The skin’s structure
In surface area, the skin is the largest human organ. In adults, it covers a surface of about 1.5 – 1.8 square metres (15-20 % of the body weight). It is only 0.1 – 0.7 millimetres thick and its total weight is from 10 to 12 kilograms.
It forms the outer surface of the body and fulfils a wide range of functions. It demarcates the body from its surroundings and protects it from environmental influences. The blood circulation and the lymphatic and nervous systems link it closely to the rest of the body. It thus also assumes important tasks associated with communication and perception, and also with metabolism and the body’s immune defences.
Unlike almost all other organs in our body, the skin is a genuinely versatile talent.
One reason is its three-layer structure.
The top layer (epidermis)
The distinct cell layers in the epidermis are named from top to bottom their stratification expresses their various functions.
They are called:
Keratin layer (stratum corneum)
Clear layer (stratum lucidum)
Granular layer (stratum granulosum)
Prickle cell layer (stratum spinosum)
Basal cell layer (stratum basale)
The process of keratinization begins in the lowest layer of the epidermis, the basal cell layer. Here the skin cells are produced by cell division. Then the skin cells migrate through the skin over a period of about four weeks. During this migration to the skin surface, the cells change their shape, flatten, die off and keratinize. This process is called differentiation.
In the prickle cell layer, the skin cells are already slowly flattening and assuming a prickly appearance.
In the granular layer the keratin starts to enrich the cells. Under the microscope “granules” are visible. At the same time, the cells flatten and lose their nuclei and cytoplasm.
The clear layer is present only where the epidermis is thick, like the palms of the hands and the soles of the feet. The stratum lucidum functions as a barrier against anything that might penetrate the skin.
The keratin layer is the part of the epidermis which is visible from outside. It consists of about twenty layers of keratin cells, packed tightly one above the other like roof tiles; at the lower part they are linked with their neighbours by protein bridges (desmosomes). In the upper part there are no desmosomes. The keratin cells lie only loosely on top of each other and are shed. This shedding (desquamation) of the keratin cells preserves the average layer thickness of the skin.
Its main task is to protect the living layers of skin by means of their keratin cells packed tightly one above the other. It also forms a protective barrier against moisture loss, injuries and harmful substances.
The corium (dermis)
This is a complex network of collagen fibres, sweat glands, hair roots, nerve cells, nerve strands and blood and lymph vessels. The dermis supplies the epidermis, the latter having no blood vessels. Cone-shaped loops of collagen fibres, in the form of papillae of connective tissue, project into depressions in the epidermis.
Above these papillae, the epidermis and dermis are dovetailed in such a way that mechanical movements like pushing, stretching or pressure can be absorbed without damage. The most important structural feature of the dermis is a fibrous network of proteins. This consists for the most part of collagen fibres and also, to a lesser extent, of elastin fibres. These give the skin its elasticity and firmness.
The protein fibres are formed from specific cells in the dermis: the fibroblasts. The fibrous proteins are embedded in a gel-like ground substance. This binds well to water and is chiefly responsible for skin turgor, that is the plumpness of the skin.
The subcutis forms the basis for the overlying layers of skin and contains the larger blood vessels and nerves. It consists of loose connective tissue subdivided into chambers by bands of connective tissue fibres and is rich in fat. It represents the energy and fat stores or the body’s nutrient reserve. It also gives protection from cold.