https://www.nature.com/articles/s41467-025-65143-x
This is slightly older, interesting and has been appearing on geronotolgy news feeds
chatGPT(5.6paid):
Overall assessment
This paper presents the most detailed live map so far of cell movement within intact, growing human hair follicles. Its principal biological finding is that hair growth may not be driven mainly by proliferating matrix cells pushing the hair shaft upward. Instead, the authors propose that an actin-dependent mechanical process in the outer root sheath (ORS) pulls the companion layer and inner follicular layers upward.
The imaging observations are strong and genuinely novel. The specific claim that the ORS provides the pulling force is plausible and well motivated, but not yet definitively demonstrated because the actin intervention is not ORS-specific and the force itself was not measured.
Paper: Tissot et al., Nature Communications (2025)
1. What the authors did
The authors microdissected anagen scalp hair follicles from human surgical tissue and maintained them ex vivo. They developed a custom multiphoton-imaging system that could image follicles longitudinally or transversely in 3D over time.
They combined:
- autofluorescence, largely from NADH;
- Hoechst nuclear labelling;
- second-harmonic generation for collagen;
- third-harmonic generation for cellular boundaries;
- manual single-cell tracking;
- particle-image velocimetry;
- quantification of mitotic location and orientation;
- cell-volume measurements;
- a continuum fluid-dynamics model;
- surgical removal of the follicular bulb;
- inhibition of mitosis with colcemid;
- actin disruption with latrunculin B.
The principal cell-tracking dataset contained 371 tracks from 22 follicles obtained from 14 donors. Most quantitative live-imaging analysis was restricted to approximately the first 20–22 hours, before signs of anagen-to-pseudo-catagen transition appeared.
2. Main findings
Outer-root-sheath cells move downward in a spiral-like pattern
Cells in the suprabulbar ORS showed a conspicuous circumferential or helical movement. The average perpendicular speed was approximately:
- ORS circumferential component: 12.0 ± 7.7 μm/h
- ORS downward axial component: −6.9 ± 3.8 μm/h
The circumferential motion became increasingly axial as cells approached the bulb:
- suprabulbar trajectory angle: approximately −5°
- bulbar trajectory angle: approximately −38°
Thus, ORS cells appear to spiral around and down the follicle before entering its lower region.
Inner follicular layers flow upward at different speeds
Cells feeding the inner root sheath moved upward faster than cells contributing to the cortex:
- cortex: approximately 4.2 ± 1.8 μm/h
- IRS: approximately 5.6 ± 1.8 μm/h
Within matched follicles, the IRS moved about 1.8 ± 1.3 μm/h faster than the cortex; 16 of 17 follicles showed this relationship.
This creates an “onion-shaped” velocity field: different concentric layers move at different rates rather than the entire follicle behaving as a single rigid column.
Mitosis is spatially organized
Most mitoses occurred below the Auber line, especially in the proximal matrix adjacent to the dermal papilla:
- proximal matrix: 13.1 ± 4.2 mitoses/h
- distal matrix: 5.7 ± 2.5 mitoses/h
After scaling from the sampled volume, the authors estimated approximately 62.7 mitoses per hour in the whole matrix.
Division orientation also varied systematically with location and broadly aligned with the subsequent direction of cellular flow. This supports the rodent-derived model in which spatially arranged progenitors feed particular concentric follicular layers.
Cells enlarge as they migrate and differentiate
THG imaging and nucleus-to-nucleus spacing indicated that cortex and IRS cells increased in volume during their first approximately 150 μm of upward migration. Their volume then plateaued, probably corresponding to advanced differentiation.
Consequently, follicle material is generated by both:
- cell proliferation; and
- post-mitotic cell enlargement.
Mitosis alone cannot reproduce the observed velocity field
The authors modelled the inner follicle as an incompressible, viscous cellular material. Inputs included measured proliferation, cell enlargement and the downward ORS flux.
A model in which the boundary between the ORS and companion/inner layers was stationary did not reproduce the observed velocity profiles. A model imposing an upward-moving boundary at approximately 6.75 μm/h fitted the observations much better.
They interpret this moving boundary as an active mechanical force exerted at or near the ORS–companion-layer interface.
Growth continues without the matrix
After surgically removing the bulb and proliferative matrix, the inner follicular layers continued moving upward for at least 24 hours. Immunostaining suggested that sliding occurred within or at the inner side of the ORS rather than between the entire follicle and connective-tissue sheath.
This is important because it shows that ongoing matrix proliferation is not necessary for short-term upward extrusion.
Actin inhibition has a much larger effect than mitotic inhibition
In intact follicles:
- colcemid inhibition of mitosis reduced elongation by approximately 24%;
- latrunculin B reduced elongation by approximately 82%;
- latrunculin B plus colcemid produced a similarly large reduction.
In bulb-removed follicles, latrunculin B reduced continued extrusion by about 80%.
The authors therefore conclude that an actin-dependent pulling mechanism is a major driver of hair-fibre extrusion, whereas proliferation mainly supplies new cellular material.
3. What is genuinely novel
A global dynamic map of the human follicle
Earlier lineage and live-imaging work was dominated by mouse follicles or was limited to local observations. This study integrates the human bulb, matrix, ORS, IRS and cortex into one dynamic map at approximately single-cell resolution.
That is probably the strongest and least controversial novelty.
Direct visualization of helical downward ORS migration
The observation that human ORS cells undergo coordinated circumferential and downward motion—and that this becomes more axial near the bulb—is striking. The bidirectional clockwise and anticlockwise movements also suggest collective organization rather than a single imposed chirality.
Quantitative coupling between mitotic geography and layer-specific flow
The paper links local division rates and orientations to the velocities of the layers those progenitors appear to supply. This provides dynamic human evidence supporting lineage arrangements previously inferred mainly from rodent experiments and static human histology.
Reframing proliferation as supply rather than propulsion
The conceptual novelty is the distinction between:
- production: mitosis and cell enlargement create material; and
- propulsion: an actin-dependent outer-layer mechanism moves that material upward.
That is a useful revision of the common intuitive picture in which matrix proliferation simply pushes the hair shaft outward.
Integration of imaging, modelling and perturbation
None of the three components alone would establish the proposed mechanism. Their combination makes a reasonably coherent case:
- observed differential cell flows;
- modelling showing that proliferation alone is insufficient;
- continued motion after bulb removal or mitotic arrest;
- marked suppression by actin disruption.
4. Critical evaluation
The pulling force is inferred, not directly measured
The model requires an upward-moving boundary to reproduce the velocity profile, but this is effectively an imposed boundary condition. It shows that such a boundary is sufficient to explain the data; it does not establish what generates it.
No traction-force microscopy, tension sensor, laser ablation, recoil experiment or direct stress measurement was performed. Consequently, “active boundary motion” is better supported than the stronger term “pulling force.”
Latrunculin B does not localize the mechanism to the ORS
Latrunculin B acts throughout the follicle. It could affect:
- ORS migration;
- companion-layer contractility;
- IRS and cortex mechanics;
- intercellular adhesion;
- keratinocyte shape;
- epithelial integrity;
- vesicular trafficking or differentiation;
- tissue friction at several interfaces.
The authors acknowledge this. The experiment demonstrates that elongation is strongly actin-dependent, but it does not demonstrate that ORS actin specifically generates the force.
A more decisive experiment would require ORS-selective actin inhibition, local drug delivery or spatially restricted photoactivation.
Continued growth after bulb removal is not conclusive proof of active pulling
Removing the bulb eliminates ongoing proliferation but does not eliminate:
- pre-existing elastic stress;
- residual pressure within already-produced layers;
- maturation and enlargement of post-mitotic cells;
- wound-induced contraction at the cut surface;
- changes in friction or boundary conditions caused by cutting.
Continued motion for 24 hours argues strongly against instantaneous dependence on mitosis, but stored stress and continued maturation would need to be quantified before attributing all remaining motion to ORS pulling.
Colcemid addresses current mitosis, not all proliferation-derived force
Although colcemid blocked visibly completing mitoses, the follicle already contained a substantial column of cells produced before treatment. Those cells could continue to enlarge, differentiate and transmit residual pressure.
The modest 24% reduction therefore does not mean proliferation contributes only 24% of normal propulsion in vivo. It shows that acute mitotic arrest has a relatively modest short-term effect in this preparation.
Longer experiments are difficult because the follicles enter pseudo-catagen, but that limitation matters mechanistically.
Elongation is not exactly equivalent to new hair production
The measurements primarily record extrusion or displacement of existing follicular material. After bulb removal, continued lengthening cannot represent sustained production of a normal new hair fibre because the main progenitor compartment has been removed.
The paper’s title carefully refers to a mechanism of “hair growth,” but “hair-fibre extrusion” is the more precise interpretation of the mechanical experiments.
Ex vivo follicles undergo rapid physiological deterioration
Multiphoton-imaged follicles began showing pseudo-catagen-like changes at a median of about 20 hours. Microdissection also removes vascular, neural, immune, adipose and broader dermal inputs.
The authors sensibly restrict most analysis to the pre-transition interval, but early molecular or mechanical changes could precede visible signs of pseudo-catagen. The measured dynamics may therefore not be identical to those in an intact scalp follicle.
The imaging sample is selective
Only non-pigmented terminal follicles were used for multiphoton imaging because melanin interfered with imaging and increased phototoxicity. This raises questions about applicability to:
- actively pigmented follicles;
- different ethnic hair types;
- vellus follicles;
- androgen-sensitive miniaturizing follicles;
- alopecia-affected follicles.
Brightfield interventions included pigmented follicles, making the imaging and intervention populations not entirely equivalent.
Some measurements have limited effective sample size
Although hundreds of tracks were collected, tracks within one follicle and follicles from one donor are not fully independent observations. The authors did perform nested variance analysis and often used follicular means, which is a strength.
Nevertheless:
- mitotic mapping used only four follicles;
- some trajectory-angle measurements came from three or four follicles;
- PIV and cell-volume analyses used small numbers of follicles;
- intervention groups contained approximately five to nine follicles.
Plots of individual cell tracks can visually exaggerate the effective sample size if the follicle or donor, rather than the cell, is the true independent unit.
Manual tracking can introduce selection bias
Cells were selected partly because they could be followed within the imaging plane. The authors recognize that this may overestimate velocities by preferentially retaining conveniently trackable trajectories. Cells with dynamics differing from the collective layers were excluded as possible immune or non-epithelial cells.
That exclusion is reasonable for measuring epithelial flow, but it requires subjective classification and could remove genuine heterogeneous ORS behaviour. Automated 3D tracking with cell-type-specific markers would be preferable.
The continuum model simplifies a highly structured tissue
Treating the follicle as a viscous incompressible fluid is useful for describing average flow, but the real tissue contains:
- changing cell volumes;
- differentiation-dependent stiffness;
- desmosomes and other discrete junctions;
- anisotropic actin and keratin networks;
- layer-specific friction;
- local cell rearrangements and death;
- possible viscoelastic stress storage.
The model reproduces velocity fields but cannot discriminate among multiple biological mechanisms capable of producing the same boundary motion.
The proposed ORS-to-inner-lineage contribution remains provisional
The trajectories suggest that some descending ORS cells enter the lower proximal cup and may join upward-moving populations. But the authors could not follow individual cells throughout the complete transition.
Without permanent lineage labelling, the work cannot establish whether a tracked ORS cell:
- becomes a matrix progenitor;
- directly joins the companion layer;
- reverses direction without changing identity;
- or dies and is replaced.
This part of the paper should be regarded as a hypothesis supported by flow patterns, rather than human lineage tracing.
5. Experiments that would most strengthen the conclusion
The most decisive follow-up studies would be:
- Spatially restricted disruption of ORS actomyosin, leaving the matrix and inner layers intact.
- Laser ablation at the ORS–companion interface, measuring directional recoil as evidence of pre-existing tension.
- Direct force or tension measurements, using deformable probes, tension-sensitive adhesion reporters or calibrated tissue deformation.
- Separate inhibition of myosin II, Arp2/3 and formins, to identify whether contraction, migration or actin polymerization produces the effect.
- Long-term 3D lineage tracing of individual ORS cells entering the bulb.
- In vivo or perfused human-scalp validation, particularly in pigmented and androgen-sensitive follicles.
- A viscoelastic multilayer model fitted prospectively to perturbation experiments rather than only to untreated velocity fields.
Bottom line
The paper convincingly changes the description of the human hair follicle from a static stack of proliferating layers into a system of coordinated, counter-flowing cellular streams. Its strongest discoveries are the spiral-like downward ORS migration, the layer-specific upward velocities and the spatial coupling of mitosis to those flows.
It also provides strong evidence that acute matrix proliferation is not the sole—or even immediate—driver of hair-fibre extrusion, and that actin-dependent mechanics are essential. However, it does not yet prove that downward migration of ORS cells specifically generates an upward pulling force. The safest conclusion is:
Human hair-fibre extrusion contains a major actin-dependent mechanical component located in or near the outer follicular layers, while matrix proliferation primarily replenishes the material being extruded.
That is still an important and potentially field-shifting result, but the precise molecular motor and anatomical site of force generation remain unresolved.