The basic answer is that there are different hair types, which are
affected in different ways by the factors influencing baldness. This
all revolves around the response to male hormones. Some types of hair
are stimulated to grow, some types become thinner and some do not
respond at all to the hormones. It is suggested that different genes
are expressed by hair follicles in different parts of the body, which
will account for these differences.
Here are some further details with references. Most of the
information is from the keratin.com site of Kevin J. McElwee, which
contains all you want to know about hair and much more besides.
The normal hair growth cycle has three main phases: anagen, catagen
and telogen. The anagen phase is when the hair is growing actively.
This usually lasts about 6-10 years and at any time about 90% of hair
follicles will normally be in anagen phase. Then, during catagen, the
dermal papilla cells start to become inactive and the hair stops
growing. This is the telogen phase. In telogen, the hairs become
weaker and can be pulled out more easily during activities such as
combing and shampooing. A resting hair follicle can once more enter
into the anagen phase. If the old hair has survived, it will be pushed
out of the follicle by the new growth.
Information from: Human hair growth cycle
Humans have more than one type of hair, and each type differs in its
response to hormones. For example:
Many hair follicles take little notice of the general levels of
hormones those follicles of the eyebrows, lashes, and extremities of
the hands and feet are generally consistent in their production of
fiber regardless of hormone concentration in males and females. A
second category would include hair follicles responsive to female
hormone levels (low androgen concentration). These hair follicles are
generally limited to the pubic and arm pit areas plus some hair
follicles on the limbs and chest. A third category would include those
hair follicles responsive to male concentrations of androgen hormones.
Hair regions would include the beard and moustache, nasal hair, upper
back hair, chest hair, and scalp vertex hair.
However, sensitivity to hormones does not have the same results in
different hair types.
Beard hair, for example, is stimulated to grow more by male hormones,
while the growth of scalp hair is inhibited.
Information from: Types of hair fiber/follicle
Male hormones affect the normal hair growth cycle described above.
Women also produce male hormones, and so are also affected.
Despite its standard name of male pattern baldnessandrogenetic
alopecia is also the most common form of hair loss in women.
Androgenetic alopecia develops as a gradual reduction of scalp hair
follicle size, and reduced time in the anagen active growth phase,
leading to more hair follicles in the telogen resting stage of the
hair cycle. In men, the hair loss is limited to the top of the head
and can involve thinning and/or receding hair lines. In women the
presentation is different with just diffuse thinning over the top of
the head and sometimes thinning over the entire scalp
From Introduction to androgenetic alopecia
Over time, the effect of the hormones is to cause the anagen phase to
become shorter and shorter. Even when the follicles enter the anagen
phase again, they remain smaller: 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
From Pattern baldness affected hair cycle
However, numerous factors, and not just hormone levels, will determine
how and whether hair is affected by male hormones. These factors
* Concentration of low and high potency androgens in the blood
* Concentration of enzymes in the hair follicles that can convert
low potency hormones (testosterone) to high potency hormones
* Hair follicles that are androgen independent or androgen
* Androgen dependent hair follicles that respond with
proliferation or with miniaturization and inactivity
* Number of androgen receptors in androgen dependent hair
* Sensitivity of androgen receptors in androgen dependent hair
* Nature and concentration of factors induced by androgen receptor
* Androgen antagonist activity (such as sex hormone binding
* The rate of androgen metabolism/breakdown and removal from the
Information from: Can naturally high testosterone levels trigger
These factors differ between different hair types on different parts
of the body, and also between individuals.
For example, when scientists compared dermal papilla cells in hair
follicles from balding scalps with those from non-balding scalps, they
found that the cells from balding scalps had more receptors for male
hormones than cells from non-balding scalps. A higher level of
receptors is likely to increase sensitivity to the substance that
binds with the receptor, in this case, the male hormones.
Journal of Endocrinology 1998, Vol 156, pages 59-65
Balding hair follicle dermal papilla cells contain higher levels of
androgen receptors than those from non-balding scalp.
Hibberts NA, Howell AE, Randall VA.
Department of Biomedical Sciences, University of Bradford, UK.
The target tissue androgens, testosterone, and dihydrotestosterone
can circulate systemically to skin or can be formed locally in hair
follicles and sebaceous glands by specific enzymes in the steroid
.. The levels of these enzymes differed between men and women, and
from frontal versus occipital sites within the same patient,
indicating that similar steroid mechanisms may be taking place in men
and women, but the amount or level of enzymes vary, perhaps explaining
why men have more severe patterns of hair loss than women.
Skin Pharmacology, 1994, Vol 7 pges 5-7.
Biochemical mechanisms regulating human hair growth.
SUNY Brooklyn Health Science Center.
These authors suggest that the differences occur because hair
follicles are induced to express different genes during embryonic
development depending to which part of the body they are assigned:
Androgens are the main regulator of normal human hair growth. After
puberty, they promote transformation of vellus follicles, producing
tiny, unpigmented hairs, to terminal ones, forming larger pigmented
hairs, in many areas, e.g. the axilla [arm pit] However, they have no
apparent effect on the eyelashes, but can cause the opposite
transformation on the scalp leading to the replacement of terminal
hairs by vellus ones and the gradual onset of androgenetic alopecia.
.. A mesenchyme-derived dermal papilla enclosed within the hair bulb
at the base controls many aspects of follicle function. In the current
hypothesis for androgen regulation, the dermal papilla is also
considered the main site of androgen action
.. All dermal papilla cells from androgen-sensitive sites contain
low capacity, high affinity androgen receptors. However, only some
cells formed 5alpha-dihydrotestosterone, e.g. beard but not axillary
cells, in line with hair growth in 5alpha-reductase deficiency.
Incubation with androgens also stimulated the mitogenic capacity of
beard cell media but inhibited that produced by scalp cells. This
suggests that the paradoxical differences are due to differential gene
expression within hair follicles, presumably caused during
Hormone Research 2000, Vol 54: pages 243-50.
The hair follicle: a paradoxical androgen target organ.
Randall VA, Hibberts NA, Thornton MJ, Hamada K, Merrick AE, Kato S,
Jenner TJ, De Oliveira I, Messenger AG.
Department of Biomedical Sciences, University of Bradford, UK
Search strategy: 1. androgenetic alopecia mechanisms on Medline
2. male "hair loss" head body
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