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Pattern
baldness affected hair cycle
The gradual changes that occur in the androgenetic alopecia affected
scalp skin is the same whether describing male of female baldness.
Androgenetic alopecia does not develop in all hair follicles at
the same time. Some are more quickly affected than others. When
looking at a punch biopsy under the microscope neighboring hair
follicles can be seen to be variably affected. Some will be normal
healthy terminal hairs with an average diameter of 0.06mm, others
will be miniaturized vellus hair follicles with an average diameter
of 0.03mm. So, one parameter of androgenetic alopecia is a decrease
in the density of terminal hairs and an increase in the vellus hair
count. Hairs in an intermediate state between terminal and vellus
hairs will also be observed.
Over time the terminal scalp hair follicles undergo progressively
shorter and shorter cycles involving reduced anagen growth periods.
This applies regardless of whether the hair follicles are terminal,
intermediate, or vellus hairs. Although periods of anagen are reduced
catagen and telogen time periods remain the same. The net effect
is that androgenetic alopecia is characterized by a gradual increase
in the number of resting telogen hair follicles present at any one
time. In unaffected scalp the percentage of hair follicles in telogen
is up to 10%. In the early stages of androgenetic alopecia affected
scalp the number of telogen stage hair follicles can be up to 20%
of the total. As androgenetic alopecia progresses the total number
of hair follicles can be reduced as the hair follicles are irreversibly
destroyed.
In androgenetic alopecia the terminal hair follicles reduce size
both in length and diameter. The hair bulb moves upwards in the
dermis yielding a small vellus hair follicle. These vellus hair
follicles can be affected by fibrosis. Beneath the miniaturized
vellus hair follicle a fibrous tract, known as a streamer or follicular
stela, can be observed marking the terminal hair follicle bulb’s
original position to the base of the current vellus hair dermal
papilla. In many cases of androgenetic alopecia an inflammatory
infiltrate can be seen around the affected hair follicles especially
the upper hair follicles. Normal unaffected scalp hair follicles
can also have mild inflammation but the inflammation in androgenetic
alopecia is often much more intense with many more cells involved.
The diagram below shows a hair follicle passing through anagen to
telogen and then to a new anagen growth phase. This occurs for all
human hair follicles, but as shown below, in pattern baldness the
subsequent anagen hair follicle is smaller than the previous follicle.
As pattern baldness progresses, the hair follicles go through several
hair cycles and with each one the follicles become shorter, finer,
and less pigmented until the initially large terminal hair follicle
has become a small vellus hair follicle.
Pattern
baldness affected hair cycle references
- Kligman AM. The comparative histopathology
of male-pattern baldness and senescent baldness. Clin Dermatol.
1988 Oct-Dec;6(4):108-18.
- Pelfini C, Cerimele D, Pisanu G. Aging
of the skin and hair growth in man. In: Montagna W, Dobson RL
(Eds) Hair growth. Pergamon Press Ltd, London, 1969: p153-160.
- Orfanos CE, Happle R. Hair and Hair Diseases
(eds), Berlin, Springer-Verlag, 1990.
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