Human
hair growth cycle
Under normal circumstances hair growth in each hair follicle
occurs in a cycle. There are three main phases of the hair growth
cycle;
anagen, catagen and telogen with anagen further subdivided into
proanagen, mesanagen and metanagen. Anagen is the active growth
phase when hair fiber is produced. Proanagen marks initiation
of growth with RNA and DNA synthesis in a follicle which then
quickly
progresses through mesanagen to metanagen and maximum follicle
length and girth. In this mature state of proliferation and differentiation
the hair follicle consists of a total of eight concentric layers
of different cell types and melanogenesis occurs within pigmented
hair follicles. Anagen
is followed by catagen, a period of controlled regression of the
hair follicle. Ultimately the hair follicle enters telogen, when
the follicle is in a so-called resting state.
Anagen is the longest phase with up to 90% of follicles on a
normal human scalp in this active hair growth state at any given
time and correspondingly telogen hair follicles comprise up to
10% on the scalp. The average rate of hair fiber growth is around
0.35mm a day but this rate varies depending on the site of the
hair follicle and the age and sex of the individual. The length
of the anagen growth phase for scalp hair is usually 3-6 years
while telogen lasts just 30-90 days and catagen is best estimated
at 14-21 days. In many young mammals the anagen growth phase occurs
in a wave like pattern across the skin surface, but the hair follicles
of humans can run through the normal cycles of growth apparently
independently of neighboring follicles.
Normally this cycle of hair production and inactivity will continue
for the duration of the individual's life but other factors can
influence and inhibit hair production and in some cases lead to
physical destruction of the hair follicle. Factors may include
adverse reactions to drugs and cosmetics, or as a result of scarring,
tumors, radiation, the genetics of the individual, hormones and/or
their immune system.
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| Diagram showing regression of a mature
anagen hair follicle. On entering catagen the dermal papilla
condenses as the cells become inactive. With a lack of dermal
papilla cell stimulation, the hair fiber and root sheaths
stop growing. In telogen the dermal papilla can become isolated
in the dermis and the hair fiber can easily be pulled out
(by combing, shampooing, or brushing) |
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| Diagram showing a resting hair follicle
returning from resting telogen to growing anagen. If the old
fiber has not already fallen out it is pushed out by the new
hair fiber growing underneath |
Non-human
hair growth cycle
The normal cycle of growth and rest in human hair follicles occurs
in a mosaic pattern. That is, each individual hair follicle determines
its own anagen growth time period and cycles without paying much
attention to the growth activity of neighboring hair follicles.
In other species a different pattern of hair growth may occur where
the hair growth activity in one hair follicle affects the growth
activity in neighboring follicles. For example, in rodents (except
guinea pigs which have a mosaic hair growth pattern), hair growth
over the skin occurs in a wave pattern. From birth onwards rodent
hair follicles go through a "Mexican wave" of growth and rest. Starting
at the head, the wave of growth moves backwards to the tail and
from the belly to the back. This happens every few weeks. This is
very different to what occurs in humans.
Unlike humans rodent hair follicles communicate with each other.
One hair follicle that is growing takes note of what its neighbors
are doing and responds accordingly. After a period of growth the
hair follicle returns to its resting state. Because of this difference
in hair growth pattern, rodents' hair follicles are mostly in
the telogen state. Only around 10% of rodent hair follicles are
in
anagen
at any one time and the period of growth only lasts a week or
two. As the growth wave passes, the hair follicles enter the
resting telogen
stage but unlike humans the hair does not fall out. It stays in
place wedged in the hair follicle. The hair then gets pushed
out
as the next wave of growth runs by. This difference in growth
pattern has important consequences for any laboratory tests on
hair growth
treatments that involve rodents - a fact many pharmaceutical companies
seem to ignore.
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| Because rodent hair
follicles grow in a wave pattern it is easy to find several
hair follicles in the same stage of hair growth located together.
The top picture shows mouse hair follicles in telogen. The middle
picture shows hair follicles in the earliest stages of anagen
where the hair follicles are just reforming and growing down
into the dermis. At the bottom is a picture of mouse hair follicles
in a mature anagen state. These pictures are all taken at the
same magnification so you can see that there is a considerable
cell growth during the hair cycle to transform from a small
telogen follicle into a large anagen follicle. |
Other species have still more patterns of hair growth. Merino
sheep have hair follicles permanently in the anagen growth stage,
they
do not cycle through growth and rest. The result is Merino sheep
must be regularly sheared regardless of whether the wool is needed
or not. Some species have hair growth cycles that take their cue
from the seasons. Arctic foxes will produce a summer coat and
a
winter coat so their hair follicles have to go through two complete
hair cycles each year and switch the type of hair fiber that
they
produce. What coordinates the change in hair production and how
the seasons act on the hair follicles is not fully understood,
but is probably based on changes in hormones like prolactin and
estrogen in response to changes in daylight.
Hair
growth cycle references
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LS, Coggle JE, Wells J, Charles MW. The influence of the hair
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FA, Titkemeyer CW, Lovell JE.Hair follicle cycle and shedding
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RD, Oliver RF. Morphological changes associated with the growth
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- Young
RD. Morphological and ultrastructural aspects of the dermal papilla
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