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Apart from corneocytes, the epidermal lipids are
a central element of the skin's barrier function. They form the "mortar",
or horny layer cell cement, that goes between the "bricks" - the corneocytes.
The skinīs own lipids are synthesized in cells
of the epidermis from intermediate products of metabolism or from
essential, meaning supplied from without, fatty acids. These include
apart from mono-, di- and triglycerides also cholesterol, ceramides
and phospholipids. |
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Phospholipids are
an important component of the cell membrane. As phosphorus in form
of phosphate is an essential substance for the organism, phospholipids
are degraded in the course of differentiation (cornification). |
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Composition
of the epidermal lipids
The composition of the lipids forming the membranes in the horny layer
differs from that of the membranes in the living epidermis which consist
mainly of phospolipids. During differentiation (cornification) of
the keratinocytes they are degraded. Therefore, ceramides, cholesterol
and free fatty acids predominate in the horny layer.
Formation of the epidermal lipids
The precursors of the epidermal lipids are formed in the Golgi apparatus
of the keratinocytes in the prickle cell layer. Then they are stored
in the cell as bilayer lipid membranes in microscopically small granules,
membrane-enclosed vacuoles known as the Odland bodies. In the upper
layer of the stratum granulosum (granular layer) the Odland bodies
expel these bilayer lipid membranes by exocytosis into the intercellular
space.
In the course of this maturation process, the polar glycolipids, phospholipids
and the sterol esters are converted by enzymes to non-polar lipids
like the ceramides and free fatty acids. Thus the functional semipermeable
corneocyte lipid barrier also known as the permeability barrier forms. |
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The epidermal lipids are produced in the Golgi apparatus of the keratinocytes
in the upper layers of the prickle cell layer.
The lipids are slacked in the Odland bodies and
finally emptied into the intercellular space of the upper granular
layer (exocytosis).
The epidermal lipids form the cement that holds
the corneocytes together (brick and mortar model).
1 Odland bodies
2 Golgi apparatus
3 Exocytosis
4 Cell of the stratum granulosum
5 Corneocytes
6 Bilayer lipid membrane |
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Consequences of damage
to the permeability barrier
If the uppermost corneocyte layers are removed,
for example with an adhesive plaster, the epidermal lipids are lost
with them. Then water, chemicals and pathogenic microorganisms penetrate
into the deeper layers of the skin, and more water is lost from the
lower skin layers - transepidermal water loss (TEWL) increases.
Influences on the regeneration mechanism
Activation of the skin's barrier regeneration
system is subject to various influences: increased cholesterol, fatty
acid and sphingolipid synthesis leads to a restoration of the barrier
function. However, it can be shown that after widespread damage to
the horny skin layer, for example, from "stripping" or by oil-removing
acetone, the natural horny layer barrier can only be restored by acidifying
the skin surface. A neutral or alkaline environment noticeably retards
the regeneration process. |
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Scanning electron microscope
image of a freeze-dried section of the stratum corneum.
1 Corneocytes
2 IIntercellular space, partially filled with skin lipids. |
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SUMMARY:
The epidermal lipids, as the corneocyte cement form together
with the corneocytes the skin's permeability barrier. Because
of their special composition (40% ceramides, 25% free fatty
acids and 25% cholesterol) they play a central role in the regulation
of the water and fluid balance in the skin and the entire body. |
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