It serves as a covering organ to delimit inside and outside, protect against environmental influences and maintain homeostasis (inner balance). In addition, the skin assumes essential functions in the area of metabolism , heat regulation and the immune response ; it has a variety of adaptation and defense mechanisms.
In addition, the skin represents the largest organ of sensory perception in terms of surface area , that of surface sensitivity . The mechanoreceptors of the skin include numerous different sensory cells for touch, pressure or vibration as qualities of the sense of touch . Thermoreceptors convey sensations of warmth or cold, nociceptors convey sensations of pain.
Skin contact in body contact is not only vital for young mammals and carries real social ties. In addition, paleness or reddening and swelling of certain areas of the skin due to changes in the blood circulation in the skin can represent special signals in intra-species communication.
A distinction should be made here between arbitrarily produced skin changes of various types in humans; they can be used as a sign of social affiliation or demarcation and serve for self-presentation. The skin becomes a representatively designed surface for one's own eyes or those of others.
Diseases originating from the skin or symptoms related to the skin are referred to as dermatogenic .
Structure of the human skin
The thickness of the human skin is 1.5 to 4 mm. The body surface (skin area) of adults is on average 1.73 m². It weighs around 10 to 14 kg.
Layers / components of the skin
The outer skin is divided into three essential layers: epidermis (upper skin), dermis (leather skin, Latin corium ) and subcutis (lower skin). In this case, the epidermis and dermis together form the cutis (or cutis ).
The epidermis is one of the epithelial tissues . It is a multilayered keratinizing squamous epithelium that is usually between 0.03 and 0.05 millimeters thick. On the palms of the hands and the soles of the feet, the horny layer is up to several millimeters thick and is colloquially called the "cornea" (see also callus ).
The following layers are distinguished from outside to inside:
- Horny layer ( stratum corneum )
- Glossy layer ( stratum lucidum ) (only present on the groin skin of the palm of the hand and foot)
- Granule cell layer ( stratum granulosum )
- Prickly cell layer ( stratum spinosum )
- Basal layer ( stratum basale )
Dermis (dermis, corium)
The dermis consists mainly of connective tissue fibers and serves to nourish and anchor the epidermis. Here the finely capillarized blood vessel system supplies the border zone to the epidermis. The lower dermis contains the smooth muscles and blood vessels that are important for temperature regulation .
The dermis is divided into a stratum papillare (papillary layer, cone layer , papillary body ) and a stratum reticulare (mesh layer).
The skin appendages include various structures, such as the scales of reptiles , the feathers of birds, the hair of mammals and other formations emerging from the skin such as horns , nails , claws and hooves , whose substance also consists essentially of keratins .
In addition to these structures, there are also skin glands that open onto the upper skin (epidermis) and are anchored in the dermis. These include human sebaceous glands , eccrine sweat glands and scent glands ; the mammary gland is a specialized skin gland. The hair follicle muscle, arrector pili , is also an appendage to the skin; Contractions of the hair follicle muscles lead to goose bumps in humans , in spiny hedgehogs they make their hair coat an effective weapon of defense.
Subcutis (subcutaneous tissue)
The subcutis (or subcutis ) forms the base for the overlying skin layers and contains the larger blood vessels and nerves for the upper skin layers as well as the subcutaneous fat and loose connective tissue. Sensory cells for strong pressure stimuli are located in the subcutis, for example the lamellar bodies .
Surface structure of the skin
If you look at the skin more closely or with a magnifying glass, a fine relief becomes visible. According to this, the skin is divided into two types.
Inguinal skin occurs on the fingers, the palm of the hand (palmar) and the sole of the foot (plantar). The epidermis shows fine papillary lines (skin ridges ), which are caused by the fact that the dermis papillae are arranged in longitudinal rows. Each skin ridge is underlaid by two rows of papillary bodies. The skin strips form an individual pattern from different geometric figures (swirl, arch, bow, double bow). These patterns are used for forensic purposes in dactyloscopy (fingerprint recognition) as a form of biometric data . Apart from many sweat glands, the inguinal skin does not contain any skin appendages.
Field skin covers the rest of the skin. Here the surface shows rhombic fields delimited by fine furrows ( Areolae cutaneae ). The furrows appear on the papilla-free epidermal areas and disappear when the skin is tense. They serve as reserve folds, since the epidermis is less elastic than the dermis. The size of the skin fields varies depending on the body region. The field skin contains the skin appendages and is less than 0.1 mm thick. It is thinnest around the eyes and the genital organs.
Functions of the skin
Functionally, the skin is the most versatile organ. Among other things, it protects against heat loss and external influences and serves to absorb sensory stimuli.
Functions of components of the skin
The individual components of the skin fulfill specialized functions.
Skin appendages and layers:
- Hair : protection against heat loss, sun rays, moisture; Communication and camouflage functions through pigmentation
- Feathers : the same functions as hair, plus additional support for flight
- Horny layer : protection against injuries and dehydration
- Fatty tissue in the subcutaneous tissue : protection from pressure and hypothermia
- Germ layer : replicates skin cells
- Sweat glands : production of sweat, protection against overheating through evaporation
- Sebum glands : production of sebum (fat)
- Hair follicle muscle : erecting the vellus hair
- Melanocytes : Protection of the genetic information in the cell nucleus from UV radiation
- Blood vessels : temperature regulation and supply of the skin cells with nutrients and oxygen
- Cold receptors : reception of temperature stimuli "cold"
- Heat receptors : reception of temperature stimuli "warm"
- Free nerve endings : absorption of touch stimuli and pain sensation
- Sensory corpuscles : recording of touch stimuli
- Lamellar bodies : absorption of pressure stimuli
The skin as a border organ
The skin protects the organism against the penetration of pathogens and gaseous, liquid or solid foreign substances in the broadest sense, against mechanical or physical injuries (e.g. radiation damage ), but also against the loss of fluids, electrolytes and protein, which occurs with extensive skin damage such as B. severe burns, assume life-threatening proportions. It is colonized by bacteria and fungi , the so-called resident skin flora ; but mites can also be found on the skin or the skin appendages. The Langerhans cells function as antigen-presenting cells in the skin .
Skin substance is lost on the surface through peeling / flaking, mechanical wear and tear and chemical corrosion - for example through strong alkalis - and is newly formed through regrowth at the lower limit of the epidermis. If the epidermis is largely worn out, the nerve cells in the skin become extremely sensitive. With permanent moderate stress, the cornea is strengthened by local callus formation . If the skin is injured locally, the body tries to glue the wound with fibrin . Crusts on the skin dry up, contract and with it the edges of the wound. By excessive elongation by body fat storage or pregnancy connective tissue under the skin can tear repeated transverse to the stretch direction, which as to reduce the body volume stretch marks can remain visible.
The exchange of substances in the skin takes place by means of microcirculation : the dermis is supplied with oxygen and nutrients by the blood stream. From there, the substances from the blood capillaries reach the non-perfused epidermis via the tissue water. Metabolic products are transported with the tissue water back into the dermis and the lymph and blood capillaries located there , each of which opens into the lymph vessels and venules .
In addition, there is constant, but imperceptible, evaporation of water (Perspiratio insensibilis), which diffuses through the skin . During perceptible sweating (perspiration, perspiratio sensibilis), water and dissolved salts and other metabolic products are washed to the surface via the sweat glands. Both together result in the transepidermal water loss .
Penetration of active ingredients
In the external dermatological or cosmetic treatment of the skin, the active ingredients have to penetrate the barrier zone of the horny layer (the stratum corneum) so that they can reach the lower skin layers. This succeeds z. B. highly soluble active ingredients with low molecular weight , moderate lipophilicity and a melting point below 200 ° C. Heat, occlusion, and solvents promote penetration. Solvents intervene in the skin structure by, among other things, dissolving fats from the horny layer, loosening the keratin structure in the horny cells or removing the upper horny cell layers. The latter can e.g. B. caused by a chemical peel . Penetration-accelerating substances or processes can damage the skin structure, which leads to increased transepidermal water loss and - depending on the intensity - cause irritation of the skin.
There are three different routes of transport through the cornea:
- The diffusion path through glandular openings and hair follicles. More recent studies ascribe major importance to this path in connection with nanoparticles and some medium-sized and very large molecules, although the area proportion of these skin appendages to the total area of the skin is relatively small.
- The transcellular transport route through the corneocytes. Due to the dense molecular structure in the corneocytes, this path is not considered to be of major importance.
- The intercellular route through the lipid matrix between the corneocytes is considered to be the most important transport route for small molecules with a lipophilic character.
The body can regulate its heat balance through the skin . The skin with the sweat glands counteracts overheating . Through the production of sweat and the evaporation possible as a result, heat is dissipated from the capillaries that run close under the skin and are wide open for this purpose (see sweating ). With the help of the subcutaneous fatty tissue and, to a lesser extent, the hair , heat is retained. When it is cold, the skin and subcutaneous fatty tissue are only slightly supplied with blood; both act as an insulating layer. The hair can only take on a small insulating function due to the small amount of coat humans have. Nevertheless, one can still clearly observe the working principle of a fur dress. When it comes to goose bumps when it's cold, the arrector pili muscle straightens the hair. A closed hair allows a much better protection against hypothermia .
Protection against UV radiation
The strength of the incident UV radiation on the earth's surface depends on the time of day, the geographical location, the season, the altitude, the respective thickness of the ozone layer , the cloud cover and many other local parameters. The following protective mechanisms exist against the harmful effects of UV radiation on the skin and the underlying tissue:
- While the coat ( fur ) of mammals or the plumage of birds provides very effective protection against the adverse effects of UV radiation, as it absorbs or reflects most of the UV radiation, the unclothed person is largely unprotected.
- The stratum corneum of the human skin normally absorbs and reflects around 10% of UVB and half of UVA radiation. The skin initially reacts to constant increased UV exposure by thickening the horny layer. As a "light callus", this is particularly strong after sunburn .
- The protection of the skin through pigmentation is based on the physical absorption of UV rays by pigments . Many animals have skin pigmentation. In the animal kingdom, however, the variable pigmentation of human skin represents a unique option for adapting and protecting against UV radiation. There are hardly any animals whose skin is able to change the pigmentation as much as humans.
- As so-called "instant tanning" ( English immediate pigment darkening ) refers to a short-term, lasting only a few hours tanning of the skin after UV exposure. The instant tanning is based on both a change in the chemical conformation of the melanin molecules and a redistribution of the pigment bodies in the epidermis; it has almost no protective effect against UV radiation.
- The (delayed) UV tanning does not start until approx. 72 hours after the UV exposure. The melanocytes of the skin react to UV radiation by increasing the production and release of eumelanin (or pheomelanin in people of skin type 1), which gives the skin a brown (pheomelanin: reddish) hue, and absorbs UV to a large extent, with pheomelanin being essential absorbs less UV. The ethnically different skin colors of the people result from the respective skin types.
- The sweat of the human body contains urocanic acid which absorbs UVA radiation .
The first hominids may have had poorly pigmented skin covered in dark hair, similar to today's chimpanzees. Relatively soon in the hominid evolution, naked, darkly pigmented skin was likely to have developed, which served as UV protection. With the spread to the north, which is less sunny, the pigmentation could decrease, presumably in order to be able to generate better vitamin D. This could have resulted in survival benefits, particularly during pregnancy and breastfeeding.
The skin as a contact and sensory organ
The skin represents the visible part of the human body. As such, the skin fulfills a number of communicative functions. The skin is equipped with different types of receptors for the absorption of stimuli and thus for surface sensitivity:
- Pain receptors : They are located in the dermis and epidermis. Their density varies depending on the body region (up to 200 / cm² skin).
- Pressure receptors ( Vater-Pacini bodies ): They are used to perceive pressure sensations and are located in the subcutaneous tissue. Their density is up to 100 / cm².
- Thermoreceptors ( free nerve endings ): They are particularly close to the chin, nose , auricle , ear lobes (9 to 12 / cm²) and lips (> 15 / cm²). The human skin has a total of around 250,000 cold receptors. The number of heat receptors is only about 1/10 of that, and they also work significantly more slowly than cold receptors.
- Stretch receptors ( Ruffini bodies ): They register the stretching state of the skin and are located in the dermis (stratum reticulare). Their density is up to 2 / cm² skin.
- Touch receptors ( Meissner bodies and Merkel cells ): Touch receptors are found in hairless skin. They are particularly dense (distance: 1 to 5 mm) in the fingertips , lips , tongue , nipples , the external genital organs and the anus region .
- Hair follicle receptors: They register the position of the hair (see also vibrissae ).
The psychogalvanic skin reaction gives conclusions about emotional processes.
The skin as a stem cell reservoir
The skin contains adult stem cells by four additionally by retroviruses infiltrated genes in pluripotent can be converted stem cells. The skin could thus serve as a source for regenerative medicine therapies .
The skin as an organ of representation
Since the skin has a strong impact on human appearance, it is the main object of cosmetics . Freckles , liver spots and age spots are natural manifestations . The appearance of the skin is artificially changed by UV radiation in the solarium, tattoos , scarification , burn scars , body painting or skin lightening . The skin is also the wearer of all types of body jewelry .
According to studies by scientists at Jacobs University in Bremen, people with smooth skin appear credible and serious.
The warlike ritual of scalping , as peeling off the scalp and the rind underneath, is historically reported from America .
The use of human tanned skin - such as those of the executed - as material for book covers can be proven in the 19th century.
The skin of animals
In the slightly pink-colored skin of the domestic pig there are few thick bristles compared to other fur hair. The pores they form are a characteristic feature of pigskin.
Skin of polar bears is very dark on the snout and under the skin to black. In combination with the white hair, it is possible to thermally absorb solar radiation well and to keep losses to the ambient air through wind and convection low.
Typical light blue and red skin tones appear on the head and buttocks of the mandrill monkey .
The skin of the whales , especially the fast-swimming dolphins , has a fine relief that, together with the damping of the vortices by the deformation of the underlying fat and suspected muscle reaction in the skin, reduces the flow resistance and thus enables faster swimming.
The skin of the amphibians is thin, bare and damp. Their surface texture is smooth in frogs and salamanders or warty in toads and toads. The skin of amphibians shows a great variety of colors. Some species, like the native tree frog , even have the ability to change colors similar to chameleons . Responsible for this property are special pigment cells below the epidermis, which store different dyes, such as melanin (brown to black), pteridine (yellow) and carotenoids (orange to red).
From time to time the epidermis of the amphibians is renewed ( molting ). The old skin is blasted off in frogs, but stripped off as a whole in tailed amphibians (especially newts). Some areas of the skin of amphibians are particularly elastic and allow the formation of sound bubbles to generate sounds.
These skin properties have advantages and disadvantages. Advantages are:
- The thin skin enables oxygen to be absorbed directly through the body surface (skin breathing), as well as water absorption.
- Smooth skin has less flow resistance and thus enables faster swimming.
- In many amphibians, especially frogs, the skin is moistened with a slippery layer of mucus that helps them escape from predators.
- The amphibian skin glands are able to secrete skin toxins; These represent an effective protection against eating. Above all, they protect the damp skin from fungal and bacterial infections - this is said to be the main reason even for the extremely strong poisons of the poison dart frogs .
- The thin skin is more vulnerable to injury.
- Increased risk of dehydration in warm sunshine due to the moisture of the skin of most amphibians. This leads to their increased nighttime activity.
- The water absorption capacity of thin skin also facilitates the absorption of toxins. Pesticides used in fields, artificial fertilizers, but also liquid manure and acid rain quickly lead to death during the spawning migration.
Exchange of substances in animals
Substances from the environment are absorbed and released to varying degrees via the body surface of different animals. These can be gaseous, liquid or solid (dissolved in an aqueous medium). The exchange of substances can take place actively (with expenditure of energy) or passively (in the direction of an osmotic gradient).
The gases can be the absorption of oxygen and the release of carbon dioxide ( skin respiration ), but also nitrogen and inert gases . Water can be absorbed or released to regulate water and serve as a transport medium for dissolved gaseous or solid substances. Dissolved substances can be salts (absorption or release), metabolic products, but also toxic substances from the environment (such as organic lead compounds ).
Fish skin and surrounding water
Fish in fresh water, the tissue of which contains dissolved salt, constantly absorb water through their semi-permeable skin through osmosis , which they have to excrete again via their organ system in order to stabilize the salt content in their body and not suffer from osmotic overpressure. Conversely, fish in the more salty sea water continuously lose the more mobile water molecules through the same process. These fish have to gain fresh water with the use of energy and actively excrete salt. Salmon live alternately and for longer periods of time in fresh and salt water, which means that they both need organ skills.
There are numerous skin diseases that are based on direct damage to the skin, for example through infection, but also those that result from other organ diseases or general diseases. In dermatology - the medical specialty of skin diseases - skin changes are referred to as efflorescences .
Further damage to the skin can be caused by infections caused by diabetes mellitus (decubitus, diabetic foot syndrome), bite wounds, burns, gunshot and stab wounds, infections after injuries in seawater, infections from rare pathogens or damage from plant constituents. Very rarely there are also congenital skin diseases such as B. the aplasia cutis congenita .
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- Thickness of human skin
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