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Why does hair grow on your scalp? The answer is
very simple: you inherit it from your mother and your father....
that is to say; it is in your genes. The genes involved in the
formation of your hair follicles are present when your mother's
egg is fertilised by your father's sperm.
After the hair follicle is formed in the embryo, it continues
to acquire the signalling pathways it will need to produce, maintain
and cycle hair for its lifetime. Production of hair requires
synthesis of proteins (keratins) and other molecules, and
these will determine the pattern of your hair (ie: will it be
straight, wavy, curly, or frizzy) and the colour of your hair
(blond, red,
brown, black).
DNA variations that alter this protein and molecular synthesis
will result in hair abnormalities. Although a number of hair
abnormalities are recognised, very little is known regarding
their actual cause and/or their genetic transmission (inheritability).
Some congenital hair abnormalities are associated with congenital
physical and mental developmental anomalies that have been mapped
onto specific chromosomes. Genes are defined as autosomal dominant
or autosomal recessive.
The dominant gene is all-powerful, which means that a person
with autosomal dominant conditions will have one affected parent,
and the condition is transmitted to future generations. Males
and females are affected equally, and both males and females
can
transmit the condition. When a person carrying the autosomal
dominant gene(s) that causes the condition has children with
a person who does not carry the gene(s), the condition will appear
in about 50% of their children.
The recessive gene can be dominated by other genes, and therefore
both parents must carry the gene(s) that cause a condition, although
neither parent necessarily exhibits the condition. The child
inherits the gene(s) from both parents, and the condition appears
in the
child.
X-linked recessive: this occurs only in males, but the gene
is actually transmitted by carrier females who carry one gene
for
the specific condition. An affected male cannot transmit the
condition to his sons, but all of his daughters will be carriers
of the gene.
Unfortunately the human condition cannot be explained by these
simple genetic terms. DNA is one of the most complex and
complicated things to describe. Medical scientists have been
trying to decipher DNA for the last 50 years, and will probably
still be trying to understand what is going on with the DNA for
the next 500 years. Even someone who is so-called
normal, his/her DNA will have up to 10 million variants (also
known as mutations). All of this results in the variations that
will occur between various family members. |
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| The hair follicle has a complex anatomical structure and a
complex physiological function that is conducted under the influence
of genetic and hormonal controls. It has a limited effective
lifetime: each follicle completes 10 to 20 anagen-catagen-telogen
cycles in its lifetime. In other words, each follicle produces
10 to 20 hairs in its lifetime. Completion of each scalp hair
cycle can take from 2 to 8 years. |
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The hair follicle, irrespective of whether it is
on the scalp or elsewhere on the body, has a characteristic anatomical
structure.
The follicle is a slim, hollow shaft extending from the surface
of the skin into layers of underlying skin and fat cells. There
are about 5 million hair follicles on a human, of which about
100,000 to 150,000 occur on the scalp.
In a cross-section view, the follicle is seen to be organized
in concentric compartments. The outermost layer of cells is the
outer root sheath, beneath it the inner root sheath and then
the hair shaft. The hair shaft itself has three concentric compartments:
the outermost cuticle, the underlying cortex and the innermost
medulla.
Longitudinally, the follicle is anatomically organized from top
to bottom as (1) the hair canal, which becomes indistinct after
birth, (2) infundibulum that extends to the opening of the sebaceous
(oil) gland duct, (3) sebaceous gland that produces the oil that
keeps hair lubricated, (4) the isthmus that begins at the sebaceous
gland and ends at (5) the bulge, where the arrector pili muscle
inserts [the little muscle that makes hair stand on end in response
to cold or fear] and where follicular epithelial stem cells are
located [stem cells that may be capable of generating new matrix
cells and initiating new follicles], (6) the keratinization zone
where the hair shaft acquires its tough coating of keratinocytes,
and (7) the hair bulb that contains the hair matrix where a new
hair is generated, and melanocytes that contribute to hair colour. |
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The normal hair cycle is associated with the temporary loss
of hair and the growth of new hair from the same follicles. The
hair growth cycle in humans occurs in three phases:
- Anagen-growth phase
- Catagen-degradation phase
- Telogen-resting phase
There is no break or discontinuity between phases; the hair cycle
is a continuous process that occurs during the life of the hair
follicle. A complete scalp hair cycle stretches over 2 to 8 years:
- Anagen (growth)-2 to 8 years
- Catagen (degradation)-2 to 4 weeks
- Telogen (rest)-2 to 4 months
The "dead" hair that was degraded during the catagen
(degradation) phase is pushed out of the follicle when a new
hair emerges in anagen (growth) phase. A healthy person with
a full head of hair will shed on average 50 to 100 "dead" hairs
per day. Shedding can be influenced by internal factors such
as age, change in hormonal or nutritional status, skin disease
and stress, and by external factors such as cancer chemotherapy,
ionizing radiation, and exposure to some types of industrial
chemicals.
All follicles, everywhere on the body, undergo hair cycles. The
duration of each cycle may vary by body site; on the same person,
scalp hair cycling may be slightly different in duration from
body hair cycling. |
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| Androgenic (male) hormones are essential for the growth of
human hair everywhere on the body to a variable extent. In boys,
especially, the appearance of body and facial hair occurs in
parallel with increase in levels of androgenic hormones as the
body matures sexually. The relative absence of heavy hair growth
on the bodies and faces of girls is an external indication of
the relatively lower levels of androgenic hormones in girls. |
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Under genetic control, hair follicles are formed and develop
to produce different kinds of hair for different purposes. Hair
can be body-site specific: nostril hair, eyelashes, eyebrows,
hair in the genital area, torso hair and scalp hair have different
physical qualities specific to their function. The physical characteristics
of hair may change over time or under the influence of hormonal
changes in the body.
Genetic ancestry influences the physical qualities of hair-heavy,
fine, straight, curly, colour. For example, Asian ancestry is
associated with black, large-diameter, straight hair; African
ancestry is associated with black, tightly curled hair; Scandinavian
ancestry is associated with red or blond hair of fine texture.
The number of genes and the specific genes involved in determination
of hair qualities is not known with certainty.
Hair is not "alive" like other body tissues. Hair is
a non-living fibre made up of biological components. The components
are assembled into a hair in the hair follicle. The major components
of a hair shaft are the cortex and the cuticle.
The cortex constitutes the bulk of the hair fibre. It is made
up largely of keratins, a family of proteins that also provide
the tough outer sheath of skin
cells. The cuticle is the armour of the hair shaft. Made up of
thin scales of dense keratin, it protects the cortex from physical
and chemical damage. Seen under a microscope, the layered scales
of cuticle resemble the armoured scales of a reptile. When the
cuticle is damaged by chemicals or physical trauma, the cortex
is exposed and open to damage. A typical appearance of such damage
is broken hair and "split ends".
The colour of hair is determined by follicular melanocytes (cells
containing the colouring pigment melanin). The melanocytes are
located in the matrix area of the follicle. |
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