Eucerin®: Skin and Cells

The outermost layer of the epidermis - the horny layer - consists of a dense segment (pars compacta) with 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 brick and mortar modell
Between the cells lie the epidermal lipids, the horny skin cells are thought of as bricks, and then lipids fill the spaces between the cells like mortar or cement (brick and mortar model).

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 these membrane-coated vacuoles - the Odland bodies - are the precursors of the skin-specific lipid barrier in the form of lamellar bilayer-lipid membranes. Through exocytosis, the contents of the Odland bodies are released into the extracellular space. Only then are the epidermal lipids formed: as a horny cell cement these bilayer-lipid membranes lend the horny layer stability.

At the same time these intercellular lipid membranes are the decisive permeability barrier 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.

Composition of the epidermal lipids
Ceramides form the largest fraction 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 the 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.

Schematic diagram of the brick and mortar model.

1 Horny layer cells (corneocytes)
2 Epidermal lipids

Schematic diagram of the synthesis of epidermal lipids

1 Odland bodies
2 Stratum granulosum cells
3 Exocytosis
4 Bilayer lipid membrane
5 Stratum corneum cells

The permeability barrier
The epidermal lipids comprise 10 to 30 percent of the total volume of the horny layer (stratum corneum). That means they make up a 100 to 200 times more of the total volume of intercellular substance than that of other tissues. Accordingly, the horny layer makes an effective permeability barrier which fulfils two important functions:

The acid pH of the skin plays an essential role in the creation of the permeability barrier.

	It prevents invasion by certain substances such as microorganisms, chemical substances and allergens.
	It minimizes transepidermal water loss (TEWL) and thus protects the body from dehydration.

If horny skin 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 that occur physiologically in the body and that retain water in the horny layer, are called natural moisturizing factors (NMF). The substances originate from the cornification (differentiation) of the keratinocytes (e.g. pyrrolidine carboxylic acid) and secretions from the sweat and sebaceous glands (including urea, salts, 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 as scales. This unperceived, continuous process is called desquamation. An adult human sheds approximately 10 grams of skin scales a day.

Transepidermal water is the water diffusing to the skin surface. There it is removed from the body by means of evaporation. The less intact the horny skin layer is, the higher the loss rate.