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General |
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The
hair follicles start to develop in
the third month after conception.
We are born with on average 100,000
hair follicles on our head. On the
head of a newborn, there is an average
of 1,000 hair follicles per cm²
. At the age of 25, this number has
diminished to about 500 hair follicles
per cm². Between the ages of
30 to 50 years, the hair density decreases
to about 300 hair follicles per cm².
If there is no question of a balding
process, the hair density will decrease
mildly as you age. The hair follicles
grow in arbitrary groups, divided
over the scalp. These groups are called
" follicular units ". These
follicular units exist of between
1 and 5 hairs. The average number
of hairs per follicular unit is 2
to 3 hairs.
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The
anatomy of the hair
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A
hair consists of keratin, a heavily
structured protein. It is the same
kind of protein which is responsible
for the epidermis and the composition
of our nails. Keratin is a molecule
that is structured from smaller units,
namely amino acids. The structure
of these amino acids is comparable
to the structure of a chain. The diameter
of a single string of hair varies
from person to person. The average
diameter is between 0.05mm and 0.09mm.
The hair follicle
The hair root is the part of the hair
that finds itself in the follicle
or hair sac under the skin, and is
the living part of the hair. The outer
part of the hair follicle, which is
called the hair bulb, is the part
that is nestled in the skin. In the
basis of each hair bulb is the dermal
papilla, which contains capillaries,
or tiny blood vessels. The hair follicles
are covered in the skin with inner
and outer sheaths. The outer sheath
of a hair follicle goes all the way
up to the epidermis or the layer of
overskin.
The hair shaft
The hair shaft is the part of the
hair that is visible above the skin
and is technically dead, with no blood
vessels or nerves. This also explains
why we feel no pain when we get our
hair cut.
The
hair shaft consists of 3 layers:
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The
inner layer or the medulla (marrow)
consists of a soft keratin, which
is only present in thick, broad
hairs. |
| 2. |
The
middle layer, better known as
the cortex, is responsible for
the strength, elasticity and texture
of the hair. The cortex also produces
the substance of melanin, which
determines the natural hair color. |
| 3. |
The
outer layer is called the cuticle,
also known as layer of scales.
The cuticle is thin and colorless
and is composed of 6 to 10 overlapping
separate layers. They serve to
protect the cortex. |
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The
hair growth cycle
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Human
hairs do not grow evenly and constant.
They grow in phases, in always returning
cycles. A single hair cycle consists
of 3 phases:
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anagen phase |
The
catagen phase |
The
telogen phase |
The anagen phase or the growing phase
The
first phase, or growing phase, is also
known as the active phase, because the
cells in the roots of the hair only
produce hair in this phase. In this
phase, the hair bulb finds itself within
the skin, and it will remain there until
the resting phase. Around 85% of all
hairs are in the growing phase at the
same time. The anagen phase or growing
phase can last for 2 to 6 years. On
average, the hair will grow around 10cm
per year, and an individual hair can
become longer than 1 meter. The hairy
scalp consists of around 1 million hair
follicles / hair roots, but on average
only 100,000 up to 150,000 hairs are
visible, which is how the portion of
non-active hair follicles can be estimated.
The catagen phase or the transitional
phase
At the end of the growing phase, the
hairs are in the transitional phase,
which lasts for around 1 to 2 weeks.
During this phase, the hair follicles
prepare themselves for the resting period.
Around 2% of all hairs are in the transitional
phase simultaneously. During this phase,
the production of hair stops and the
hair follicle shrinks to around 1/6th
of its normal length.
The telogen phase or resting phase
The resting phase follows the transitional
phase and lasts for on average 5 to
6 weeks. During this period, the hairs
do not grow but they do stay attached
to the follicle, while the dermal papilla
below is in a phase of resting. Around
13% of all hairs are in the resting
phase simultaneously - however, this
percentage may vary from 4% up to 24%.
At the end of this phase, the aged hair
will fall out and the newly formed hair
will find its way to the outside of
the skin, and will remain there for
several years. The hair bulb will return
to its place and the hair will grow
further in a healthy manner.
During a human life, this hair growth
cycle will repeat itself on average
20 times from birth on. |
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