Stratum corneum corneocyte protein envelope

The cornified cell envelope is the outermost layer of a corneocyte, and mainly consists of tightly bundled keratin filaments aligned parallel to the main face of the corneocyte. The envelope consists of both protein and lipid components in that the lipid is attached covalently to the protein envelope. The envelope lies adjacent to the interior surface of the plasma membrane. " The corneocyte protein envelope appears to play an important role in the structural assembly of the intercellular lipid lamellae of the stratum corneum. The corneocyte possesses a chemically bound lipid envelope comprised of A-co-hydroxyceramides, which are ester linked to the numerous glutamate side chains provided possibly by both the ot-helical conformation and p-sheet conformation of involucrin in the envelope protein matrix. In the absence of A-oo-hydro-xyceramides, the stratum corneum intercellular lipid lamellae were abnormal and permeability barrier function was disrupted. 

The outermost layer of the skin, the cornified layer or stratum corneum, has been identified as the principal diffusion barrier for substances, including water [2,3]. It is approximately 10 to 20 pm thick when dry but swells to several times this thickness when fully hydrated [17], It contains 10 to 25 layers lying parallel to the skin surface of nonviable cells, the corneocytes, which are surrounded by a cell envelope and imbedded in a lipid matrix. This architecture is often modeled as a wall-like structure, with the corneocytes as protein bricks embedded in a lipid mortar [18]. Similarly to the viable epidermis, desmosomes (corneodesmosomes) contribute to the cell cohesion. 

Several classes of ceramides have been described in human skin.14 Today it is considered that the ceramides are essential for the barrier properties. It has been suggested that the lower amount of ceramides found in stratum corneum in atopic dermatitis26,27 explains the increased TEWL seen in dry atopic skin. In this context it is of special interest to note that part of the long-chain ceramides of the horny layer are covalently bound to the proteins forming the corneocyte envelope.25 This suggests that such lipids constitute anchors of the hydrophobic phase to the corneocytes and thereby add to the cohesion of the cells of the horny layer. 

The remarkable barrier function of the skin is primarily located in the stratum corneum (SC), the thin, outermost layer of the epidermis. The SC consists of several layers of protein-filled corneocytes (i.e., terminally differentiated keratinocytes) embedded in an extracellular lipid matrix. Attached to the outer cor-neocyte envelope are long-chain covalently bound cer-amides that interact with the lipids of the extracellular space. These lipids are composed primarily of free fatty acids, ceramides, and cholesterol arranged in multiple lamellae.f Passive permeation across the SC is believed to occur primarily via the intercellular... 

Fig. 5. a Bilayer organisation of the stratum corneum lipids. In the corneocyte, a protein envelope has replaced the lipid cell membrane of the cell of the viable epidermis and some lipids are covalently bound to this envelope (not represented here). Between the hydrophilic head groups of the stacked bilayers resides a water tablet, within which diffusion transport may take place parallel to the skin surface, b A water molecule passing through the skin barrier is visualised to take a conspicuously meandering way through the liquid-crystalline zones of the... 

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