GO:0042633 hair cycle: Phases, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0042633 (hair cycle) describes the cyclical phases of growth (anagen), regression (catagen), quiescence (telogen), and shedding (exogen) in the life of a hair.
• The hair cycle is driven by complex epithelial-mesenchymal interactions and is regulated by a wide array of molecular signals, including Wnt, BMP, FGF, and Shh pathways.
• Immune cells and circadian clock genes are increasingly recognized as critical regulators of hair cycle progression.
• Dysregulation of the hair cycle is linked to common hair loss disorders such as androgenetic alopecia and alopecia areata, as well as to chemotherapy-induced alopecia.
• Accurate staging of the hair cycle is essential for experimental reproducibility and is based on well-defined histological criteria.
• CRISPR-based models (knockout, knock-in, point mutation, overexpression) enable causal testing of hair cycle genes and are supported by EDITGENE services.
Description
The hair cycle (GO:0042633) is a fundamental biological process that governs the cyclical growth, regression, rest, and shedding of hair follicles. This process is not merely a cosmetic concern; it represents a dynamic model of organ regeneration, stem cell activation, and tissue remodeling that has broad implications for developmental biology, immunology, and regenerative medicine. Understanding the hair cycle is essential for researchers studying skin biology, hair disorders, and the molecular mechanisms of cyclical organ transformation. The hair cycle is orchestrated by a complex interplay of signaling pathways, transcription factors, and environmental cues, with each phase characterized by distinct morphological and molecular signatures. Recent advances have highlighted the roles of immune cells, circadian rhythms, and exosomes in modulating hair cycle progression, opening new avenues for therapeutic intervention. This article provides a comprehensive overview of the hair cycle, its regulation, associated genes, and the research methods used to study it, with a focus on CRISPR-based approaches for functional validation.
hair cycle At A Glance
| GO ID | GO:0042633 |
|---|---|
| GO term | hair cycle |
| Ontology | biological_process |
| Synonym | none |
| Major function | Cyclical growth, regression, quiescence, and shedding of hair follicles |
| Key phases | Anagen (growth), catagen (regression), telogen (quiescence), exogen (shedding) |
| Regulatory pathways | Wnt, BMP, FGF, Shh, Notch, and immune signaling |
| Associated disorders | Androgenetic alopecia, alopecia areata, chemotherapy-induced alopecia |
What Is GO:0042633?
The hair cycle (GO:0042633) is defined as the cyclical phases of growth (anagen), regression (catagen), quiescence (telogen), and shedding (exogen) in the life of a hair, which is one of the collection or mass of filaments growing from the skin of an animal, forming a covering for a part of the head or for any part or the whole of the body. In simpler terms, it is the repeating life cycle of a hair follicle, from active growth to rest and eventual shedding, driven by coordinated molecular and cellular events.
Why Is hair cycle Important in Cell Biology?
The hair cycle is important because it serves as a paradigm for understanding how adult stem cells are activated and how organs undergo cyclical regeneration. Dysregulation of the hair cycle underlies numerous hair loss disorders, which affect millions of individuals worldwide and have significant psychological impact. Moreover, the hair follicle is an accessible and well-characterized model system for studying stem cell biology, cell-cell communication, and tissue engineering. Insights into hair cycle regulation can inform the development of therapies for alopecia, and may also have broader implications for wound healing and regenerative medicine.
• Hair cycle disruption is a hallmark of androgenetic alopecia, the most common form of hair loss.
• Alopecia areata is an autoimmune disease that targets anagen hair follicles, leading to hair cycle arrest.
• Chemotherapy often induces premature catagen and hair shedding by disrupting rapidly dividing matrix cells.
• The hair cycle is a model for studying stem cell quiescence and activation in the bulge region.
• Circadian clock genes modulate the hair cycle and may influence the timing of hair growth.
• Exosomes derived from various cell types can promote hair growth by modulating the hair cycle.
• Immune cells, including macrophages and T cells, regulate hair cycle progression and regeneration.
• Understanding the hair cycle aids in the development of treatments for hair loss and skin regeneration.
• Accurate hair cycle staging is critical for reproducibility in dermatological research.
What Happens During hair cycle?
Anagen: The Growth Phase
In simple terms: Anagen is the active growth phase when the hair follicle produces a new hair shaft.
Anagen is characterized by rapid proliferation of matrix cells in the hair bulb and differentiation into the hair shaft. This phase can last for years in humans and is divided into sub-stages (anagen I-VI) based on morphological criteria. During anagen, the dermal papilla signals to epithelial stem cells to initiate and sustain growth, with key roles for Wnt, Shh, and FGF signaling. The duration of anagen determines the maximum length of the hair.
Catagen: The Regression Phase
In simple terms: Catagen is the transitional phase when the hair follicle stops growing and shrinks.
Catagen involves apoptosis-driven regression of the lower follicle, including the matrix and dermal papilla, and the formation of a club hair. This phase lasts a few weeks and is marked by cessation of proliferation and melanocyte activity. Molecularly, catagen is associated with decreased Wnt signaling and increased BMP and neurotrophin signaling. The follicle retracts upward, bringing the dermal papilla to rest below the bulge.
Telogen: The Resting Phase
In simple terms: Telogen is the resting phase when the hair follicle is dormant and the hair is retained.
During telogen, the hair follicle is quiescent, and the club hair remains anchored in the follicle. The dermal papilla is compact and located beneath the bulge, and stem cells in the bulge are maintained in a quiescent state. Telogen can last for months, and its duration varies by body site and individual. The transition from telogen to anagen (activation) is a critical step regulated by factors such as BMP inhibitors and Wnt agonists.
Exogen: The Shedding Phase
In simple terms: Exogen is the active shedding of the old hair shaft.
Exogen is the final phase of the hair cycle, where the club hair is released from the follicle and shed. This phase is distinct from telogen and involves proteolytic degradation of the anchoring structures. Exogen can be influenced by mechanical and chemical factors, and its dysregulation may contribute to hair loss disorders.
Molecular Regulation of Phase Transitions
In simple terms: The shift between hair cycle phases is controlled by a balance of activating and inhibitory signals.
Phase transitions are governed by a complex interplay of signaling pathways, including Wnt/beta-catenin (promotes anagen), BMP (promotes telogen), FGF, and Shh. Immune cells and circadian clock genes also modulate these transitions. For example, macrophage-derived Wnt ligands can activate hair follicle stem cells, while regulatory T cells influence stem cell proliferation. Circadian clock disruption alters the timing of anagen onset and catagen progression.
Key Genes Involved in GO:0042633 hair cycle
The following genes are key regulators of the hair cycle, with roles spanning stem cell activation, signaling, and structural integrity.
| Gene | Major Role | Research Relevance |
|---|---|---|
| WNT3A | Activates Wnt/beta-catenin signaling to promote anagen | Studied for hair follicle stem cell activation |
| CTNNB1 | Beta-catenin, central mediator of Wnt signaling | Essential for hair follicle morphogenesis and cycling |
| BMP4 | Inhibits hair follicle stem cell activation, promotes telogen | Key regulator of quiescence |
| SHH | Sonic hedgehog, regulates anagen progression and follicle growth | Critical for hair follicle development and cycling |
| FGF7 | Fibroblast growth factor, modulates hair follicle growth | Involved in epithelial-mesenchymal interactions |
| TGFB1 | Transforming growth factor beta, promotes catagen | Induces apoptosis in hair follicle regression |
| VDR | Vitamin D receptor, regulates hair cycle | Mutations cause alopecia in humans and mice |
| HR | Hairless, transcriptional cofactor | Mutations cause alopecia universalis |
| KRT5 | Keratin 5, structural component of hair follicle | Marker of basal keratinocytes |
| KRT14 | Keratin 14, structural component of hair follicle | Marker of basal keratinocytes |
| CD200 | Stem cell marker in the bulge | Used to isolate hair follicle stem cells |
| CD34 | Stem cell marker in the bulge | Enriches for hair follicle stem cells |
| LHX2 | Transcription factor in hair follicle stem cells | Regulates stem cell quiescence and activation |
| NFATC1 | Transcription factor downstream of BMP signaling | Maintains stem cell quiescence |
| SOX9 | Transcription factor in hair follicle stem cells | Regulates stem cell maintenance |
| TCF7L2 | Wnt signaling transcription factor | Mediates Wnt effects on hair cycle |
| LEF1 | Wnt signaling transcription factor | Essential for hair follicle development |
| GLI1 | Hedgehog signaling transcription factor | Mediates Shh signaling in hair follicle |
How Is hair cycle Regulated?
The hair cycle is regulated by a complex network of signaling pathways, including Wnt/beta-catenin, BMP, FGF, Shh, and Notch, which act in a stage-specific manner. Immune cells, such as macrophages and regulatory T cells, modulate hair follicle stem cell activity and hair cycle progression. The circadian clock also influences the hair cycle, with clock genes regulating the timing of anagen and catagen. Additionally, exosomes and microRNAs have emerged as regulators of hair growth and cycling.
hair cycle and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| AR | Androgenetic alopecia | Knockout or point mutation in dermal papilla cells |
| VDR | Vitamin D-dependent rickets with alopecia | Knockout mouse model |
| HR | Alopecia universalis | Knockout or knock-in mouse model |
| FOXP3 | Alopecia areata (immune dysregulation) | Knockout mouse model |
| CD8A | Alopecia areata (T cell-mediated) | Knockout or overexpression models |
Androgenetic Alopecia
Androgenetic alopecia is characterized by progressive miniaturization of hair follicles and shortening of the anagen phase, leading to a reduced hair cycle duration. Dihydrotestosterone (DHT) binds to androgen receptors in dermal papilla cells, altering the expression of hair cycle regulators such as Wnt and TGF-beta. This results in premature catagen and increased telogen shedding.
Alopecia Areata
Alopecia areata is an autoimmune disorder in which cytotoxic T cells attack anagen hair follicles, causing hair cycle arrest and hair loss. Immune privilege of the hair follicle is disrupted, leading to presentation of autoantigens and inflammatory signaling that prematurely terminates anagen. Therapies targeting immune pathways, such as JAK inhibitors, can restore hair cycle progression.
Chemotherapy-Induced Alopecia
Chemotherapy agents target rapidly dividing matrix cells in anagen hair follicles, inducing apoptosis and premature catagen. This results in rapid hair shedding (anagen effluvium). Understanding the hair cycle is crucial for developing protective strategies, such as topical cooling or pharmacological interventions.
From hair cycle-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Is gene X required for anagen initiation? | Knockout mouse or conditional knockout |
| Does a specific point mutation in gene Y alter hair cycle timing? | Point mutation knock-in mouse |
| Can overexpression of gene Z prolong anagen? | Transgenic overexpression or viral delivery |
| Where is protein X expressed during the hair cycle? | Tagged knock-in (e.g., GFP) reporter |
| What is the effect of gene W on hair follicle stem cell quiescence? | Inducible knockout in stem cells |
| Can CRISPR activation of gene V promote hair growth? | CRISPRa overexpression model |
How to Study the hair cycle Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Histology | Follicle morphology and cycle stage | Classification of anagen, catagen, telogen |
| RNA-seq | Global gene expression | Identification of cycle-associated genes |
| Single-cell RNA-seq | Cell-type-specific expression | Mapping cell populations in hair follicle |
| Lineage tracing | Stem cell fate and migration | Tracking stem cell progeny during cycling |
| Intravital imaging | Real-time follicle dynamics | Observing growth and regression in vivo |
| Exosome profiling | Cargo of exosomes | Identifying hair growth regulators |
| CRISPR screening | Gene function at scale | Discovering novel hair cycle regulators |
Histology and Hair Cycle Staging
Accurate staging of the hair cycle is performed using histological sections of skin, with criteria established by Müller-Röver et al.. This method allows classification of anagen, catagen, and telogen based on follicle morphology. It is essential for reproducible research and for evaluating genetic models.
Transcriptomics and Single-Cell RNA Sequencing
RNA sequencing of hair follicles at different cycle stages reveals dynamic gene expression changes. Single-cell RNA sequencing has identified distinct cell populations and their signaling interactions during the hair cycle. These approaches uncover novel regulators and biomarkers.
Lineage Tracing and Live Imaging
Lineage tracing using Cre-lox systems in mice allows tracking of stem cell progeny during the hair cycle. Intravital imaging enables real-time observation of hair follicle growth and regression in live animals. These techniques provide spatial and temporal resolution of cellular dynamics.
Exosome and MicroRNA Profiling
Exosomes and microRNAs are emerging as key regulators of hair growth and cycling. Profiling their cargo in different hair cycle phases can identify therapeutic candidates. Functional studies using exosome delivery or microRNA mimics/inhibitors can validate their roles.
How CRISPR Can Be Used to Study GO:0042633 hair cycle
Knockout
CRISPR knockout (KO) is used to ablate candidate genes in hair follicle cells or mouse models to determine their requirement for hair cycle progression. For example, KO of Vdr or Hr in mice results in alopecia, confirming their essential roles. Conditional KO allows spatial and temporal control, avoiding developmental lethality.
Point Mutation
Point mutations can be introduced via CRISPR base editing or homology-directed repair to model specific human variants associated with hair disorders. This approach helps distinguish pathogenic mutations from benign polymorphisms and reveals mechanistic insights into protein function.
Knock-in
Knock-in of reporter genes (e.g., GFP) or epitope tags enables visualization and purification of specific cell populations or proteins during the hair cycle. Knock-in of human disease alleles into mouse models facilitates translational research.
Overexpression
CRISPR activation (CRISPRa) or transgenic overexpression allows gain-of-function studies to test whether a gene is sufficient to promote hair growth or alter cycle timing. Overexpression of Wnt ligands or growth factors can prolong anagen and enhance hair regeneration.
How EDITGENE Supports hair cycle Research
Researchers studying hair cycle-related genes often need to determine whether a candidate gene is causally involved in hair follicle cycling or is merely a bystander. EDITGENE provides a comprehensive suite of CRISPR-based services to enable precise genetic manipulation in relevant cell models and animal models, accelerating functional validation and therapeutic development.
Contact EDITGENE today to design your custom CRISPR model for hair cycle research.
Frequently Asked Questions About hair cycle
What is the hair cycle (GO:0042633)?
The hair cycle is the cyclical process of growth (anagen), regression (catagen), quiescence (telogen), and shedding (exogen) in the life of a hair follicle.
What are the phases of the hair cycle?
The hair cycle consists of four main phases: anagen (growth), catagen (regression), telogen (rest), and exogen (shedding).
What genes are involved in the hair cycle?
Key genes include WNT3A, CTNNB1, BMP4, SHH, FGF7, TGFB1, VDR, HR, and many others that regulate stem cell activation and follicle growth.
How is the hair cycle regulated?
The hair cycle is regulated by signaling pathways such as Wnt, BMP, FGF, and Shh, as well as immune cells and circadian clock genes.
What diseases are associated with hair cycle disruption?
Disorders include androgenetic alopecia, alopecia areata, and chemotherapy-induced alopecia.
How do researchers study the hair cycle?
Methods include histology for staging, RNA-seq, single-cell RNA-seq, lineage tracing, and CRISPR-based genetic models.
What is anagen?
Anagen is the active growth phase of the hair cycle, during which the hair follicle produces a new hair shaft.
What is telogen?
Telogen is the resting phase of the hair cycle, when the hair follicle is quiescent and the club hair is retained.
Can CRISPR be used to study hair cycle genes?
Yes, CRISPR knockout, knock-in, point mutation, and overexpression models enable functional studies of hair cycle genes.
What services does EDITGENE offer for hair cycle research?
EDITGENE provides CRISPR knockout, point mutation, knock-in, overexpression, library screening, and bioinformatics services for hair cycle research.
Conclusion
The hair cycle (GO:0042633) is a dynamic and tightly regulated biological process that serves as a model for stem cell biology and organ regeneration. Understanding its molecular controls is essential for developing therapies for hair loss disorders and for advancing regenerative medicine. CRISPR-based models and EDITGENE services provide powerful tools to dissect the genetic basis of hair cycling and to translate findings into clinical applications.
References
- 1. Müller-Röver S et al.. 2001. A comprehensive guide for the accurate classification of murine hair follicles in distinct hair cycle stages.. J Invest Dermatol 117(1):3-15 PMID: 11442744
- 2. Zhou Y et al.. 2024. Exosomes for hair growth and regeneration.. J Biosci Bioeng 137(1):1-8 PMID: 37996318
- 3. Wang ECE et al.. 2020. Immune cell regulation of the hair cycle.. Exp Dermatol 29(3):322-333 PMID: 31903650
- 4. Stenn KS et al.. 2001. Controls of hair follicle cycling.. Physiol Rev 81(1):449-494 PMID: 11152763
- 5. Ji S et al.. 2021. Functional hair follicle regeneration: an updated review.. Signal Transduct Target Ther 6(1):66 PMID: 33594043
- 6. Niu Y et al.. 2023. Overview of the Circadian Clock in the Hair Follicle Cycle.. Biomolecules 13(7) PMID: 37509104
- 7. Oh JW et al.. 2016. A Guide to Studying Human Hair Follicle Cycling In Vivo.. J Invest Dermatol 136(1):34-44 PMID: 26763421
- 8. Messenger AG. 2011. Hair through the female life cycle.. Br J Dermatol 165 Suppl 3:2-6 PMID: 22171678
