Hair Growth Cycle Phases
The hair growth cycle consists of four distinct phases: anagen (growth phase lasting 2-7 years), catagen (regression phase lasting 2-3 weeks), telogen (resting phase lasting 2-4 months), and kenogen (empty follicle phase lasting 2-7 months after hair shedding). 1, 2
Anagen (Growth Phase)
The anagen phase represents active hair growth where the follicle is producing the hair shaft. This is the longest phase of the cycle, typically lasting 2-7 years for scalp hair 3, 4. During this phase, hair follicles are firmly anchored and demonstrate strong cohesion forces correlated with hair thickness 5. The duration of anagen determines the maximum length hair can achieve, which explains why scalp hair grows much longer than body hair.
Catagen (Regression Phase)
Catagen is an apoptosis-driven regression phase lasting approximately 2-3 weeks 3, 4. During this brief transitional period, the hair follicle undergoes controlled involution, the hair shaft stops growing, and the follicle begins to detach from its blood supply. This phase marks the end of active hair production and prepares the follicle for the resting phase.
Telogen (Resting Phase)
The telogen phase represents a period of relative quiescence lasting 2-4 months 3, 4. During telogen, the hair remains in the follicle as a "club hair" but is no longer growing. The follicle is preparing for either the next anagen phase or hair shedding. Telogen hair are firmly anchored to the hair follicle and retain cellular elements of the outer root sheath 5.
Kenogen (Empty Follicle Phase)
Kenogen is a physiological interval where the hair follicle remains empty after the telogen hair has been shed (teloptosis) and before a new anagen hair emerges 2. This phase was more recently characterized and represents an alternative route in the hair cycle.
Kenogen Duration and Clinical Significance:
- Normal individuals: Affects approximately 8% of follicles, lasting about 2 months in prepubertal children 2
- Androgenetic alopecia (AGA) patients: Can affect up to 80% of hair cycles in balding males, lasting 4-7 months 2
- Female AGA: Involves 16-22% of follicles, lasting 3-12 months 2, 6
The increased frequency and duration of kenogen in androgenetic alopecia may account for progressive baldness, as more follicles spend extended periods empty rather than producing hair 2, 6. In women with progressing AGA, kenogen phases increase in parallel with vellus hair transformation and decreased normal hair cycles 6.
Clinical Pitfall:
The kenogen phase is often overlooked in clinical practice but is crucial for understanding pattern hair loss. Minoxidil's mechanism of action specifically targets prolongation of anagen and promotes telogen-to-anagen switching, which effectively shortens or bypasses the kenogen phase 1. This explains why minoxidil remains effective across multiple hair loss conditions where follicles are spending excessive time in kenogen rather than producing terminal hairs.