THE LIFECYCLE OF HAIR FOLLICLES
The anagen phase is the longest one in the hair of our head, with duration between 2 and 6 years.
Hair grows in cycles consisting of various phases.
- A long active growth phase, called the anagen phase.
- A transition phase, which lasts 2 to 4 weeks and it is called the catagen phase.
- A resting phase, which lasts 2 to 4 months and it is called the telogen phase.
The anagen phase duration determines the final hair length, which varies depending on the body area. Both the catagen and the telogen phases show less variation among the various parts of the human body.
The hair on the scalp has the longest anagen phase, with a duration of 2-6 years.
Hairs of the scalp are not synchronised, i.e. they are at different growth phases. At any time, the majority of our hair follicles (up to 90%) are in the anagen phase, while a much lower number of hair follicles (around 10%) are in the telogen phase and only 1% in the catagen phase.
New follicles and hair formation at the beginning of the anagen phase is actually a repetition of follicular morphogenesis (formation of the follicle) on the scalp of the fetus. Anagen phase consists of seven stages. During the anagen phase the life of hair is 2 to 6 years, depending on the area, while hair follicles are located in the subcutaneous fat, deep below the main layer of the skin, below the dermis.
Extensive deterioration of the lower part of hair follicles denotes the initiation of the catagen phase. Keratinocytes interrupt suddenly each mitotic activity and undergo apoptosis. The duration of the catagen phase is approximately 2 to 4 weeks. Hair follicles shrink in size, the lower follicle regresses gradually and the outer epithelial sheath disappears.
Telogen and exogen phase
During the telogen phase, hair follicles become inactive, their growth stops and shedding occurs. Telogen phase is a heterogeneous state, given that 1% of telogen hair sheds on a daily basis. Telogen phase, which is also called exogen phase, is a particularly controlled and stable in time process observed in all mammals, while a characteristic is the seasonality of hair shedding. The immediate release of telogen hair could be characterised as an active phase during the lifecycle of hair.
One can easily estimate that if from the total of 100,000 hair we have on our scalp 10% is constantly in the telogen phase and 1% falls every day. Therefore normal hair loss on a daily basis affects approximately 100 hairs.
Typically the period between hair shedding and new hair growing is really short, while in patients with male pattern hair loss it lasts for months.