Biomechanics of Hair Growth
Hair is produced by hair follicles. The skin covering the human body features several million of these mini-organs. A hair follicle is shaped like an elongated pear, a few millimeters long but only a fraction of a mm wide. It has an intricate multi-layered tissue structure.
The base of the follicle is attached to the skin tissue by the dermal papilla in the centre of the bulb of the 'pear'. The periphery of the bulb of the follicle features rapidly dividing cells divide (in anagen phase). In the thinner, upper part of the follicle cells change shape, harden by cornification and undergo programmed cell death as they form the hair shaft.
What are the forces that push the hair outward?
Which follicle structures and biomechanical processes govern the genesis of the narrow hair shaft?
How come that the hair glides outward during growth, but remains firmly anchored when one tries to pluck it?
To answer these questions, we simulated the biomechanics of hair growth. We embarked on a 'multi-scale mechanical modeling approach' using finite element analysis.
Finite element modeling is a computational modeling methodology used in engineering. It allows simulating the effects of load application on structures and yields information on the way forces propagate.
We modeled various processes that characterize the hair follicle actitivies at different scales. Information gained based on cell-level simulations were used to inform tissue and organ level models.
We constructed a model based on the geometrical follicle structure, its tissue layers, and we simulated the effects of volume addition through cell division and the sliding of layers on hair growth.
The follicle layers are differentially pressurized. Living cells are under pressure, whereas cornified and dead cells have lost pressure. The cells in the different layers mature at different distances from the follicle bulb. We simulated the pressure that individual cells exert on their neighbors. Our simulations predict how cell size and intracellular spaces influence the generation of force that causes the hair shaft to be pushed outwards.
The inner layers of the follicle differentiate into hair shaft, inner root sheath and companion layer. These layers move outwards whereas the outer root sheath (ORS) and connective tissue remain stationary. Consequently, sliding occurs between the companion layer and ORS and we simulated the effect of the resulting friction.
Volume addition through cell division results in pressure on the peripheral follicle layers and on the distally forming hair shaft. We assessed the effect of the location of volume addition, the resulting friction, and follicle shape on hair protrusion.
The simulations generated multiple verifiable predictions for the functioning of hair follicles. Future experimental studies will probe their validity.
Zamil MS, Harland DP, Fisher BK, Davis MG, Schwartz JR, Geitmann A. 2021. Biomechanics of hair fibre growth: a multi-scale modelling approach. Journal of the Mechanics and Physics of Solids, in press. Accepted manuscript.
Bidhendi AJ, Geitmann A. 2019. Geometrical details matter for mechanical modeling of cell morphogenesis. Developmental Cell 50: 117-125
Bidhendi AJ, Geitmann A. 2018. Finite element modeling of shape changes in plant cells. Plant Physiology 176: 41-56
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