Discussion on how pattern baldness affects the hair growth cycles.

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Home >> Hair Biology >> Hair Growth Cycle
Hair Growth Cycles in Pattern Baldness

Pattern baldness or androgenetic alopecia is a common problem affecting both males and females. It affects the normal hair growth cycles in both the sexes and causes baldness.

The only difference between male and female pattern baldness is in the pattern of hair loss. In males, the hair loss occurs in a well-defined pattern. It typically starts above both temples and gradually the hairline recedes to form a characteristic M shape. Hair loss is also spotted at crown (near the top of the head) and it often results in either partial or complete vertex baldness.

In women the hair becomes thinner all over the top of the head and the hairline does not recede. Androgenetic alopecia resulting into total baldness is rare among women.

Causes of androgenetic alopecia

Androgenetic alopecia is triggered by increased binding of androgens to the androgen receptors in the dermal papilla of sensitive hair follicles and this ultimately brings a change in the structure of the hair follicle. The resulting hair produced from the hair follicle is shorter and finer and it provides less complete scalp coverage. Progressive shrinking of the dermal papilla results in less and less nutrition for these shortened hair follicles. The later ultimately fall creating bald spots on the scalp.

Hair growth is a cyclical process with three alternating phases anagen, catagen and telogen phase. Anagen lasts for 35 years, catagen for 2 weeks and telogen for 3 months. The usual ratio of anagen hair to telogen hair is 12:1.

Normal hair shedding occurs within the telogen or the resting phase. The telogen phase can be subdivided into exogen (the proper resting phase) and the latent phase (the transitional phase between telogen and anagen).


Androgen concentration

Androgenetic alopecia witnesses quite a high level of androgen concentration. The influence of these hormones on dermal papilla cells alters the growth factors produced in the hair follicle.

The alteration of growth factors causes a decrease in the anagen length with each cycle whereas the telogen length remains constant or is prolonged. It results in a reduction of the anagen to telogen ratio and the ratio of new hair to resting hair over the scalp. The progression of the disease finds more and more hairs in the resting phase. This prolonged resting phase often makes balding patients to experience periods of excessive hair shedding.


Reduction in anagen duration

The growth rate of hair remains relatively constant. The maximum hair length is determined by the duration of the anagen growth. Therefore with each successively foreshortened hair cycle the length of each hair shaft is reduced. Ultimately the anagen duration becomes too short for the growing hair to get sufficient length to reach the skin surface.

The above development results in an empty follicular pore. The gradual reduction in the hair follicles contributes finally to the balding process.

Hair shaft

A miniaturized hair follicle has more than tenfold reduction in its overall cell numbers. The reason for the change is somewhat obscure though it is often related to dermal adhesion. That leads to dermal papilla fibroblasts dropping off into the dermis, or dermal papilla cells migration into the dermal sheath associated with the outer root sheath of the hair follicle.

It is these changes that cause alteration of hair follicular function. Smaller follicles result in finer hairs. The hair shift diameter reduces from 0.08 mm to less than 0.06 mm. There is also a pigment production. On the scalp, which is gradually balding, three types of hairs are seen -- the terminal hair, the thin and short vellus hair, transitional indeterminate hairs which are the bridge between full-sized terminal hairs and miniaturized hairs.
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