Name | Skin |
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Image2 | HumanSkinDiagram.jpg |
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width | 300px |
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Caption2 | A diagram of human skin. |
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Dorlandssuf | }} |
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Skin is the soft outer covering of vertebrates. Other animal coverings such as the arthropod exoskeleton or the seashell have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin ''cutis'', skin). In mammals, the skin is the largest organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, and birds. All mammals have some hair on their skin, even marine mammals which appear to be hairless.
The skin is one of the most important parts of the body because it interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens and excessive water loss. Its other functions are insulation, temperature regulation, sensation, and the production of vitamin D folates. Severely damaged skin may heal by forming scar tissue. This is sometimes discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans for example, the skin located under the eyes and around the eyelids is the thinnest skin in the body at 0.5 mm thick, and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is 4 mm thick and the thickest skin in the body.
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 to passage of chemicals and is often subject to osmosis. For example, a frog sitting in an anesthetic solution could quickly go to sleep.
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).
#Heat regulation: increase perfusion and heatloss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat. Erector pili muscles are significant in animals.
#Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to fluid loss.
#Storage and synthesis: acts as a storage center for lipids and water
#Absorption: oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts; some animals use their skin as their sole respiration organ (in 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 our skin are covered by our 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.
thumb|(''See also: [[:File:HautFingerspitzeOCT.gif|image rotating (1.1 mb)'' )
Optical coherence tomogram of fingertip, depicting stratum corneum (~500 µm thick) with stratum disjunctum on top and stratum lucidum (connection to stratum spinosum) in the middle. At the bottom superficial parts of the dermis. Sweatducts are clearly visible.]]
Name | Dermis |
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Graysubject | 234 |
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Graypage | 1065 |
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Image2 | Gray940.png |
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Caption2 | A diagrammatic sectional view of the skin (''click on image to magnify''). (Dermis labeled at center right.) |
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Meshname | Dermis |
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Meshnumber | A17.815.180 |
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Dorlands | seven/000097765 |
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Dorlandsid | Skin
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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)
Keratinocytes in the
stratum basale proliferate through
mitosis and the daughter
cells move up the strata changing shape and composition as they undergo multiple stages of
cell differentiation to eventually become anucleated. During that process
keratinocytes will become highly organized, forming
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 contains no blood vessels, and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis.
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 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.
Microorganisms like ''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.
The epidermis of
fish and of most
amphibians consists entirely of live
cells, with only minimal quantities of
keratin in the
cells of the superficial layer. It is generally permeable, and, in the case of many
amphibians, may actually be a major respiratory organ. The
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 reptiles, 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.
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 (i.e. has
residual stress) like
neoprene wetsuits around the diver's
body. When deep cuts are made on the skin, it retracts, widening the slice hole.
The term "skin" may also refer to the covering of a small
animal, such as a
sheep,
goat (
goatskin),
pig,
snake (
snakeskin) etc. or the young of a large
animal.
The term hides or rawhide refers to the covering of a large adult animal such as a cow, buffalo, horse etc.
Skins and hides from different animals are used for clothing, bags and other consumer products, usually in the form of leather, but also furs.
Skin from sheep, goat and cattle was used to make parchment for manuscripts.
Skin can also be cooked to make pork rind or crackling.
Dutch artist Jalila Essaïdi is trying to create bulletproof skin.
List of cutaneous conditions
Acid mantle
Callus – thick area of skin
Cutaneous structure development
Hair – including hair follicles in skin
Intertriginous
Moult
Meissner's corpuscle
Pacinian corpuscle
Rawhide
Role of skin in locomotion
Superficial fascia
Potential of embryonic stem cells to reconstitute a skin.
Category:Soft tissue
Category:Leathermaking
Category:Skin
Category:Organs
Category:Animal anatomy
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