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Hair
- Frequently Asked Questions (FAQ) |
Overview | Hair colour | Influence of race
| Glands | Papilla
| Hair formation | Growth cycle | Shedding
Overview
The amount of hair and where it grows vary with different mammals. The
entire body of the dog, the sheep, the cow, and the horse is covered with a hairy coat.
The whale and the hippopotamus have only a few hairs. In humans, hair is not found on
palms of the hands or the soles of the feet.
The coloration and pattern of coats in animals serve both as
a camouflage for protection against enemies and as an allurement to mates. Fine and
transparent, human hair is a vestige of our hairier animal forbears, that probably evolved
from the scales of reptiles. The adult human body averages five million hairs, of which
100,000 to 150,000 are on the scalp.
Hair is composed of keratin, the same protein
that makes up nails and the outer layer of our skin. The part seen rising out of the skin
is called hair shaft or strand. Each strand consist of three layers. The outermost
protective layer (cuticle) is thin and colorless. The middle layer, or cortex , is the
thickest. It provides strength, determines your hair color and whether your hair is
straight or curly.
Hair colour
Hair color is determined by melanin from your pigment cells. The more pigment
granules there are, and the more tightly packed, the darker the hair. Two kinds of melanin
contribute to hair color. Eumelanin colors hair brown to black, and an iron-rich
pigment, pheomelanin colors it yellow-blonde to red. Whether hair is mousy, brown,
brunette or black depends on the type and amount of melanin and how densely it's
distributed within the hair. For example, deep-black African hair contains closely packed
melanin in the cortex, a few in the cuticle. Very dark European hair, quite apart from
having more melanin granules than lighter or blonde hair, has more melanin per granule.
When pigment-producing cells cease to function, the result is the uncolored white or gray
hair.
Influence of race
Scalp hair varies tremendously between races, between individuals of same
race, and even within an individual. Asians have straight hair simply because their
scalp hair has the greatest thickness and the roundest cross-section. In Caucasians the
hair is more elliptical and slender; in Blacks it is flattened, resulting in kinky curls.
Asian, both male and female, have much less pubic, axillary, facial, and body hair
than Caucasians. In Caucasians, true blonds typically have more hair (about 140,000 hair)
than brunette (about 105,000) or redhead (about 90,000).
Glands
Below your skin is the hair root which is enclosed by a sack-like
structure called the hair follicle. Tiny blood vessels at the base of the follicle provide
nourishment. A nearby gland secretes a mixture of fats (called sebum) which keep the hair
shiny and waterproof to some extent. Secretions from some sweat glands also produce a
characteristic odor. A dog can differentiate a human being by the typical scent secreted
by these glands. Two sets of glands discharge secretions through the skin:- a)
sebaceous, or oil, glands arise from the walls of hair follicles and produce an oil called
sebum that lubricates the skin and hair; and b) sweat glands, embedded in the subcutaneous
layer, are scattered over the body, particularly in the palms and soles. Sweat glands
produce moisture called perspiration that reaches the skin's surface through the pores and
evaporates to cool the body.
Papilla
At the base of the follicle is the papilla, which is the "hair
manufacturing plant." The papilla is fed by the blood-stream which carries
nourishment to produce new hair. Male hormones, or androgens, regulate hair growth. Pubic
and axillary (armpit) hair are particularly androgen-sensitive and grow at lower androgen
levels than hair on the chest or legs. In boys, most pubic hair is grown by age 15,
followed by the development of armpit hair two to three years later. In girls, too, an
increase in androgens at puberty triggers growth of pubic and armpit hair. Scalp hair, not
directly androgen-responsive, is influenced by local amounts of a testosterone derivative,
dihydrotestosterone.
Hair formation
Hair follicles initially form in-utero (i.e. before birth). No new follicles are
created after birth, and none are lost in adult life. The first hair to be produced by the
fetal hair follicles is Lanugo hair, which is fine, soft, and unpigmented. This is usually
shed in about the eighth month of gestation.
The first postnatal hair is vellus hair, which is
fine, soft, usually unpigmented, and seldom more than 2 cm long. Vellus hair remains on
the so-called hairless regions of the body, such as the forehead and balding scalp. At
puberty, the vellus hair in some areas is replaced by terminal hair, which is
longer, coarser, and pigmented. Growth starts in the pubic region; then the eyelashes and
eyebrows become thicker. Axillary hair and male facial hair appear about two years after
growth of pubic hair begins. Body hair continues to develop long after puberty, stimulated
by male hormones that paradoxically, also cause terminal hair to be replaced by vellus
hair when balding begins.
Hair growth cycle
Scalp hair fibers grow from 100,000 to 350,000 follicles which are
reported to occupy the human scalp; however, not all the follicles are productive.[1] In
each producing follicle, the duration of the hair's life cycle is influenced by age,
pathology and a wide variety of physiological factors. [1,2] The life cycle is divided
into the anagen (active), catagen (transitional) and telogen (resting) phases.
The anagen phase is the period of active hair growth where
protein synthesis and keratinization are continuously occurring. In normal subjects, this
phase lasts for up to five years, although longer durations have been documented.
The cessation of the anagen phase is characterized by a
transitory phase known as catagen. This phase lasts for two to three weeks. Following the
catagen phase, the hair enters the telogen or "resting" phase. In normal
subjects, telogen hair is retained within the scalp for up to 12 weeks before the emerging
new hair dislodges it from its follicle.
During the anagen phase, protein synthesis is the main
distinction of the hair bulb. In the telogen phase, the dermal papilla undergoes renewal.
It is at this time that structural characteristics can be modified. The new hair should be
identical to its predecessor, but with advancing age, and in some pathological states, a
strict copy is not maintained. In these circumstances, the hair may become finer and
shorter, modifying the esthetic profile. Since these effects occur over several hair
cycles, years may elapse before the affected individual recognizes the difference.
Hair shedding
Like skin cells, hair grows and is shed regularly. Shedding anywhere from 50
to100 hairs per day is considered normal. The average rate of growth is about 1/2 inch
a month. It is now known that hair grows fastest in the summer, slowest in the winter,
speeds up under heat and friction, but slows down when exposed to cold. Hair grows the
best between the ages of 15 to 30. But, hair growth begins to wind down sometime between
the ages of 40 and 50. Progressive hair loss begins naturally in both sex about age 50,
accelerating in the 70s. About 40 percent of Caucasian men lose hair to some extent by age
35.
What cause hair loss? can we do something about it? we will
discuss these matters in upcoming chapters.
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