Why does our hair grow? A new study provides a shocking answer

Scientists have discovered that human hair does not grow because it is "pushed out" by dividing cells at the bottom of the follicle. Rather, the hair is actively pulled upwards by forces associated with moving cells in the outer layers of the root. These findings call into question decades of biology textbooks and may change approaches to the study of hair loss and hair regrowth.

The study was carried out by the L'Oréal Research & Innovation team in collaboration with scientists at Queen Mary University of London and published in the journal Nature Communications.

How the "traction" mechanism of hair growth works

The team used state-of-the-art real-time 3D microscopy to track the behaviour of individual cells in living human hair follicles that had been maintained in culture.

It turned out that:

• cells in the outer root sheath (the layer surrounding the hair shaft) move in a spiral downward trajectory;

• it's in this area that the force that pulls the hair upwards is generated, as if a tiny engine of living cells is working around the shaft;

• that is, the hair is not squeezed out of the follicle by dividing cells, as previously thought, but is lifted by the co-ordinated mechanical forces of the surrounding tissues.

"Our results showed an amazing 'choreography' within the follicle," says Dr Ines Sequeira, one of the lead authors of the paper. - For decades, it was thought that hair grew by dividing cells in the bulb, which pushed it outwards. We found that it's actually being actively pulled upwards by the surrounding tissues, working almost like a micromotor."

What happens if you stop cell division

To test the hypothesis, the researchers:

• blocked cell division inside the follicle - hair growth was expected to stop;

• but growth continued at almost the same rate;

• but when the scientists interfered with actin - a protein needed for cell contraction and movement - the rate of hair growth dropped by more than 80 per cent.

Computer modelling showed that it is the traction force associated with the coordinated movement of cells in the outer layers of the follicle that is needed to explain the actual speed of hair advancement.

"Static images only provide isolated moments," explains the paper's first author Dr Nicolas Tissot from L'Oréal's Advanced Research division. - 3D timelapse microscopy allows us to see the dynamics: who is moving where and at what speed the cells are dividing. This is what gave us the opportunity to model the locally occurring forces."

A new perspective on hair loss and regeneration

Co-author Dr Thomas Bornschlögl notes:

"We now realise that hair growth is not only determined by cell division. The outer root sheath actively pulls the hair upwards. This revision of the mechanics of the follicle opens up new possibilities for studying hair diseases, testing drugs and developing tissue engineering and regenerative medicine."

Although the work was carried out on follicles grown in the laboratory, it provides important clues for:

• developing new treatments for baldness,

• creating treatments that take into account not only the biochemistry but also the mechanical forces within the follicle,

• testing drugs on living follicles thanks to new imaging techniques.

The authors emphasise: the study demonstrates the growing role of biophysics in biology - at the micro level, tissue mechanics can determine how organs like hair work.