Anagen
Anagen is the phase of active hair production, initiated when dermal papilla cells release the Wnt, GAS6, and FGF-7 signals that activate hair follicle stem cells in the bulge and secondary hair germ, driving the regeneration of the follicle’s transient inferior segment from the previous telogen position upward. The matrix – the rapidly proliferating epithelial population encasing the dermal papilla at the base of the newly formed bulb – has the highest mitotic rate of any tissue in the human body, producing the differentiating cells that build the hair shaft and inner root sheath continuously throughout this phase.
Duration and Its Clinical Meaning
Anagen duration varies by body site: scalp hair follicles remain in anagen for two to seven years; eyebrow follicles for three to six months; eyelash follicles for one to six months. This is not a passive biological constant but an actively maintained state, governed by the ongoing balance of Wnt pathway activity, IGF-1 and FGF signalling, and BMP suppression within the dermal papilla. The DP acts as a molecular clock, and anagen ends when its Wnt output progressively declines to a threshold that tips the follicle into catagen.
The practical consequence is that longer anagen means longer potential hair length, more of the follicle population producing hair at any one time, and greater overall scalp density. In androgenetic alopecia, DHT shortens anagen by driving early withdrawal of DP Wnt signalling and upregulation of TGF-β2 – not by killing follicles, but by progressively compressing the productive phase until the follicle miniaturises. In telogen effluvium, the HPA axis-driven suppression of GAS6 output prevents anagen from being initiated in recovered follicles; not a shortening of anagen once started, but a delay in its initiation that compounds across the affected follicle population.
The Anagen Subphases
Anagen is not a single undifferentiated state, it progresses through six subphases (anagen I–VI). In anagen I–III, the secondary hair germ reactivates and the new inferior segment begins elongating downward; the emerging hair shaft is not yet visible above the skin surface. By anagen IV–VI, the new shaft has reached and grown through the infundibulum, and the follicle has reached its full depth in the dermis and subcutis. The visible, measurable hair growth phase that clients experience corresponds to late anagen (IV–VI) but the biological work of re-establishing the follicle architecture begins weeks earlier in the early subphases.
Clinical Application
Most hair treatments – iPRF, CAP, minoxidil, finasteride – are ultimately attempting to do one of two things: extend anagen duration, or support the initiation of a new anagen phase in follicles delayed in telogen. Understanding which goal applies to a given client helps frame both treatment selection and expectations accurately.
For androgenetic alopecia, the goal is anagen extension – counteracting the DHT-driven compression of the DP molecular clock that progressively shortens each successive cycle. iPRF delivers PDGF, IGF-1, and VEGF directly to the DP and perifollicular zone, supporting the molecular environment that sustains Wnt signalling and delays the anagen-to-catagen tipping point. CAP activates Wnt/β-catenin in DPCs directly, targeting the same molecular clock from a different biological angle. Finasteride addresses the DHT source upstream; minoxidil extends anagen through potassium channel opening and perifolicular IGF-1 enhancement. These are not competing approaches, they address the same anagen duration deficit through different mechanisms at different points in the pathway.
For telogen effluvium, the goal is anagen initiation – re-establishing GAS6-mediated HFSC activation in follicles held in extended telogen by glucocorticoid suppression. iPRF timed to the recovery phase, once cortisol normalisation is underway, supports DPC reactivation and provides the growth factor milieu that recovering DPCs need to resume their normal GAS6 and Wnt signalling output. Treating during active glucocorticoid suppression is less productive – the inhibitory signal is still present and the DP is not yet in a position to initiate anagen regardless of what growth factors are available.
For both conditions, the early anagen subphases are invisible. Clients will not see new hair above the surface for several weeks after anagen has actually been initiated. Managing expectations around this lag, and explaining that the treatment is working at the level of the follicle clock and stem cell activation rather than immediately visible growth, is one of the most useful clinical applications of the biology in client conversations.
Laser hair removal: anagen as the primary target
Laser hair removal operates on the principle of selective photothermolysis – the laser emits a wavelength absorbed by melanin, which converts the light energy to heat and diffuses it to the surrounding follicular structures. [1] The two structures destroyed by this heat diffusion are the hair matrix and the bulge stem cells, and it is the bulge destruction that makes laser reduction lasting rather than temporary. Without the bulge’s regenerative capacity, the follicle cannot produce a new inferior segment; without the matrix, it cannot produce a hair shaft in the current cycle. Both targets need to be reached for permanent reduction rather than temporary inhibition. [1]
Anagen is the preferred treatment phase because the melanin concentration in the matrix is at its highest, the follicle is at its greatest depth in the dermis, and the dermal papilla is maximally close to the melanin source, all of which maximise heat delivery to the targets that matter. jcadonline.com It is worth noting that the classical picture of laser working only in anagen was based partly on animal models where follicles lose their melanin almost entirely in telogen. In humans, follicles retain pigmentation throughout the cycle; the anagen advantage is one of degree, with higher melanin concentration producing more efficient energy absorption, rather than an absolute phase-dependent on/off effect. thepmfajournal.com
The clinical consequence of the 15–20% anagen ratio at any given moment is the multiple-session requirement: each session targets the proportion of follicles currently in anagen; successive sessions progressively reduce the remaining active follicle pool until density falls below the threshold of noticeable regrowth. Spacing sessions at intervals matched to the site’s typical anagen cycle – shorter intervals for facial hair with its faster cycling; longer for areas with more extended anagen – ensures each session catches the maximum proportion of follicles that have transitioned into the treatable phase since the last treatment.
References
Vaidya T, Hohman MH, Kumar D D (2026). Laser Hair Removal. StatPearls Publishing. ncbi.nlm.nih.gov/books/NBK507861
Also Known As
- anagen phase
Pathway Connections
Regulators & Triggers
- this Stimulated by Dermal Papilla Evidence: FGF-7 secreted by DPCs that acts… to prolong anagen, protect follicular keratinocytes from apoptosis, and sustain the growth phase
- this Stimulated by Platelet-derived growth factor Evidence: PDGF signalling sustains DPC function required for anagen maintenance; PDGF deficiency causes hair follicle dermal stem cell depletion and disrupted cycling. Chojnacki 2017 PMC5665619; Shin 2024 BBRC.
- this Stimulated by Vascular endothelial growth factor Evidence: VEGF expression in DPCs drives perifollicular vascularisation during anagen; vasculature expands during anagen, regresses during catagen/telogen. Entity text; Yano et al. 2001 JCI PMC199257.
- this Affected by Tissue regeneration Evidence: Tissue regeneration in hair follicles proceeds through anagen re-entry; wound-induced hair anagen and wound-induced follicle neogenesis are models of regeneration where anagen is the productive growth phase (PMC3772651; PMC5378973).
- this Required by Hair follicle Evidence: Entity text: anagen is the phase during which follicle actively produces hair shaft; the follicle functionally requires anagen cycling to fulfil its biological role.
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