Hair is much more complicated than it appears. 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. This article explains what exactly hair is and how
it works.
Hair Origin
A developing fetus has all of its 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.
Hair Anatomy
Hair has two separate structures - the follicle in the
skin and the shaft we see.
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 its characteristic color.
Hair Growth Cycle
Hair on the scalp grows about .3-.4 mm/day 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.
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.
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.
Hair Shape
The amount of natural curl a hair has is determined by its
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.
