A diagram of human skin.
Skin is the soft outer covering of vertebrates. Other 
Fur is dense hair. Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier, especially regarding the passage of chemicals via skin and is often subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated quickly, as the chemical diffuses through its skin.
- Functions 1
Mammalian skin layers 2
- Epidermis 2.1
- Basement membrane 2.2
- Papillary region 2.3.1
- Reticular region 2.3.2
- Hypodermis 2.4
- Fish and amphibians 3
- In birds and reptiles 4
- Mechanics 5
- Human uses and culture 6
- Detailed cross section 7
- See also 8
- Additional images 9
- References 10
Skin performs the following functions:
- Protection: an anatomical barrier from pathogens and damage between the internal and external environment in bodily defense; Langerhans cells in the skin are part of the adaptive immune system.
- Sensation: contains a variety of nerve endings that jump to heat and cold, touch, pressure, vibration, and tissue injury (see somatosensory system and haptic perception).
- Thermoregulation: eccrine (sweat) glands and dilated blood vessels (increased superficial perfusion) aid heat loss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat. Erector pili muscles in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair or fur.
- Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.
- Storage and synthesis: acts as a storage center for lipids and water
- Absorption: humans, the cells comprising the outermost 0.25–0.40 mm of the skin are "almost exclusively supplied by external oxygen", although the "contribution to total respiration is negligible")
- Water resistance: The skin acts as a water resistant barrier so essential nutrients aren't washed out of the body. The nutrients and oils that help hydrate the skin are covered by the most outer skin layer, the epidermis. This is helped in part by the sebaceous glands that release sebum, an oily liquid. Water itself will not cause the elimination of oils on the skin, because the oils residing in our dermis flow and would be affected by water without the epidermis.
Mammalian skin layers
The distribution of the bloodvessels in the skin of the sole of the foot. (Corium – TA alternate term for dermis – is labeled at upper right.)
A diagrammatic sectional view of the skin (click on image to magnify). (Dermis labeled at center right.)
Mammalian skin is composed of two primary layers:
- the epidermis, which provides waterproofing and serves as a barrier to infection; and
- the dermis, which serves as a location for the appendages of skin;
The epidermis is composed of the outermost layers of the skin. It forms a protective barrier over the body's surface, responsible for keeping water in the body and preventing pathogens from entering, and is a stratified squamous epithelium, composed of proliferating basal and differentiated suprabasal keratinocytes. The epidermis also helps the skin regulate body temperature.
Keratinocytes are the major cells, constituting 95% of the epidermis, while Merkel cells, melanocytes and Langerhans cells are also present. The epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer):
- Stratum corneum
- Stratum lucidum (only in palms and soles)
- Stratum granulosum
- Stratum spinosum
- Stratum germinativum (also called the stratum basale)
cellular junctions (desmosomes) between each other and secreting keratin proteins and lipids which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin. Keratinocytes from the stratum corneum are eventually shed from the surface (desquamation).
The epidermis and dermis are separated by a thin sheet of fibers called the basement membrane, and is made through the action of both tissues. The basement membrane controls the traffic of the cells and molecules between the dermis and epidermis but also serves, through the binding of a variety of cytokines and growth factors, as a reservoir for their controlled release during physiological remodeling or repair processes.
The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils, microfibrils, and elastic fibers, embedded in proteoglycans.
It harbors many mechanoreceptors (nerve endings) that provide the sense of touch and heat. It also contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as for the epidermis.
The dermis is tightly connected to the epidermis through a basement membrane and is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep thicker area known as the reticular region.
The papillary region is composed of loose areolar connective tissue. This is named for its fingerlike projections called papillae that extend toward the epidermis. The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of collagenous, elastic, and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength, extensibility, and elasticity. Also located within the reticular region are the roots of the hair, sebaceous glands, sweat glands, receptors, nails, and blood vessels.
The hypodermis is not part of the skin, and lies below the dermis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (the hypodermis contains 50% of body fat). Fat serves as padding and insulation for the body. Another name for the hypodermis is the subcutaneous tissue.
Staphylococcus epidermidis colonize the skin surface. The density of skin flora depends on region of the skin. The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut and urogenital openings.
Fish and amphibians
The epidermis of dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods. Instead, in most species, it is largely replaced by solid, protective bony scales. Apart from some particularly large dermal bones that form parts of the skull, these scales are lost in tetrapods, although many reptiles do have scales of a different kind, as do pangolins. Cartilaginous fish have numerous tooth-like denticles embedded in their skin, in place of true scales.
Sweat glands and sebaceous glands are both unique to mammals, but other types of skin gland are found in other vertebrates. Fish typically have a numerous individual mucus-secreting skin cells that aid in insulation and protection, but may also have poison glands, photophores, or cells that produce a more watery, serous fluid. In amphibians, the mucus cells are gathered together to form sac-like glands. Most living amphibians also possess granular glands in the skin, that secrete irritating or toxic compounds.
Although melanin is found in the skin of many species, in the reptiles, the amphibians, and fish, the epidermis is often relatively colourless. Instead, the colour of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as chameleons and flounders may be able to change the colour of their skin by adjusting the relative size of their chromatophores.
In birds and reptiles
The epidermis of birds and reptiles is closer to that of mammals, with a layer of dead keratin-filled cells at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial amphibians such as toads. However, in all of these animals there is no clear differentiation of the epidermis into distinct layers, as occurs in humans, with the change in cell type being relatively gradual. The mammalian epidermis always possesses at least a stratum germinativum and stratum corneum, but the other intermediate layers found in humans are not always distinguishable. Hair is a distinctive feature of mammalian skin, while feathers are (at least among living species) similarly unique to birds.
Birds and reptiles have relatively few skin glands, although there may be a few structures for specific purposes, such as pheromone-secreting cells in some reptiles, or the uropygial gland of most birds.
Skin has a soft tissue mechanical behavior when stretched. The intact skin is prestreched like wetsuits around the diver's body. When deep cuts are made on the skin, it retracts, widening the slice hole.
Human uses and culture
Dutch artist Jalila Essaïdi is trying to create bulletproof skin.
Detailed cross section
- List of cutaneous conditions
- Acid mantle
- Callus – thick area of skin
- Cutaneous respiration - gas exchange conducted through skin
- Cutaneous structure development
- Hair – including hair follicles in skin
- Meissner's corpuscle
- Pacinian corpuscle
- Role of skin in locomotion
- Skin flora
- Superficial fascia
Illustration of Integumentary System
- "Skin care" (analysis), Health-Cares.net, 2007
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- McCracken, Thomas (2000). New Atlas of Human Anatomy. China: Metro Books. pp. 1–240.
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- The Ageing Skin – Structure. pharmaxchange.info. March 3, 2011
- Breitkreutz, D; Mirancea, N; Nischt, R (2009). "Basement membranes in skin: Unique matrix structures with diverse functions?". Histochemistry and cell biology 132 (1): 1–10.
- Iozzo, RV (2005). "Basement membrane proteoglycans: From cellar to ceiling". Nature reviews. Molecular cell biology 6 (8): 646–56.
- Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 129–145.
- Human skin strengthened with spider silk can stop a bullet (2:16). Reuters (2011-09-20)