Corpus: Skin

The skin is a surface organ that forms the organism's boundary to the outside world. It protects the human body from pathological germs, sunlight and dehydration, among other things.

Human skin has a total surface area of about 1.8 m² and is about 1.5 to 4 mm thick. Its total weight is approx. 3.5 to 10 kg. If the fatty tissue is included, a weight of approx. 20 kg is possible.

Macroscopically, two functionally different skin forms can be distinguished on the human body:

• The groin skin on the palms of the hands and soles of the feet
• Field skin on the rest of the body, which also has hair and glands.

The skin is supplied by numerous epifascial arterial and venous blood vessels, which are connected to each other in the subcutis by collateral vessels. Its sensory innervation takes place via superficial cutaneous nerves.

The skin is made up of different layers:

• Cutis, consisting of
  • Epidermis (epidermis)
  • Dermis or corium (leather skin)
• Subcutis (hypodermis)

The skin contains skin appendages (hair, nails, sweat, sebaceous and scent glands), blood vessels, nerve endings, thermoreceptors and mechanoreceptors, which serve to protect the skin, but also to interact with the environment.

The border area between the dermis and epidermis is known as the dermoepidermal junction zone.

The squamous epithelium of the skin is subject to a constant renewal process in which stem cells located in the depths of the epithelium give rise to new keratinocytes through cell division. They are slowly pushed upwards by the succeeding cells and undergo a controlled maturation process. Their cell morphology and cell metabolism change fundamentally. The cells become flatter and massively accumulate the structural protein keratin in their cytoplasm. The resulting dead corneocytes are nucleus-less, no longer contain any cell organelles, and are eventually shed.

The skin has a variety of functions. These include:

• Establishment of physical integrity
• Protection of the organism from environmental influences such as
  • Radiation, through the division of melanocytes
  • chemicals
  • Pathogens, through skin flora and acid mantle
  • Dehydration, through sebum formation
• Mechanical protection of the subcutaneous structures (e.g. vessels), through the elasticity of the corium and the subcutaneous fatty tissue
• Thermal regulation (heat release through vasodilation or heat retention through vasoconstriction, sweating, goose bumps, etc.)
• Energy reserve in the form of fat stored in the skin
• Sensory perceptions of sensitivity, e.g. sense of touch, pressure, pain and vibration
• Synthesis of vitamin D under UV radiation
• Perspiration (skin respiration, in certain animals)
• Absorption of substances, e.g. morphine via transdermal patches
• Unconscious communication with the sexual partner through pheromones

Its function as a covering for all parts of the body is also the origin of its functional-anatomical name "integumentum commune", meaning "common covering" (of all body parts).

The skin color of humans varies from pink-white to black-brown. It is the result of natural selection processes, which are mainly due to the different UV radiation in the areas where humans live. Today (2024), however, migration processes have largely cancelled out the evolutionary geographical classification.

Essentially, skin color is determined by 4 different pigments that are localized in different layers of the skin. These include

• Melanin: Brown pigment in the basal cell layer of the epidermis.
• Melanoids: They are similar to melanin, but are diffusely distributed in the epidermis.
• Carotenes: They have a yellowish to orange color and are found in the stratum corneum and in the fat cells of the dermis and subcutis.
• Haemoglobin: The erythrocyte blood pigment circulates in the skin capillaries and gives them a red to violet color depending on the oxygen saturation.

The diagnosis and treatment of skin diseases is the domain of dermatology. Important skin diseases are

• Skin mycoses
• Eczema
• Psoriasis

The skin can absorb numerous drugs that exert their effect locally and/or systemically. The administration of drugs with a systemic effect via the skin is known as transdermal application. It plays a role, for example, in the administration of painkillers.

In order to be absorbed by blood vessels in the dermis or subcutis, the active ingredients have to pass through the stratum corneum, i.e. the horny layer of the skin. This can occur via three different transport routes:

• Paracellular diffusion via glandular orifices and hair follicles. This route is very important, although the surface area of these openings is relatively small compared to the total surface area of the skin.
• Intercellular transport through the lipid matrix between the corneocytes. This transport route is particularly relevant for small, lipophilic active ingredients.
• Transcellular transport through the corneocytes. Due to the density of the horny layer, this transport mechanism only plays a subordinate role.