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The hair shaft
seen with the light microscope: light is reflected from the colorless
cuticle and bent as it passes through the hair - this effect gives
hair the color we see
levels of their hormones (androgens) are high. In
many teenagers, a massive surge in hormone levels leads to raised
grease production. This results in a tendency to greasy hair, which
many young people know all too well. The good news is that most
of them outgrow it
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Structure of the hair shaft
Your smooth, glossy hairs have a more complicated structure than
you might think. Each one can be compared to a tree: all its moisture
lies in its centre, behind a tough outer layer of protective bark.
If the 'bark' of the hair is well looked after the whole hair remains
in good condition. If the 'bark' is stripped off to expose the centre
the hair may break.
The centre part of the hair, called the cortex,
makes up most of the hair shaft. It is the cortex that gives hair
its special qualities such as elasticity and curl. The cortex is
packed with strands of keratin, lying along the length of the hair.
These keratin fibres are made of the low-sulphur keratins, and are
compressed into
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HAIR
FACTS
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The chemistry of hair
As we have seen, the main constituent of hair is the protein
keratin. Keratin is a remarkable protein which is
resistant to wear and tear. It is the protein that makes
up feathers, claws, nails and hoofs, as well as hair. Like
other proteins, keratin has very large molecules made up
of smaller units called amino acids, joined together
in chains like beads on a string.
Hair also contains fats, pigment (melanin),
small amounts of vitamins, and traces of zinc and other
metals. Hair also contains water which, although it makes
up only 10-13% of the hair, is extremely important for its
physical and chemical properties.
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bundles of larger fibres. These are held together by a mass of sulphur-rich
keratins, the matrix. The fibre-matrix combination is extremely
strong and resists stretching and other strains such as twisting,
much as does the glass fibre-resin mixture from which many boats
are built.
The cortex also contains granules of the hair pigment
melanin, produced when the hair was growing in its follicle. The
granules are of two types: smooth, dark granules which tend to be
regularly positioned within the cortex, and lighter granules that
are more irregular in shape and which are scattered randomly through
the cortex. A hair may
contain just one type of granule or a mixture
(see also pages 16 and 59).
In some of the terminal hairs, especially grey (unpigmented)
ones, the cortex has a central hollow core, the medulla.
There are medullae in the hairs of many animals, and they play a
part in the regulation of body temperature. It may be that the presence
of this air space in some human hairs is an evolutionary 'throw-back'
to a time when our ancestors needed extra heat insulation.
The outer layer of the hair (the 'bark') is called the
cuticle. It is made up of between six and ten overlapping
layers of long cells. Each of these cells or scales is about 0.3
micrometres thick and around 100 micrometres long, and about 10
micrometres across. (1 micrometre,
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