Epidermal Lipids
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 skin's own lipids are synthesized in cells of the epidermis from intermediate products of metabolism or from essential, meaning supplied from without, fatty acids. Apart from mono-, di- and triglycerides, these also include cholesterol, ceramides and phospholipids.
Composition of the epidermal lipids
Membranes in the living epidermis consist mainly of phospholipids, essential to the body in the form of phosphate. During differentiation (cornification) of the keratinocytes, the membranes degrade, making ceramides, cholesterol and free fatty acids predominant 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. 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.
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. Water, chemicals and pathogenic microorganisms can penetrate into the deeper layers of the skin, and more water is lost from the lower skin layers - transepidermal water loss (TEWL) increases.
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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).
The epidermal lipids are produced in the Golgi apparatus of the keratinocytes
The lipids are slacked in the Odland bodies and finally emptied into the intercellular space of the upper granular layer (exocytosis).
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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. 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 slows the regeneration process.
Scanning electron microscope image of a freeze-dried section of the stratum corneum.
1 Corneocytes
2 Intercellular space, partially filled with skin lipids.
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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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