Hair Follicle Regeneration and Ageing
Key Takeaways
- A hair follicle repeatedly rebuilds its lower, hair-producing portion through growth, regression, and rest; this normal cycling is different from creating a new follicle. [1] [2]
- Regeneration depends on communication among epithelial stem cells, their progeny, the dermal papilla, extracellular matrix, nerves, adipose tissue, vasculature, and immune cells. [1] [3]
- Ageing can alter both stem-cell behaviour and the niche, producing longer resting periods, weaker activation, changes in lineage fate, and less coordinated growth in mouse models. [4] [5] [6]
- De novo follicle formation after wounding is well demonstrated in mice, but it is not equivalent to routine human scalp renewal and has not established a general method for restoring aged human follicles. [9] [10] [12]
Hair follicles are small epithelial-mesenchymal organs that undergo repeated structural remodelling. During each cycle, relatively quiescent stem cells and early progenitors generate the transient lower follicle, whose matrix cells produce a new hair shaft under signals from the dermal papilla. Much of the permanent upper follicle remains in place while the lower portion grows and regresses. [1] [2] [3]
Who This Is Useful For
This page is useful for readers who want to distinguish hair cycling, age-related follicle decline, hair greying, repair after injury, and the experimental creation of new follicles. It also provides a framework for interpreting studies that use mice, cultured human follicle cells, or human scalp samples.
Cycling Is Regeneration, but Not Follicle Neogenesis
In anagen, the lower follicle expands and produces a shaft; in catagen, much of this cycling compartment undergoes controlled regression; and in telogen, the follicle rests before renewed activation. Human follicles follow this sequence asynchronously, and human scalp anagen is much longer than the rapidly coordinated hair-growth waves commonly studied in mouse skin. [1] [2]
This cycle reuses an existing follicular architecture and its resident niches. By contrast, follicle neogenesis requires epithelial and mesenchymal cells to recreate the spatial organisation of a new appendage. The distinction matters because reactivating an existing resting follicle is biologically different from replacing a follicle that has been lost. [3] [9] [12]
Regenerative Components at a Glance
| Component | Role in Cycling | Age-Related Evidence | Evidence Limit |
|---|---|---|---|
| Epithelial stem cells | Bulge and hair-germ populations supply progenitors that rebuild the cycling follicle [1] | Aged mouse cells show delayed activation, stronger quiescence, and altered fate or maintenance in several models [4] [5] | Markers and age effects do not map identically between mouse pelage and human scalp follicles [1] [2] |
| Dermal papilla | This mesenchymal niche regulates progenitor activity, hair-shaft properties, and the frequency of regeneration [3] | Changes in niche-cell number or signalling can alter follicle size and cycling in experimental systems [3] | Experimental depletion is not a measurement of the normal contribution of papilla ageing in humans |
| Extracellular matrix | Provides attachment, mechanical context, and signals that help maintain stem-cell state [1] [7] | The hair-follicle stem-cell niche stiffens with age in mice, reducing regenerative potential through altered chromatin accessibility [7] | The causal experiment was performed principally in mouse follicles and engineered matrices |
| Inflammatory environment | Cytokines and immune cells form part of the wider niche that modulates stem-cell activity [1] | Aged mouse epidermis shows cytokine imbalance associated with poorer follicular stem-cell function and stress tolerance [11] | Inflammatory signatures are context-dependent and do not identify one universal cause of human hair ageing |
| Melanocyte stem cells | Generate pigment-producing cells that colour newly formed hair shafts during anagen [8] | Defective melanocyte stem-cell maintenance and premature differentiation are associated with greying in mice and ageing human follicles [8] | Loss of pigmentation is distinct from loss of the epithelial machinery that produces the shaft |
Ageing Changes Activation as Well as Cell Number
An older follicle is not necessarily one that has simply run out of stem cells. In aged mice, hair-follicle stem cells can be more numerous yet less functional, or remain present while responding slowly to cues that normally initiate growth. Increased BMP-NFATc1 signalling, altered inflammatory signalling, and reduced stress tolerance have each been linked to this lower activation state in particular mouse models. [4] [11]
A separate mouse and human study described progressive loss of COL17A1 in ageing epithelial stem cells. In that model, DNA-damage responses promoted a change in stem-cell fate, terminal epidermal differentiation, and eventual elimination from the skin, accompanying follicle miniaturisation. Forced COL17A1 maintenance prevented this sequence in mice, which establishes a mechanism in that model but not a demonstrated way to reverse ordinary human scalp ageing. [5]
The Older Niche Can Constrain Otherwise Responsive Cells
Follicular stem cells receive cues from neighbouring epithelial cells, dermal fibroblasts, the dermal papilla, immune cells, adipocytes, nerves, vessels, and extracellular matrix. Age-related change in any of these components can therefore affect cycling without requiring irreversible failure inside every stem cell. [1] [3]
In one study, aged mouse hair-follicle stem cells preserved lineage identity and could respond to injury, yet regenerated less effectively within aged skin. Some deficits were reduced in culture or after exposure to a younger dermal environment, supporting a niche contribution. Another mouse study linked age-related matrix stiffening to reduced accessibility of genes needed for stem-cell activation. These results show environmental plasticity in experimental systems, not established rejuvenation of aged human follicles. [6] [7]
Hair Greying Is a Related but Distinct Stem-Cell Problem
Epithelial stem cells rebuild the structure that makes a hair shaft, whereas melanocyte stem cells replenish the pigment-producing lineage. Work using ageing human follicles and lineage-traced mice linked greying to incomplete maintenance of melanocyte stem cells and inappropriate differentiation within their niche. A follicle can therefore continue to produce a shaft after pigment regeneration has declined. [8]
De Novo Follicle Formation After Wounding
Large full-thickness wounds in adult mice can form new follicles in their centres. Lineage and signalling experiments showed that this wound-induced hair neogenesis partly recapitulates embryonic development and requires Wnt signalling after re-epithelialisation. The resulting follicles establish stem-cell compartments, make shafts, and enter later cycles. [9]
Immune context is one reason that this finding does not transfer directly across species. In mice, dermal gamma-delta T cells provide FGF9 that amplifies Wnt activity and follicle formation after wounding; human skin lacks a comparably abundant resident dermal gamma-delta T-cell population. Reviews therefore treat wound-induced neogenesis as a valuable regeneration model while emphasising substantial unresolved barriers to controlled human application. [10] [12]
Evidence Quality and Interpretation
Confidence is strong that hair follicles are cyclically regenerated by coordinated epithelial and mesenchymal compartments, and that human follicles pass through recognisable anagen, catagen, and telogen states. This is supported by lineage experiments, histology, transplantation studies, and direct analysis of human scalp follicles. [2] [3]
Confidence is also strong that ageing changes follicular stem cells and their niches, but the relative importance of individual mechanisms in normal human scalp ageing remains less certain. Many causal perturbations use synchronously cycling mouse back skin, whereas human scalp follicles cycle asynchronously and remain in anagen for much longer. [1] [2] [4]
Evidence for controlled de novo regeneration of aged human follicles is more limited still. Mouse wound neogenesis demonstrates biological possibility, but species-specific immune composition, wound scale, and difficulty preserving human dermal-papilla inductive identity outside its native architecture remain translational constraints. [3] [10] [12]
What This Does Not Mean
- It does not mean every age-related change in hair density is caused by one stem-cell defect; intrinsic cell state, niche signals, inflammation, matrix mechanics, hormones, and disease can overlap. [1]
- It does not mean a resting follicle has disappeared, because telogen is a normal phase of the regenerative cycle. [2]
- It does not mean greying and shaft loss are the same process; they involve related but distinct stem-cell lineages. [8]
- It does not mean a pathway that increases mouse hair growth or wound neogenesis has been shown to regenerate aged human scalp follicles. [9] [10] [12]
Related Reading
Summary
Hair-follicle ageing reflects changes in a regenerative system rather than failure of a single switch. Epithelial and melanocyte stem cells, dermal-papilla cells, extracellular matrix, immune signals, and the wider skin environment contribute different parts of cycling and pigmentation. Existing follicles retain substantial regenerative machinery, but ageing can make activation slower, niches less supportive, and follicle maintenance less stable. De novo follicle formation remains most clearly established as an experimental wound response in mice. [1] [5] [6] [9]
References
- Jang, H. et al. "Aging of hair follicle stem cells and their niches." BMB Reports (2023). https://pmc.ncbi.nlm.nih.gov/articles/PMC9887102/
- Oh, J. W. et al. "A Guide to Studying Human Hair Follicle Cycling In Vivo." Journal of Investigative Dermatology (2016). https://pmc.ncbi.nlm.nih.gov/articles/PMC4785090/
- Morgan, B. A. "The Dermal Papilla: An Instructive Niche for Epithelial Stem and Progenitor Cells in Development and Regeneration of the Hair Follicle." Cold Spring Harbor Perspectives in Medicine (2014). https://pmc.ncbi.nlm.nih.gov/articles/PMC4066645/
- Keyes, B. E. et al. "Nfatc1 orchestrates aging in hair follicle stem cells." Proceedings of the National Academy of Sciences (2013). https://pmc.ncbi.nlm.nih.gov/articles/PMC3870727/
- Matsumura, H. et al. "Hair follicle aging is driven by transepidermal elimination of stem cells via COL17A1 proteolysis." Science (2016). https://pubmed.ncbi.nlm.nih.gov/26912707/
- Ge, Y. et al. "The aging skin microenvironment dictates stem cell behavior." Proceedings of the National Academy of Sciences (2020). https://pmc.ncbi.nlm.nih.gov/articles/PMC7071859/
- Koester, J. et al. "Niche stiffening compromises hair follicle stem cell potential during ageing by reducing bivalent promoter accessibility." Nature Cell Biology (2021). https://www.nature.com/articles/s41556-021-00705-x
- Nishimura, E. K., Granter, S. R., Fisher, D. E. "Mechanisms of hair graying: incomplete melanocyte stem cell maintenance in the niche." Science (2005). https://pubmed.ncbi.nlm.nih.gov/15618488/
- Ito, M. et al. "Wnt-dependent de novo hair follicle regeneration in adult mouse skin after wounding." Nature (2007). https://www.nature.com/articles/nature05766
- Gay, D. et al. "Fgf9 from dermal γδ T cells induces hair follicle neogenesis after wounding." Nature Medicine (2013). https://www.nature.com/articles/nm.3181
- Doles, J. et al. "Age-associated inflammation inhibits epidermal stem cell function." Genes & Development (2012). https://pmc.ncbi.nlm.nih.gov/articles/PMC3465736/
- Oak, A. S. W., Cotsarelis, G. "Wound-Induced Hair Neogenesis: A Portal to the Development of New Therapies for Hair Loss and Wound Regeneration." Cold Spring Harbor Perspectives in Biology (2023). https://pmc.ncbi.nlm.nih.gov/articles/PMC9899649/
This content is provided for educational purposes only and does not constitute medical advice.