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'''Skin''' is the soft outer tissue covering ]s. '''Skin''' is the soft outer tissue covering ]s.

Other ], such as the ], have different ], structure and ]. The adjective '''cutaneous''' means "of the skin" (from Latin ''cutis'', skin). In ]s, the skin is an ] of the ] made up of multiple layers of ] ], and guards the underlying ]s, ]s, ]s and ]s. Skin of a different nature exists in ]s, ]s, and ]s.<ref>{{cite journal | vauthors = Alibardi L | year = 2003 | title = Adaptation to the land: The skin of reptiles in comparison to that of amphibians and endotherm amniotes | url = | journal = J Exp Zoolog B Mol Dev Evol. | volume = 298 | issue = 1| pages = 12–41 | doi = 10.1002/jez.b.24 | pmid = 12949767 }}</ref> All mammals have some hair on their skin, even ]s like ]s, ]s, and ]s which appear to be hairless.
The skin 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 ] against ]s<ref name="Proksch">{{cite journal | vauthors = Proksch E, Brandner JM, Jensen JM | year = 2008 | title = The skin: an indispensable barrier | journal = Exp Dermatol | volume = 17 | issue = 12| pages = 1063–72 | pmid = 19043850 | doi=10.1111/j.1600-0625.2008.00786.x}}</ref> and excessive water loss.<ref name="Madison">{{cite journal | vauthors = Madison KC | year = 2003 | title = Barrier function of the skin: "la raison d'être" of the epidermis | url = http://www.nature.com/jid/journal/v121/n2/pdf/5601872a.pdf | format = PDF | journal = J Invest Dermatol | volume = 121 | issue = 2| pages = 231–41 | doi = 10.1046/j.1523-1747.2003.12359.x | pmid = 12880413 }}</ref> Its other functions are ], ] regulation, sensation, and the production of ] folates. Severely damaged skin may heal by forming ]. 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&nbsp;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&nbsp;mm thick and is the thickest skin on the body. The speed and quality of wound healing in skin is promoted by the reception of ].<ref>{{cite journal|url=http://faculty.washington.edu/andchien/PDFs/HuBio/estrogen.pdf|journal=Experimental Dermatology|title=The biological actions of estrogen in skin|author=Thornton MJ|year=2002}}</ref><ref>{{cite journal|journal=The American Journal of Pathology|title=Topical Estrogen Accelerates Cutaneous Wound Healing in Aged Humans Associated with an Altered Inflammatory Response|author1=Gillian S. Ashcroft |author2=Teresa Greenwell-Wild |author3=Mark W. J. Ferguson |last-author-amp=yes |year=1999|pmc=1867002|volume=155|issue=4|pages=1137–1146|pmid=10514397|doi=10.1016/S0002-9440(10)65217-0}}</ref><ref>{{cite journal|url=http://www.medscape.com/viewarticle/524313_3|author=Desiree May Oh, MD, Tania J. Phillips, MD|title=Sex Hormones and Wound Healing|journal=Wounds|year=2006}}</ref>

] is dense hair.<ref>{{cite web|url=http://www.thefreedictionary.com/fur|title=fur|publisher=|accessdate=4 March 2017|via=The Free Dictionary}}</ref> Primarily, fur augments the insulation the skin provides but can also serve as a ] or as ]. On some animals, the skin is very hard and thick, and can be processed to create ]. ]s and ] have hard protective scales on their skin for protection, and ]s have hard feathers, all made of tough β-]s. ] skin is not a strong barrier, especially regarding the passage of chemicals via skin and is often subject to ] and diffusive forces. For example, a ] sitting in an ] solution would be sedated quickly, as the chemical diffuses through its skin. Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.<ref name=clarke>{{cite journal|last1=Clarke|first1=BT|title=The natural history of amphibian skin secretions, their normal functioning and potential medical applications.|url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=651&fileId=S0006323197005045|journal=Biological reviews of the Cambridge Philosophical Society|year=1997|volume=72|issue=3|pages=365–379|pmid=9336100|doi=10.1017/s0006323197005045}}</ref>

==Structure in humans and mammals==
{{See also|Human skin}}
{{anchor|Skin layers|Layers|Layer}}
<!-- This section is linked from redirect "]"; please adjust when changing section title -->
{{Infobox anatomy
| Name = Dermis
| Latin =
| GraySubject = 234
| GrayPage = 1065
| Image = Gray942.png
| Caption = 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.)
| Width =
| Image2 = Gray940.png
| Caption2 = A diagrammatic sectional view of the skin (''click on image to magnify''). (Dermis labeled at center right.)
| ImageMap =
| MapCaption =
| Precursor =
| System =
| Artery =
| Vein =
| Nerve =
| Lymph =
| MeshName = Dermis
| MeshNumber = A17.815.180
| Code =
| Dorlands = seven/000097765
| DorlandsID = Skin
}}
]'')<br/>
] of fingertip, depicting ] (~500&nbsp;µm thick) with stratum disjunctum on top and ] (connection to stratum spinosum) in the middle. At the bottom superficial parts of the dermis. Sweatducts are clearly visible.{{citation needed|date=January 2011}}]]
Mammalian skin is composed of two primary layers:
*the '']'', which provides waterproofing and serves as a barrier to infection; and
*the '']'', which serves as a location for the ] of skin;

===Epidermis===
{{main|Epidermis}}

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 ] from entering, and is a stratified squamous ],<ref name="Rooks">{{cite book
| last1=McGrath |first1=J.A. |last2=Eady|first2=R.A. |last3=Pope|first3=F.M.
| title=Rook's Textbook of Dermatology
| year=2004 | edition=7th
| publisher=Blackwell Publishing | isbn=978-0-632-06429-8
| pages=3.1–3.6
}}</ref> composed of ] basal and ] suprabasal ]s.

]s are the major ], constituting 95% of the ],<ref name="Rooks" /> while ]s, ]s and ]s are also present. The ] can be further subdivided into the following ''strata'' or layers (beginning with the outermost layer):<ref name="ageingskin">. pharmaxchange.info. March 3, 2011</ref>
*]
*] (only in ] and ])
*]
*]
*] (also called the ])
]s in the ] proliferate through ] and the daughter ] move up the strata changing shape and composition as they undergo multiple stages of ] to eventually become anucleated. During that process, ]s will become highly organized, forming ] (]) between each other and secreting ] ] and ] which contribute to the formation of an ] and provide mechanical ] to the skin.<ref name="Pmid">{{cite journal | pmid=19333614 | year=2009 | last1=Breitkreutz | first1=D | last2=Mirancea | first2=N | last3=Nischt | first3=R | title=Basement membranes in skin: Unique matrix structures with diverse functions? | volume=132 | issue=1 | pages=1–10 | doi=10.1007/s00418-009-0586-0 | journal=Histochemistry and cell biology}}</ref> ]s from the ] are eventually shed from the surface (]).

The ] contains no ]s, and ] in the deepest layers are nourished by diffusion from ] ] extending to the upper layers of the ].

===Basement membrane===
{{main|Basement membrane}}

The ] and ] are separated by a thin sheet of ] called the ], and is made through the action of both ].
The basement membrane controls the traffic of the ] and ] between the dermis and epidermis but also serves, through the binding of a variety of ]s and ], as a reservoir for their controlled release during ] remodeling or ] processes.<ref>{{cite journal | pmid=16064139 | year=2005 | last1=Iozzo | first1=RV | title=Basement membrane proteoglycans: From cellar to ceiling | volume=6 | issue=8 | pages=646–56 | doi=10.1038/nrm1702 | journal=Nature Reviews Molecular Cell Biology}}</ref>

===Dermis===
{{main|Dermis}}

The '''dermis''' is the layer of skin beneath the ] that consists of ] and cushions the body from ] and strain.
The ] provides tensile ] and ] to the skin through an ] composed of ], ], and ], embedded in ] and ]s.<ref name="Pmid" /> Skin proteoglycans are varied and have very specific locations.<ref>{{cite journal | vauthors = Smith MM, Melrose J | title = Proteoglycans in normal and healing skin | journal = Adv. Wound Care | volume = 4 | issue = 3 | pages = 152–73 | year = 2015 | pmid = 25785238 | doi = 10.1089/wound.2013.0464 | pmc=4352701}}</ref> For example, ], ] and ] are present throughout the ] and ] ], whereas ] and ] are only found in the ].

It harbors many ]s (nerve endings) that provide the sense of ] and ] through ]s and ]s. It also contains the ]s, ]s, ]s, ], ]s and ]s. The ]s in the ] provide nourishment and waste removal from its own ] as well as for the ].

The ] is tightly connected to the ] through a ] and is structurally divided into two areas: a superficial area adjacent to the ], called the ''papillary region'', and a deep thicker area known as the ''reticular region''.

====Papillary region====
The papillary region is composed of loose ].This is named for its fingerlike projections called ''papillae'' that extend toward the ]. The papillae provide the ] with a "bumpy" surface that interdigitates with the ], strengthening the connection between the two layers of skin.

====Reticular region====
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular ], and receives its name from the dense concentration of ]ous, ], and ] fibers that weave throughout it. These ] fibers give the ] its properties of ], ], and ].
Also located within the reticular region are the ], ]s,sebaceous glands ], ], and ]s.

===Subcutaneous tissue===
{{main|Subcutaneous tissue}}

The ] (also hypodermis) is not part of the skin, and lies below the ]. Its purpose is to attach the skin to underlying ] and ] as well as supplying it with ]s and ]s. It consists of loose ] and ]. The main ] types are ]s, ]s and ]s (the subcutaneous tissue contains 50% of ]). ] serves as padding and insulation for the body.

] like ''] epidermidis'' colonize the skin surface. The density of ] depends on region of the skin. The disinfected skin surface gets recolonized from ] residing in the deeper areas of the ], ] and ] openings.

===Detailed cross section===
{{wide image|Skin_layers.svg|800px|Skin layers, of both the hairy and hairless skin}}

==Structure in Fish, Amphibians, Birds, and Reptiles==

===Fish===
{{see also|Fish scales}}

The epidermis of ] and of most ]s consists entirely of live ], with only minimal quantities of ] in the ] of the superficial layer. It is generally permeable, and in the case of many ], may actually be a major respiratory organ. The ] of ] typically contains relatively little of the ] found in ]s. Instead, in most species, it is largely replaced by solid, protective bony ]s. Apart from some particularly large dermal bones that form parts of the ], these ]s are lost in ], although many ]s do have ]s of a different kind, as do ]s. ] have numerous tooth-like ]s embedded in their skin, in place of true ]s.

]s and ]s are both unique to ], but other types of skin gland are found in other ]. ] typically have a numerous individual ]-secreting skin ] that aid in insulation and protection, but may also have ] ], ]s, or ] that produce a more watery, ] fluid. In ], the ] ] are gathered together to form sac-like ]. Most living ] also possess ''granular ]'' in the skin, that secrete irritating or toxic compounds.<ref name=VB/>

Although ] is found in the skin of many species, in the ]s, the ], and ], the ] is often relatively colourless. Instead, the colour of the skin is largely due to ]s in the ], which, in addition to ], may contain ] or ] ]. Many species, such as ]s and ]s may be able to change the colour of their skin by adjusting the relative size of their ].<ref name=VB/>

=== Amphibians ===
''See also: ]s''

==== Overview ====
Amphibians possess two types of ]s, ] and granular (serous). Both of these glands are part of the ] and thus considered ]. Mucous and granular glands are both divided into three different sections which all connect to structure the gland as a whole. The three individual parts of the gland are the duct, the intercalary region, and lastly the alveolar gland (sac). Structurally, the duct is derived via ]s and passes through to the surface of the ] or outer skin layer thus allowing external secretions of the body. The gland alveolus is a sac shaped structure which is found on the bottom or base region of the granular gland. The cells in this sac specialize in secretion. Between the alveolar gland and the duct is the intercalary system which can be summed up as a transitional region connecting the duct to the grand alveolar beneath the epidermal skin layer. In general, granular glands are larger in size than the mucous glands, however mucous glands hold a much greater majority in overall number.<ref name=":0">{{Cite journal|url=http://www.sciencedirect.com/science/article/pii/030096299598515I|title=Cutaneous granular glands and amphibian venoms - ScienceDirect|language=en|access-date=2017-04-27|doi=10.1016/0300-9629(95)98515-I|volume=111|journal=Comparative Biochemistry and Physiology Part A: Physiology|pages=1–29 | last1 = Toledo | first1 = R.C.}}</ref>
[[File:FrogSkin.png|thumb|616x616px|'''Frog Gland Anatomy-''' A: Mucous gland (alveolus), B: Chromophore, C: Granular Gland (alveolus), D: Connective Tissue, E: Stratum Corneum, F: Transition Zone (intercalary region), G: Epidermis (Where the duct resides), H: Dermis
Photo Credit: John Houseman
License: ]]
]]

==== Granular Glands ====
Granular glands can be identified as ]ous and often differ in the type of toxin as well as the concentrations of secretions across various orders and species within the amphibians. They are located in clusters differing in concentration depending on ] taxa. The toxins can be fatal to most vertebrates or have no effect against others. These glands are alveolar meaning they structurally have little sacs in which venom is produced and held before it is secreted upon defensive behaviors.<ref name=":0" />

Structurally, the ducts of the granular gland initially maintain a cylindrical shape. However, when the ducts become mature and full of toxic fluid, the base of the ducts become swollen due to the pressure from the inside. This causes the epidermal layer to form a pit like opening on the surface of the duct in which the inner fluid will be secreted in an upwards fashion.<ref name=":1">{{Cite book|url=https://books.google.com/?id=UklQAQAAMAAJ&pg=PA635&lpg=PA635&dq=necturus+integument+google+book#v=onepage&q=necturus%20integument%20google%20book&f=false|title=Journal of Morphology|date=1920-01-01|publisher=Wistar Institute of Anatomy and Biology .|language=en}}</ref>

The intercalary region of granular glands are more developed and mature in comparison with mucous glands. This region resides as a ring of cells surrounding the basal portion of the duct which are argued to have an ]al muscular nature due to their influence over the lumen (space inside the tube) of the duct with dilation and constriction functions during secretions. The cells are found radially around the duct and provide a distinct attachment site for muscle fibers around the gland's body.<ref name=":1" />

The gland alveolus is a sac that is divided into three specific regions/layers. The outer layer or tunica fibrosa is composed of densely packed connective-tissue which connects with fibers from the spongy intermediate layer where elastic fibers as well as nerves reside. The nerves send signals to the muscles as well as the epithelial layers. Lastly, the epithelium or tunica propria encloses the gland.<ref name=":1" />

==== Mucous Glands ====
Mucous glands are non-venomous and offer a different functionality for amphibians than granular. Mucous glands cover the entire surface area of the amphibian body and specialize in keeping the body lubricated. There are many other functions of the mucous glands such as controlling the pH, thermoregulation, adhesive properties to the environment, anti-predator behaviors (slimy to the grasp), chemical communication, even anti-bacterial/viral properties for protection against pathogens.<ref name=":0" />

The ducts of the mucous gland appear as cylindrical vertical tubes which break through the epidermal layer to the surface of the skin. The cells lining the inside of the ducts are oriented with their longitudinal axis forming 90 degree angles surrounding the duct in a helical fashion.<ref name=":1" />

Intercalary cells react identically to those of granular glands but on a smaller scale. Among the amphibians, there are taxa which contain a modified intercalary region (depending on the function of the glands), yet the majority share the same structure.<ref name=":1" />

The alveolor of mucous glands are much more simple and only consist of an epithelium layer as well as connective tissue which forms a cover over the gland. This gland lacks a tunica propria and appears to have delicate and intricate fibers which pass over the gland's muscle and epithelial layers.<ref name=":1" />

===Birds and reptiles===
{{Main|Reptile scales}}
The ] of ]s and ]s is closer to that of ], with a layer of dead keratin-filled ] at the surface, to help reduce ] loss. A similar pattern is also seen in some of the more terrestrial ] such as ]s. However, in all of these ] there is no clear ] of the ] into distinct layers, as occurs in ]s, with the change in ] type being relatively gradual. The ]ian ] always possesses at least a ] and ], but the other intermediate layers found in ]s are not always distinguishable.
] is a distinctive feature of ]ian skin, while ]s are (at least among living species) similarly unique to ]s.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 129–145|isbn= 0-03-910284-X}}</ref>

]s and ]s have relatively few skin ]s, although there may be a few structures for specific purposes, such as ]-secreting ] in some ]s, or the ] of most ]s.<ref name=VB/>

==Development==
{{Cn span|Cutaneous structures arise from the ] and include a variety of features such as hair, feathers, claws and nails. During embryogenesis, the epidermis splits into two layers: the periderm (which is lost) and the ]. The basal layer is a ] layer and through asymmetrical divisions, becomes the source of skin cells throughout life. It is maintained as a stem cell layer through an ] signal, ], and through ] signal FGF7 aka ] (KGF) produced by the dermis below the basal cells. In mice, over-expression of these factors leads to an overproduction of granule cells and thick skin.|date=December 2017}}

{{Cn span|Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system. This ] combines an activator, ], with an inhibitor, BMP4 or BMP2, to form clusters of cells in a regular pattern. Sonic hedgehog-expressing epidermal cells induce the condensation of cells in the mesoderm. The clusters of mesodermal cells signal back to the epidermis to form the appropriate structure for that position. BMP signals from the epidermis inhibit the formation of placodes in nearby ectoderm.|date=December 2017}}

{{Cn span|It is believed that the mesoderm defines the pattern. The epidermis instructs the mesodermal cells to condense and then the mesoderm instructs the epidermis of what structure to make through a series of reciprocal inductions. Transplantation experiments involving frog and newt epidermis indicated that the mesodermal signals are conserved between species but the epidermal response is species-specific meaning that the mesoderm instructs the epidermis of its position and the epidermis uses this information to make a specific structure.|date=December 2017}}

==Functions==
Skin performs the following functions:

#Protection: an anatomical barrier from ] and damage between the internal and external ] in bodily defense. (See ].) ]s in the skin are part of the ].<ref name="Proksch"/><ref name="Madison"/>
#]: contains a variety of ] that jump to ], ], ], ], and ] ] (see ] and ]).
#Thermoregulation: ] (]) glands and dilated blood vessels (increased superficial ]) aid heat loss, while constricted ] greatly reduce cutaneous ] and conserve ]. ]s in ] adjust the angle of ] shafts to change the degree of insulation provided by hair or ].
#Control of ]: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.<ref name="Madison"/>
#Storage and ]: acts as a storage center for ]s and ]
#]: ], ] and ] can diffuse into the ] in small amounts; some ]s use their skin as their sole ] (in ]s, the ] comprising the outermost 0.25–0.40&nbsp;mm of the skin are "almost exclusively supplied by external ]", although the "contribution to total ] is negligible")<ref>{{cite journal | vauthors = Stücker M, Struk A, Altmeyer P, Herde M, Baumgärtl H, Lübbers DW | title = The cutaneous uptake of atmospheric oxygen contributes significantly to the oxygen supply of human dermis and epidermis | journal = J. Physiol. | volume = 538 | issue = 3 | pages = 985–994 | year = 2002 | pmid = 11826181 | doi = 10.1113/jphysiol.2001.013067 | pmc=2290093}}</ref> Some ].
#] resistance: The skin acts as a ] resistant barrier so essential ] 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 ]. This is helped in part by the sebaceous glands that release ], 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.<ref>{{cite book|last=McCracken|first=Thomas|title=New Atlas of Human Anatomy|year=2000|publisher=Metro Books|location=China|isbn=1-58663-097-0|pages=1–240}}</ref>
#], whether the skin is naked or covered in fur, scales, or feathers, skin structures provide protective coloration and patterns that help to conceal animals from predators or prey.<ref>{{cite web| title=Camouflage |url=http://www.nationalgeographic.org/encyclopedia/camouflage/ |publisher=National Geographic |accessdate=27 February 2017}}</ref>

===Mechanics===
{{main|Soft tissue}}
Skin is a soft tissue and exhibits key mechanical behaviors of these tissues. The most pronounced feature is the J-curve stress strain response, in which a region of large strain and minimal stress exists, and corresponds to the microstructural straightening and reorientation of collagen fibrils.<ref>{{Cite journal |last=Sherman |first=Vincent R. |title=The materials science of collagen |journal=Journal of the Mechanical Behavior of Biomedical Materials |volume=52 |pages=22–50 |year=2015 |doi=10.1016/j.jmbbm.2015.05.023 |pmid=26144973}}</ref> In some cases the intact skin is prestreched, like wetsuits around the diver's body, and in other cases the intact skin is under compression. Small circular holes punched on the skin may widen or close into ellipses, or shrink and remain circular, depending on preexisting stresses.<ref>{{Cite journal |last=Bush |first=James A. |title=Skin tension or skin compression? Small circular wounds are likely to shrink, not gape |journal=Journal of Plastic, Reconstructive & Aesthetic Surgery |pmid=17652049 |doi=10.1016/j.bjps.2007.06.004 |volume=61 |issue=5 |pages=529–34|year=2008 }}</ref>


==Society and culture== ==Society and culture==

Revision as of 23:37, 5 December 2017

This article is about the outer covering of animals. For skin in humans, see human skin. For other uses, see Skin (disambiguation).
Skin
Skin of an elephant
Details
Identifiers
LatinCutis
MeSHD012867
TA98A16.0.00.002
TA27041
Anatomical terminology[edit on Wikidata]

Skin is the soft outer tissue covering vertebrates.

Society and culture

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 the different animals are used for clothing, bags and other consumer products, usually in the form of leather, but also as 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.

See also

References

External links

Physiology of skin
Skin physiology
Hair
Superficial fascia and areolar connective tissue
Development of skin
Skin
Skin appendage
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