The Horny Layer (Stratum Corneum)
The outermost layer of the epidermis - the horny layer - consists of a dense segment (pars compacta) of about 15 to 20 cell layers. The horny skin cells (corneocytes) are connected by a small number of desmosomes - protein-rich appendages of the cell membrane ("adhesive plates").
The functions of the horny layer
The brick and mortar model
Found between the cells are the epidermal lipids: If the horny skin cells are thought of as bricks, then the lipids filling the spaces between the cells are the mortar or cement (brick and mortar model).
Schematic diagram of the horny layer:
The brick and mortar model
1 Horny cells (corneocytes)
2 Epidermal lipids
Formation and function of the epidermal lipids
The lipid composition and moisture content of the epidermis change with increasing differentiation of the skin cells. Lipids are formed in the Golgi apparatus of the keratinocytes. Stored in the membrane-coated vacuoles known as the Odland bodies are the precursors of the skin-specific barrier lipids in the form of lamellar bilayer lipid membranes. The contents of these Odland bodies are released into the extracellular space by exocytosis where they are further processed to epidermal lipids: As a horny cell cement these bilayer lipid membranes give the horny layer stability.
At the same time these intercellular lipid membranes are the decisive permeability barrier (i) of the horny layer: Regulation of the water and fluid content is its most important function, as elasticity and firmness of the horny layer depend on moisture content.
Schematic diagram of the epidermis:
During differentiation the basal cells become the flat horny cells with no nucleus.
1 Odland body with bilayer membrane
2 Keratinocyte
3 Exocytosis
4 Epidermal lipids
5 Horny cells
Composition of the epidermal lipids
Ceramides form the largest fraction of lipids in the horny layer with 40 percent. Also found are free fatty acids (25%) and cholesterol (25%) as well as cholesteryl sulphate. The ceramides are primarily responsible for the barrier-forming and moisture-binding functions of this complex lipid mixture. Chemically, the ceramides are a group of sphingolipids. These are compounds formed from high-molecular weight alcohols, primarily sphingosine, and various fatty acids such as linoleic acid.
The permeability barrier (i)
The epidermal lipids comprise 10 to 30 percent of the total volume of the horny layer (stratum corneum). That means they make up 100 to 200 times more of the total volume of intercellular substance than in other tissues. Accordingly, the stratum corneum makes an effective permeability barrier (i) which performs two important functions:
- It prevents invasion by microorganisms and certain substances such as chemicals and allergens.
- It minimizes transepidermal water (i) loss (TEWL) and thus protects the body from dehydration.
If horny cell layers are removed and with them the epidermal lipids, the skin becomes more permeable to water (TEWL) and other substances, including toxins and allergens.
Natural moisturizing factors (NMF)
The ability of the skin to store water depends in large part on the make-up of the barrier lipids in the horny layer. The protein structure of the horny cells, including the presence of the amino acid arginine, also influences the water-binding capacity of the skin. These substances, which occur naturally in the body and retain water in the horny layer, are called natural moisturizing factors (i) (NMF). They originate from the cornification process (differentiation) of keratinocytes (e.g. pyrrolidine carboxylic acid) and from sweat and sebum (e.g. urea (i), salts, and organic acids).
Desquamation and skin renewal
Towards the surface the horny layer of the skin becomes increasingly fragile. The individual cells split apart from each other (pars disjunctiva), loosen and are sloughed off unnoticed as scales. This unperceived, continuous process is called desquamation. An adult human sheds approximately 10 grams of skin scales a day.
