A little more about the horny layer of the skin

As the outermost layer of the skin, the horny layer plays a pivotal role in protecting the body from the environment and limiting the amount of water both absorbed into and emitted from the epidermis. Several factors allow the horny layer to fulfill this function:.

1. Water binding
The horny layer can bind with water, which helps maintain its elasticity, firmness and suppleness. Besides keratin, the horny layer contains several other substances, referred to as natural moisturising factors, which give it the ability to bind with water. The most important of these are lactic acid (lactate), pyrrolidonecarboxylic acid, urea, urocaninic acid and carbohydrates bound to scleroproteins. If these factors are depleted from the skin, as may occur with excessive washing with soap, the horny layer loses its capacity to bind with water and moisture is lost. When the moisture of the horny layer falls to values below 8% - 10%, the layer becomes rough, dry and prone to cracking. Moisture from perspiration of sweat through the sweat glands also keeps the horny layer moistened. The water penetrates into the spaces between the cells thus moisturising the horny layer from within.

2. Epidermal lipids
In the deeper portions of the horny layer, the cells are bound together by a cement-like mixture of fats (lipids), much the way cement holds together bricks in a wall. This mixture forms a secure but elastic bond between the cells, which helps preserve loss or too much absorption of moisture. The lipids are produced by the Golgi apparatus within the horny cells. About 40% of these lipids are ceramides, which are responsible for creating the permeability barrier and maintaining the moisture-binding function of the horny layer. Fatty acids, cholesterol and cholesterol sulphate make up the rest of the lipid mixture.

3. Desquamation & Regeneration
The cells in the outermost layer of the horny layer are constantly being sloughed off in a process called desquamation. Since new epidermal cells are continually being formed and pushed upward to form new horny cells, the thickness of the horny layer remains constant. When skin is exposed to exogenous (outside) mechanical, chemical and physical stimuli, the horny layer thickens. For example, when calluses form on the hands and feet when exposed to repeated mechanical stress or due to frequent bathing with lipid solvents and water.

The Brick & Mortar Model


1 Horny layer cells
2 Epidermal lipids