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Hair is
far more complex than it appears on the surface. We all know that it not only
plays a vital role in the appearance of both men and women, but it also helps
to transmit sensory information as well as create gender identification.
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THE
ORIGINS OF HAIR
By week
22, a developing foetus has all of its hair follicles formed. At this stage of
life there are about 5 million hair follicles on the body. There are a total of
one million on the head, with one hundred thousand of those follicles residing
on the scalp. This is the largest number of hair follicles a human will ever
have, since we do not generate new hair follicles anytime during the course of
our lives.
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Most people will notice that the density of scalp hair is reduced as
they grow from childhood to adulthood. The reason: Our scalps expand as we
grow.
THE
TYPES OF HAIR
The head
contains two types of hair. One is called ‘Vellus’ which is fine and soft that
is almost invisible. The other is called ‘Terminal’ and this is what you see on
your head. It can grow long and is relatively coarse and more pigmented.
Vellus
hair can develop into terminal hair, as in the beards of adolescent males.
On
the other hand, terminal hair may become vellus, as happens during the
development of common baldness, a process that is called
‘Miniaturization’.
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THE
FOLLICULAR UNIT
The hair
of the scalp naturally grows in clusters of 1, 2, 3 and seldom, four
hairs. These clusters we call ‘Follicular units’. This small group of hairs is
separated from each other by hairless intervening skin.
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THE
HAIR FOLLICLE
Hair has
two distinct structures - first, the follicle itself, which resides in the
skin, and second, the shaft, which is what is visible above the scalp.
The hair
follicle is a tunnel-like segment of the epidermis that extends down into the
dermis. The structure contains several layers that all have separate functions. At the base of the follicle is the Papilla, which contains capillaries, or tiny
blood vessels that nourish the cells. The living part of the hair is the very
bottom part surrounding the papilla, called the Bulb. The cells of the bulb
divide every 23 to 72 hours, remarkably faster than any other cell in the
body. Two sheaths, an inner and outer sheath, surround the follicle.
These structures
protect and form the growing hair shaft. The inner sheath follows the hair
shaft and ends below the opening of a sebaceous (oil) gland, and sometimes an
apocrine (sweat) gland.
The outer sheath continues all the way up to
the gland. A muscle called an erector pili muscle attaches below the gland to a fibrous
layer around the outer sheath. When this muscle contracts, it causes the hair
to stand up which also causes the sebaceous gland to secrete oil.
The
sebaceous gland is vital because it produces sebum, which conditions the hair
and skin.
After puberty our body produces more sebum but as we age we
begin to make less sebum. Women have far less sebum production than men do
as they age
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THE
HAIR SHAFT
The hair
shaft is made of a hard protein called Keratin and is made in three
layers. This protein is actually dead, so the hair that you see is not a living
structure. The inner layer is the Medulla.
The second layer is the Cortex and
the outer layer is the Cuticle. The cortex makes up the majority of the hair
shaft. The cuticle is a tightly formed structure made of shingle-like
overlapping scales. It is both the cortex and the medulla that holds the hair’s
pigment, giving it its colour. |
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HAIR
GROWTH CYCLES
Hair on
the scalp grows about 0.3 to 0.4 mm/day or about 12 cm per year. Unlike other
mammals, human hair growth and shedding is random and not seasonal or
cyclical. At any given time, a random number of hairs will be in one of three stages of
growth and shedding: Anagen, Catagen, and Telogen.
ANAGEN
Anagen is
the active phase of the hair, and about 90% of all hairs are in this phase at
any time The cells in the root of the hair are dividing rapidly. A new hair is
formed and pushes the club hair (a hair that has stopped growing or is no
longer in the anagen phase) up the follicle and eventually out. During this
phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active
phase of growth for two to six years. Some people have difficulty growing their
hair beyond a certain length because they have a short active phase of
growth. On the other hand, people with very long hair have a long active phase of
growth. The hair on the arms, legs, eyelashes, and eyebrows have a very short
active growth phase of about 30 to 45 days, explaining why they are so much
shorter than scalp hair.
CATAGEN
The
catagen phase is a transitional stage. This phase lasts for about two to three
weeks. Growth stops and the outer root sheath shrinks and attaches to the root
of the hair. This is the formation of what is known as a club hair.
TELOGEN
Telogen
is the resting phase and usually accounts for 10% of all hairs. This phase
lasts for about 100 days for hairs on the scalp and longer for hairs on the
eyebrow, eyelash, arm, and leg. During this phase, the hair follicle is
completely at rest and the club hair is completely formed. Pulling out a hair
in this phase will reveal a solid, hard, dry, white material at the root.
About
100-150 telogen hairs are shed normally each day
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