The Epidermal Lipids
Along with the horny cells, the epidermal lipids are essential to the barrier function of the skin. They form the “mortar”, or horny layer cell cement, between the “bricks” – the horny cells.
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. These include apart from mono-, di- and triglycerides also cholesterol, ceramides and phospholipids (i).
Scanning electron microscope image of a freeze-dried section of the stratum corneum
- Intercellular space, partially filled with skin lipids
Composition and function of the epidermal lipids
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 phospholipids (i). These are degraded during differentiation (cornification) of the keratinocytes. 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 upper 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 (i) and 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 (i) forms.
The epidermal lipids are produced in the Golgi apparatus of the keratinocytes in the upper layers of the prickle cell layer.
- Odland bodies
- Golgi apparatus
- Cell of the stratum granulosum
- Bilayer lipid membrane
The lipids are stacked in the Odland bodies and finally emptied into the extracellular space of the upper granular layer (exocytosis).
The epidermal lipids form the cement that holds the corneocytes tightly together (brick and mortar model).
Consequences of damage to the permeability barrier (i)
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 can penetrate into the deeper layers of the skin, and more water is lost from the lower skin layers; transepidermal water (i) loss (TEWL) increases.
Influences on the regeneration mechanism
Activation of the skin's barrier regeneration system is subject to various influences. Thus 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.