the root, overturning an assumption embedded in the field for decades.

The implications reach well beyond satisfying scientific curiosity. "This new view of follicle mechanics opens fresh opportunities for studying hair disorders, testing drugs and advancing tissue engineering," said Dr. Thomas Bornschlögl of L'Oréal Advanced Research. Because hair loss affects hundreds of millions of people, a mechanism-level understanding of what actually drives growth could point toward entirely new classes of treatments — ones that target the follicle's internal motor rather than simply trying to coax more cells to divide. Dr. Nicolas Tissot, the study's first author, called the 3D time-lapse approach "indispensable for truly unraveling the intricate, dynamic biological processes within the hair follicle."

The findings also help explain observations that never fit the old model neatly, such as why some treatments aimed at boosting cell division at the root have delivered disappointing results in the clinic. If the true engine of growth is coordinated mechanical movement, then therapies that support the follicle's tissue architecture and its actin-driven motor may prove more effective than those that simply try to spur proliferation. The researchers cautioned that the work was conducted on follicles grown in culture and that translating it into products for people will require further study. Still, by rewriting a passage in the biology textbooks, the study reframes a question that has vexed scientists — and frustrated the balding — for as long as anyone has tried to answer it.