About Hair Loss
Part Two
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here for part one)
The amount of hair and
where itgrows vary with different mammals. The entire body of
the dog, the sheep, thecow, and the horse is covered with a hairy
coat. The whale and the hippopotamushave only a few hairs. In
humans, hair is not found on palms of the hands or thesoles of
the feet.
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The coloration and pattern
of coats in animals serve both as a camouflage for protection against
enemies and as anallurement to mates. Fine and transparent, human
hair is a vestige of ourhairier animal forbears, that probably
evolved from the scales of reptiles. Theadult human body averages
five million hairs, of which 100,000 to 150,000 are onthe scalp.
Hair is composed of
keratin, thesame protein that makes up nails and the outer layer
of our skin. The part seenrising out of the skin is called hair
shaft or strand. Each strand consist ofthree layers. The outermost
protective layer (cuticle) is thin and colorless.The middle layer,
or cortex , is the thickest. It provides strength, determinesyour
hair color and whether your hair is straight or curly.
Hair color is determined
by melaninfrom your pigment cells. The more pigment granules there
are, and the moretightly packed, the darker the hair. Two kinds
of melanin contribute to haircolor. 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'sdistributed
within the hair. For example, deep-black African hair containsclosely
packed melanin in the cortex, a few in the cuticle. Very dark
Europeanhair, 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.
Scalp hair varies tremendouslybetween
races, between individuals of same race, and even within an individual.Mongolians
have straight hair simply because their scalp hair has the greatestthickness
and the roundest cross-section. In Caucasians the hair is moreelliptical
and slender; in African Americans it is flattened, resulting in
kinky curls.Mongolians, both male and female, have much less public,
axillary, facial, andbody 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).
Below your skin is the
hair rootwhich is enclosed by a sack-like structure called the
hair follicle. Tiny bloodvessels at the base of the follicle provide
nourishment. A nearby gland secretesa mixture of fats (called
sebum) which keep the hair shiny and waterproof tosome extent.
Secretions from some sweat glands also produce acharacteristic
odor. A dog can differentiate a human being by the typical scentsecreted
by these glands. Two sets of glands discharge secretions through
theskin. while sebaceous, or oil, glands arise from the walls
of hair follicles andproduce an oil called sebum that lubricates
the skin and hair, Sweat glands,embedded in the subcutaneous layer,
are scattered over the body, particularly inthe palms and soles.
Sweat glands produce moisture called perspiration thatreaches
the skin's surface through the pores and evaporates to cool the
body.
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At the base of the follicle
is thepapilla, which is the "hair manufacturing plant."
The papilla is fedby the blood-stream which carries nourishment
to produce new hair. Male hormonesor androgens regulate hair growth.
Pubic and axillary (armpit) hair areparticularly androgen-sensitive
and grow at lower androgen levels than hair onthe chest or legs.
In boys, most pubic hair is grown by age 15, followed by thedevelopment
of armpit hair two to three years later. In girls, too, an increasein
androgens at puberty triggers growth of pubic and armpit hair.
Scalp hair,not directly androgen-responsive, is influenced by
local amounts of atestosterone derivative, dihydrotestosterone.
Hair follicles initially
form inutero. No new follicles are created after birth, and none
are lost in adultlife. 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 eighthmonth of gestation.
The first postnatal
hair is vellushair, 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 theforehead and balding
scalp. At puberty, the vellus hair in some areas isreplaced by
terminal hair, which is longer, coarser, and pigmented. Growthstarts
in the pubic region; then the eyelashes and eyebrows become thicker.Axillary
hair and male facial hair appear about two years after growth
of pubichair begins. Body hair continues to develop long after
puberty, stimulated bymale hormones that paradoxically, also cause
terminal hair to be replaced byvellus hair when balding begins.
Scalp hair fibers grow
from 100,000to 350,000 follicles which are reported to occupy
the human scalp; however, notall the follicles are productive.[1]
In each producing follicle, the duration ofthe hair's life cycle
is influenced by age, pathology and a wide variety ofphysiological
factors. [1,2] The life cycle is divided into the anagen (active),catagen
(transitional) and telogen (resting) phases.
The anagen phase is
the period ofactive hair growth where protein synthesis and keratinization
are continuouslyoccurring. In normal subjects, this phase lasts
for up to five years, althoughlonger durations have been documented.
The cessation of the
anagen phase ischaracterized by a transitory phase known as catagen.
This phase lasts for twoto three weeks. Following the catagen
phase, the hair enters the telogen or"resting" phase.
In normal subjects, telogen hair is retained withinthe scalp for
up to 12 weeks before the emerging new hair dislodges it from
itsfollicle.
During the anagen phase,
protein sthesis is the main distinction of the hair bulb. In the
telogen phase, thedermal papilla undergoes renewal. It is at this
time that structuralcharacteristics can be modified. The new hair
should be identical to itspredecessor, but with advancing age,
and in some pathological states, a strictcopy is not maintained.
In these circumstances, the hair may become finer andshorter,
modifying the esthetic profile. Since these effects occur over
severalhair cycles, years may elapse before the affected individual
recognizes thedifference.
Like skin cells, hair
grows and isshed regularly. Shedding anywhere from 50 to 100 hairs
per day is considerednormal. The average rate of growth is about
1/2 inch a month. It is now knownthat hair grows fastest in the
summer, slowest in the winter, speeds up underheat and friction,
but slows down when exposed to cold. Hair grows the bestbetween
the ages of 15 to 30. But, hair growth begins to wind down sometimebetween
the ages of 40 and 50. Progressive hair loss begins naturally
in bothsex about age 50, accelerating in the 70s. About 40 percent
of Caucasian menlose hair to some extent by age 35.
What cause hair loss?
can we dosomething about it?
AboutViviscal
(Click for a hair loss solution)
(click
here for part one - hair loss)
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