Biology of the hair growth cycle and its control

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Duration of the hair growth cycle

Duration of the hair cycle

Hair follicles are of different types and they are localized in different places. These two factors influence the duration of the different phases of hair cycle. Under physiological conditions, 85% of the scalp hair is under the influence of anagen phase and approximately 15% are under the telogen phase. The anagen phase of scalp hair follicles typically lasts for 2-6 years. There are exceptions though; a few individuals experience anagen growth phases for much longer durations. The maximum hair length that scalp hair can grow to is mostly determined by the duration of the anagen phase. Some people can have anagen growth phase lasting 10 years or more. These people - usually women - can grow their hair very long – even down to the ground

The duration of anagen phases of hair follicles in the other parts of the body are different - the anagen phase of the eyebrows is only 70 days long, while for eyelashes it is 100-150 days. The duration of telogen in hair follicles is another determinant factor in the hair growth cycle. The body hair follicles are characterized by an increased telogen frequency and duration as compared to scalp hair follicles – so there are more eyebrow follicles in a telogen resting state and they stay in a dormant state for a longer period of time compared to scalp hair follicles. Under physiological conditions, both anagen and telogen durations are maintained in hair follicles though their lifelong growth cycling. Only if disease develops (alopecia) does the hair cycle rate change.

Hair growth and cycle regulation potential of the dermal papilla

It has long been known by scientists that dermal papilla cells have a potent hair growth inductive ability and they can even promote hair follicle formation under experimental conditions. The size of the anagen hair follicle bulb is regulated by the size and secretory power of its dermal papilla. This determines the diameter of the hair shaft produced by the hair bulb, and ultimately the rate of hair growth. The duration of anagen is also controlled by the dermal papilla. So it comes to this: dermal papilla controls the potential maximum length of hair fiber, as well as the rate of hair growth. However it is still unknown as to what determines the duration of anagen within dermal papilla cells.

What is involved in the biochemical clock mechanism inside dermal papilla cells has been considered for a long time and a number of hypotheses have been suggested. Several different concepts - still in the stages of controversy - suggest an accumulation of an endogenous inhibitor. The chemical may build up during anagen and, at a threshold level, it inhibits mitosis thereby causing transition of the hair follicles to a catagen phase. Dictation of hair cycling length by the (limited) number of mitoses of slow-cycling epithelial cells in response to mesenchymal signals are being discussed as well as the presence of so far unidentified endogenous oscillating signals or embryonic pathmakers. Which is involved is still completely unproven.

Dermal papilla induction of new hair follicles

The possibility of the epithelial component’s being entirely passive in determining the size and shape of hair follicles has largely been ruled out. It has been confirmed that the mesenchymal signalling component plays a predominating role in the size and growth cycle of the hair follicle. Some investigators have made use of the dermal papilla cells at the base of an anagen hair follicle - either fresh or after tissue culture expansion - to show that follicular dermal papilla cells can induce brand new hair follicle formation in rodents if placed in proximity to non-follicular epithelium. This process has been termed as the follicular neogenesis model or hair follicle replication or hair multiplication.

Other research has involved the incorporation of labelled cultured dermal papilla cells and cells from the bulbar connective tissue sheath of rodent vibrissae into resident pelage hair follicles, to show that they make them larger and prolong the duration of anagen. It proved that introduced cells could override the inherent properties of resident dermal papilla cells. This process has been called the morphogenetic switch model, follicular amplification, or in the case of damaged follicles, follicular reactivation. However there is only one published study so far that claims follicular replication is possible in humans. Reynolds et al transplanted microdissected connective tissue sheath from the scalp of a man onto the forearm of a woman. New follicles were sprouted at the site of transplantation with the mesenchymal portion arising from the transplanted donor cells just after three to five weeks. These and other studies clearly prove that dermal papilla cells know beforehand what size and shape of hair follicle they should form and they also retain a memory of their hair cycle clock.

hair follicle bulb diagram
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