What is Hair - the Biology
Like
your fingernails and toenails, hair is part of your
skin. Hair has more functions that most people realise.
It helps transmit sensory information and creates gender
identity. Hair is important to the appearance of men
and women. There is hair on all the major visible surfaces
of the body. It is also the only body structure that
is completely renewable without scarring. Most of the
complex activities that keep your hair growing goes
in below the surface.
Origin
of Hair
A developing fetus has all of it's
hair follicles formed by week 22. At this time there
are 5 million follicles on the body. One million of
those are on the head, and 100,000 are on the scalp.
This is the largest number of follicles we will ever
have - follicles are never added during life. As the
size of the body increases as we grow older, the density
of the hair follicles on the skin decreases.
Anatomy
of Hair
Hair has
two separate structures - the follicle in the skin and
the shaft we see.
Illustration
of Hair Follicle
Follicle
- The follicle is a stocking-like structure that contains
several layers with different jobs. At the base of the
follicle is a projection formed like sticking a finger
in the bottom of a stocking and pushing it in a small
amount. This projection is called a papilla and it contains
capillaries, or tiny blood vessels, that feed the cells.
The living part of the hair is bottom part of the stocking
surrounding the papilla called the bulb. This bottom
part is the only part fed by the capillaries. The cells
in the bulb divide every 23 to 72 hours, faster than
any other cells in the body.
The follicle
is surrounded by two sheaths - an inner and outer sheath.
These sheaths protect and mold 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 (scent) 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.
The sebaceous
gland is important because it produces sebum which is
a natural conditioner. More sebum is produced after
puberty. The sebum production decreases in women throughout
their lives. The production also decreases in men, but
not as much as in women.
Shaft
- The hair shaft is made up of dead, hard protein called
keratin in three layers. The inner layer is called the
medulla and may not be present. The next layer is the
cortex and the outer layer is the cuticle. The cortex
makes up the majority of the hair shaft. The cuticle
is formed by tightly packed scales in an overlapping
structure similar to roof shingles. Most hair conditioning
products attempt to affect the cuticle. There are pigment
cells that are distributed throughout the cortex and
medulla giving the hair it's characteristic color.
Hair
Growth Cycle - How Hair Grows
Hair
on the scalp grows about .3-.4 mm/day, 0.5 inch per
month, or about 6 inches per year. Unlike other mammals,
hair growth and loss is random and not seasonal or cyclic.
At any given time, a random number of hairs will be
in various stages of growth and shedding. There are
three stages of hair growth: catagen, telogen, and anagen.
Catagen - The catagen phase is a transitional
stage and 3% of all hairs are in this phase at any time.
This phase lasts for about 2-3 weeks. During this time
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 accounts for 10-15%
of all hairs. This phase lasts for about 100 days for
hairs on the scalp and much 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 25-100 telogen hairs are shed normally each
day.
Anagen
- Anagen is the active phase of the hair. The cells
in the root of the hair are dividing rapidly. A new
hair is formed and pushes the club hair 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 2-6 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
30-45 days explaining why they are so much shorter than
scalp hair.
Hair
grows to a length of 1.5 to 3 feet before growth stops
and the hair falls out.
Hair Shape
The
amount of natural curl a hair has is determined by it's
cross-sectional shape. Hair that is most similar to
a circle is straight and hair that is flattened and
elliptical is curly or kinky. The more circular the
shaft is, the straighter it is. The more elliptical
the shaft is, the curlier or kinkier the hair. The cross-sectional
shape also determines the amount of shine the hair has.
Straighter hair is shinier because sebum from the sebaceous
gland can travel down the hair more easily. The kinkier
the hair, the more difficulty the sebum has traveling
down the hair, therefore the more dry or dull the hair
looks.
Why
do we have hair on our heads?
Some
surprising facts about hair and ethnicity and race.
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