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For more than 100 years the function of the protective
acid mantle has been under discussion. Initially, the focus was on
indirect microbiological defence and the direct protection against
alkaline noxae. This classical knowledge of the importance of the
pH for the skin, however, has since been expanded by more recent biochemical
and molecular biological studies. |
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More
recent research findings show that the acid pH of the horny layer
plays an essential role in the formation and structure of the epidermal
lipids and with them the permeability barrier. These studies indicate
that an acidic environment is important for:
 activation of the
enzymes responsible for the synthesis of important epidermal lipids
formation of the
bilayer lipid membrane
restoration of the
horny layer following mechanical or chemical damage
From hydrolipid film to protective acid mantle
Close inspection of the hydrolipid film reveals why this protective
layer was first described by Schade and Marchionini in 1928 as the
protective acid mantle which contains:
lactic acid and various
amino acids from sweat
free fatty acids
from sebum
amino acids and pyrrolidine
carboxylic acid from the cornification process
The physiological pH of healthy skin has an average value lying between
5.4 and 5.9.
Within this pH range, the skin is populated
by a normal skin-typical flora. Pathogenic microorganisms are hindered
from spreading. In the armpits, anal folds and the genitals the pH
is approximately 6.5 (physiological gaps).
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1 Odland bodies
2 Exocytosis
3 Cells of the stratum granulosum
4 Bilayer lipid membrane
An acidic environment is essential for synthesis of the epidermal
lipids. They are comprised mainly of ceramides (40%), free fatty acids
(25%) and cholesterol (25%). Synthesis of the especially important
ceramides is catalysed by an enzyme belonging to the acidic hydrolase
group.
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The pH gives the concentration
of hydrogen ions in an aqueous solution. It can be measured with indicators
such as methyl orange or phenolphthalein that change colour at different
pH values. Today planar glass electrodes are used. |
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An
important protective mechanism: alkali-neutralizing capacity
A rise in the pH into the alkaline range - for example through excessive
use of soap - disturbs the physiological balance of the skin. If the
pH is higher than the normal physiological range for an extended period
of time, the function of the bacteriological defence mechanism of
the skin is compromised, favouring infections. |
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... the protective acid
mantle
To counteract the influence of alkaline substances the protective
acid mantle uses what are called buffer substances. These neutralize
the alkaline substances and ensure the acidic milieu is restored and
stabilized. This capability is the alkali neutralizing capacity. |
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Alkaline noxae (noxious
agents) are substances that react as a base (alkaline, pH >7) in aqueous
solution. So, for example, a soap or sodium carbonate solution can
have a pH of up to 11. |
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SUMMARY:
The hydrolipid film is made up of different substances:
sweat and sebaceous
oils
substances
derived from the cornification
desquamating
but still adhering horny cells
water from
the deepest layers that has reached the surface
Due to the presence of weakly acidic components, the aqueous
portion of the hydrolipid film forms the protective acid mantle.
This fulfils three important functions:
support of
the formation and maturing of the epidermal lipids and hence
maintenance of the barrier function.
indirect protection
against invasion by microbial pathogens
direct protection
against alkaline noxae (alkali neutralizing-capacity) |
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more
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