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Types
of hair fiber/follicle
Many hair subtypes have been classified in other species. For
example, in adult rodents hair types can be classified into vibrissa,
guard, awl, auchene, and peri-anal hairs. There are several different
types of hair follicle present in humans that are distinct from
hair types found in other species. Although the basic human hair
follicle structure remains the same, the type of fiber that is produced
can be quite different. More than one hair type can be made by the
same hair follicle at different times. Humans have several different
types of hair that can be classified depending on their body position
and form. Size, angle of penetrance through the skin, embryological
time of first appearance, and structural variations in the hair
follicles are all taken into account when classifying hair types.
Dermatologists working on embryology or pediatric development
of hair with age describe three different hair types, primary, secondary,
and tertiary hair. The categories are based on the length and diameter
of the hair follicles in the skin. Primary hair is the hair type
produced in the very first cycle of hair growth that a hair follicle
enters shortly after it develops in the embryo. Primary hair is
also correctly described as lanugo hair. During the first year of
life hair follicles go through significant structural changes. The
secondary or vellus hair gradually replaces the primary hair and
depending on position of the body where the hair follicles are present,
the vellus hair may turn to tertiary or terminal hair.
Another classification system can be made based on the active
response of hair follicles towards steroidal hormones. Many hair
follicles take little notice of the general levels of hormones those
follicles of the eyebrows, lashes, and extremities of the hands
and feet are generally consistent in their production of fiber regardless
of hormone concentration in males and females. A second category
would include hair follicles responsive to female hormone levels
(low androgen concentration). These hair follicles are generally
limited to the pubic and arm pit areas plus some hair follicles
on the limbs and chest. A third category would include those hair
follicles responsive to male concentrations of androgen hormones.
Hair regions would include the beard and mustache, nasal hair, beard
hair, upper back hair, chest hair, and scalp vertex hair.
Essentially, there are three basic groups of hair based on hair
follicle size.
A) Lanugo. Lanugo hair is the very first hair fiber to be produced
by a hair follicle. As an embryo develops in the womb the hair
follicles
form and begin to produce this type of hair. Lanugo hair is long,
unpigmented and very fine. This first wave of hair growth is
normally
shed by the embryo at around 8 months gestation while in the womb
and replaced by terminal or vellus hair ready for birth. However,
sometimes babies can be born with this coat of lanugo hair. This
rare condition is called "congenital hypertrichosis lanuginosa".
B) Vellus. Vellus hair is short fine unpigmented hair. It is the
fuzzy hair found on the nose and over the cheeks. It is some times
incorrectly referred to as lanugo hair. Sometimes vellus hair can
grow exceptionally long particularly in people with malignancies.
This excessively long vellus hair is defined as the disease "acquired
hypertrichosis lanuginosa". Strictly speaking long vellus hair is
not lanugo hair despite the disease name.
C) Terminal. Terminal hair comes from large hair follicles. It
is long, coarse, pigmented and frequently contains a medulla. A
hair follicle is capable of switching from vellus hair production
to terminal hair and switching back again. During puberty many hair
follicles around the genitals, arm pits, beard and chest in men
transform from vellus hair to terminal hair under the direction
of hormones. Equally hormones can cause terminal hairs to revert
to vellus hair production as in androgenetic alopecia.
In adults hair exists in a variety of locations, patterns, and
density over the body. Terminal hairs can be further subdivided
into different types depending on their nature and/or position of
growth on the body. The hair characteristics are genetically determined
within each follicle. Different terminal hair types include;
1) Eyebrow hair. Eyebrow hair is a protective patch of hair above
the eye sockets. It channels away sweat and other fluids and helps
reduce any excessive glare from sunlight entering the eyes. Average
maximum hair length is about 10mm. The hair fiber is curved and
coarse and includes a medulla. Hair growth rate is typically 0.16mm
per day in both men and women. This is one of the slowest growth
rates of any hair follicle type found on the human body. Slow growth
has led to some degree of superstition from many in the medical
and cosmetic industries. Some surgeons are wary of shaving eyebrows
for fear they will not grow back. They do grow but the rate of growth
is so slow that the patient may become concerned. Eyebrow follicles
are also sensitive to injury. Plucking an eyebrow follicle once
or twice may lead to extensive and irreversible destruction and
no further hair growth.
2) Eyelash hair. Eyelash hair is very similar to eyebrow hair.
Average length is around 7.5mm and the growth rate is about the
same as for eyebrows. Eyelashes are very important in protecting
the eye from dust and debris. People without eyelashes have frequent
problems with eye contamination and a possible increase in eye infection
that requires regular attention from an ophthalmologist.
3) Scalp hair. Scalp hair is terminal hair unless the hair follicles
have been affected by disease such as androgenetic alopecia. Terminal
scalp hair grows in a clockwise whorl pattern on the top of the
head with merging of this pattern into hair angled downwards and
away from the face around the ears and lower back of the scalp.
Growth rates are up to 0.35mm per day, possibly slightly faster
in females (0.36mm) than males (0.34mm).
4) Beard hair. The beard develops in response to testosterone/steroidal
hormones during puberty. These hair follicles produce thick, coarse,
terminal hair. The hair follicles are some of the fastest producers
of hair with a mean growth rate around 0.38mm per day.
5) Body hair. Some hairs on the arms legs and torso are mildly
responsive to androgens. Other neighboring follicles may not have
androgen receptors. This distinction leads to some body hair follicles
producing pigmented terminal hair in response to androgen stimulation
in adulthood when neighboring follicles continue to produce vellus
hair.
6) Whisker hair. The hair follicles on the chin are highly androgen
responsive in both men and women. Women with abnormally high androgen
levels can grow beards. Note that whiskers are not the same as the
vibrissae on rodents and cats. Vibrissae are specially modified
follicles with numerous sensory nerve endings in an around the hair
follicle structure to detect movement in the hair fiber.
7) Pubic hair. Pubic hair is a kind of terminal hair type. It
is usually pigmented and of large diameter. The kinks in it make
its appearance distinctive.
8) Peri-anal hair. Not much is known about peri-anal hairs, I
guess because no researcher wants to poke around down there! They
are terminal hairs and, as the name suggests, they surround the
anus. Peri-anal hairs are distinguished from other hair types in
the vicinity by their size and their unusually large associated
sebaceous glands. Peri-anal hairs apparently produce a lot of oil
if the size of the sebaceous glands are anything to go by.
Types
of hair fiber/follicle references
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