Human hair doesn't grow in the way scientists once thought.
A new imaging study has revealed that human hair isn't 'pushed up' from the hair follicle, as biology textbooks have described for decades. Instead, the hair is pulled upwards by a sophisticated network of moving cells previously unobservable. This discovery could change how scientists approach issues such as hair loss, tissue regeneration, and regenerative medicine.
The hidden force behind hair growth
Researchers from L'Oréal Research & Innovation and Queen Mary University of London used real-time 3D imaging technology on living cells to track individual cells inside human hair follicles cultured in the laboratory. The results, published in the journal Nature Communications , showed that cells in the outer root sheath (the layer surrounding the hair shaft) are not stationary, but move in a spiral downward motion.
This movement creates an upward pulling force, helping the hair strand emerge from the scalp, completely contradicting the old theory that hair is pushed outward by dividing cells at the root of the hair follicle.
Dr. Inês Sequeira, co-author of the study at Queen Mary, said:
"For decades, it was believed that hair grew because cells at the hair follicle continuously divided and pushed the hair upwards. But we discovered that hair is actually actively pulled upwards by the surrounding tissue acting like a tiny motor."
To test the new hypothesis, the research team blocked cell division within the hair follicle, expecting that hair growth would slow down or stop. Surprisingly, the hair continued to grow at almost the same rate.
Conversely, when they interfered with actin—a protein that helps cells contract and move—the rate of hair growth decreased by more than 80%. Subsequent computer simulations confirmed that the traction generated by the coordinated movement of cells in the outer layer of the hair follicle is a crucial factor in explaining the actual rate of hair growth.
3D imaging technology opens up a whole new perspective.
Dr. Nicolas Tissot, lead author of the study from L'Oréal's Advanced Research group, said the team used a completely new imaging method: real-time time-lapse 3D microscopy.
According to him, static images only show "disjointed snapshots," while 3D time-lapse technology truly helps decipher the complex, dynamic, and continuous biological processes inside hair follicles. This allows scientists to observe cell dynamics, movement patterns, and cell division rates, as well as accurately simulate the mechanical forces generated in situ.
Dr. Thomas Bornschlögl, another co-author from L'Oréal, noted that this finding shows that hair growth depends not only on cell division, but also heavily on the active pulling mechanism of the outer root sheath.
This new understanding of the 'mechanics of hair follicles' opens up numerous research opportunities for hair disorders, drug testing, and applications in tissue engineering and regenerative medicine. A thorough understanding of these mechanical forces, alongside biochemical factors, could help design more effective hair loss treatments in the future.
Although the research was only conducted on human hair follicles cultured in a laboratory, the authors believe these findings provide important clues for the science of hair and demonstrate the growing role of biophysics in the formation of familiar organs of the human body.
