View source for Skin

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: ''Beyond overall skin structure, refer below to: [[#See also|See-also]].''
[[Image:Skin.jpg|thumb |300px| '''Skin''' layers: [[Epidermis (skin)|epidermis]], [[dermis]], and [[subcutis]], showing a [[hair follicle]], [[sweat gland]] & [[sebaceous gland]].]]
In [[zootomy]] and [[dermatology]], '''skin''' is the largest [[organ (anatomy)|organ]] of the [[integumentary system]] made up of multiple layers of [[epithelial]] [[biological tissue|tissues]] that guard underlying [[muscle]]s and [[organ (anatomy)|organ]]s.
<ref name="Hcare">
   "Skin care" (analysis),
   Health-Cares.net, 2007, webpage:
   [http://skin-care.health-cares.net/oily-skin-care.php HCcare].
</ref>
Skin pigmentation (''see:'' [[human skin color]] or coloring) varies among populations, and '''skin type''' can range from [[dry skin]] to '''oily skin'''.

The adjective '''cutaneous''' literally means "of the skin" (from Latin ''cutis'', skin).

As the interface with the surroundings, skin plays the most important role in protecting (the body) against [[pathogen]]s. Its other main functions are [[Thermal insulation|insulation]] and [[temperature]] regulation, sensation, and synthesis of [[vitamin D]] and the protection of [[vitamin B]] folats.

Severely damaged skin will try to heal by forming [[scar|scar tissue]], often giving rise to discoloration and depigmentation of the skin.

The use of natural or synthetic cosmetics to treat the appearance of the face and condition of the skin (such as pore control and [[blackhead]] cleansing) is common among many cultures.  Oily skin is caused by hormonal fluctuations in the body, which lead to a [[Dihydrotestosterone|DHT]] sensitivity. This sensitivity means that the skin begins to lose moisture and essential fatty acids (linoleic acid in particular), causing thousands of skin cells to die, so the skin compensates for this loss of moisture by producing higher levels of oil.
<ref name="SkinMed">
   "Why Me, Why Now?"
   SkinMed.co.uk, 2007, webpage:
   [http://www.skinmed.co.uk/skinmed_aknicare/science1.html].
</ref>
Oily skin can be cleaned quickly with a mild solution of laundry detergent,<ref name=Hcare/> when pure bath soaps fail (''see below: [[#Hygiene|Hygiene]]''). Afterward, body lotions could be used to recondition cleansed skin,<ref name=Hcare/> as would be used to treat dry skin.

==Skin components==
: ''Also see below: [[#Skin layers|Skin layers]], at bottom.''

Skin has pigmentation, or [[melanin]], provided by [[melanocyte]]s, which absorb some of the potentially dangerous [[ultraviolet radiation]] (UV) in [[sunlight]]. It also contains [[DNA]] repair [[enzyme]]s which help to reverse UV damage, and people who lack the [[gene]]s for these enzymes suffer high rates of [[skin cancer]]. One form predominantly produced by UV light, [[malignant]] [[melanoma]], is particularly invasive, causing it to [[metastasize|spread]] quickly, and can often be deadly. Human skin pigmentation varies among populations in a striking manner. This has sometimes led to the classification of people(s) on the basis of [[human skin color|skin color]].

[[Mammal]]ian skin often contains hairs, which in sufficient density is called fur. The [[hair]] mainly serves to augment the insulation the skin provides, but can also serve as a [[Secondary sex characteristic|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-[[keratin]]s. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to [[osmosis]]. A frog sitting in an [[anesthetic]] solution could quickly go to sleep.

The skin is often known as the largest organ of the human body. This applies to exterior surface, as it covers the body, ''appearing'' to have the largest surface area of all the organs. Moreover, it applies to weight, as it weighs more than any single internal organ, accounting for about 15 percent of body weight. For the average adult human, the skin has a surface area of between 1.5-2.0 square meters (8-10.8 sq.ft.), most of it is between 2-3 mm (0.10 inch) thick. The average square inch (6 cm²) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than a thousand nerve endings.

==Functions==
Skin performs the following functions:

#Protection: an anatomical barrier between the internal and external environment in bodily defense; [[Langerhans cell]]s in the skin are part of the [[adaptive immune system]]
#[[Sensation]]: contains a variety of nerve endings that react to [[thermoreceptor|heat and cold]], touch, pressure, vibration, and tissue injury; see [[somatosensory system]] and [[Somatosensory system|haptics]].
#Heat regulation: the skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss while constricted vessels greatly reduce cutaneous blood flow and conserve heat. [[Erector pili muscle]]s are significant in animals.
#Control of evaporation: the skin provides a relatively dry and impermeable barrier to fluid loss. Loss of this function contributes to the massive fluid loss in [[burn (injury)|burns]].
#Aesthetics and communication:  others see our skin and can assess our mood, physical state and attractiveness.
#Storage and synthesis: acts as a storage center for lipids and water, as well as a means of synthesis of [[vitamin D]] by action of [[UV]] on certain parts of the skin.
#Excretion: The concentration of [[urea]] is 1/130th that of urine.  Excretion by sweating is at most a secondary function to temperature regulation.
#Absorption: Oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts, some animals using their skin for their sole [[respiration organ]]. In addition, medicine can be administered through the skin, by ointments or by means of adhesive [[transdermal patch|patch]], such as the [[nicotine patch]] or [[iontophoresis]]. The skin is an important site of transport in many other organisms.

==Hygiene==
Unclean skin favors the development of pathogenic organisms - the dead cells that continually slough off of the epidermis mix with the secretions of the sweat and sebaceous glands and the dust found on the skin to form a filthy layer on its surface. If not washed away, the slurry of sweat and sebaceous secretions mixed with dirt and dead skin is decomposed by bacterial flora, producing a foul smell. Functions of the skin are disturbed when it is excessively dirty; it becomes more easily damaged, the release of antibacterial compounds decreases, and dirty skin is more prone to develop infections. Cosmetics should be used carefully because these may cause allergic reactions. Each season requires suitable clothing in order to facilitate the evaporation of the sweat. Sunlight, water and air play an important role in keeping the skin healthy.

The skin supports its own [[ecosystem]]s of [[microorganism]]s, including [[yeast]]s and [[bacteria]], which cannot be removed by any amount of cleaning. Estimates place the number of individual bacteria on the surface of one square inch (6.5 square cm) of human skin at 50 million though this figure varies greatly over the average 20 feet<sup>2</sup> (1.9 m²) of human skin.  Oily surfaces, such as the face, may contain over 500 million bacteria per square inch (6.5 cm²).  Despite these vast quantities, all of the bacteria found on the skin's surface would fit into a volume the size of a pea.<ref>Theodor Rosebury. ''Life on Man'': Secker & Warburg, 1969 ISBN 0-670-42793-4</ref> In general, the microorganisms keep one another in check and are part of a healthy skin. When the balance is disturbed, there may be an overgrowth and infection, such as when [[antibiotic]]s kill [[microbe]]s, resulting in an overgrowth of yeast. The skin is continuous with the inner [[epithelial]] lining of the body at the orifices, each of which supports its own complement of microbes.

Oily skin is caused by over-active glands, that produce a substance called [[sebum]], a naturally healthy skin lubricant.<ref name=Hcare/> When the skin produces excessive [[sebum]], it becomes heavy and thick in texture. Oily skin is typified by shininess, blemishes and [[pimples]].<ref name=Hcare/> The oily-skin type is not necessarily bad, since such skin is less prone to wrinkling, or other signs of aging,<ref name=Hcare/> because the oil helps to keep needed moisture locked into the [[Epidermis (skin)|epidermis]] (outermost layer of skin).

The negative aspect of the oily-skin type is that oily complexions are especially susceptible to clogged pores, [[blackhead]]s, and buildup of dead skin cells on the surface of the skin.<ref name=Hcare/> Oily skin can be sallow and rough in texture and tends to have large, clearly visible pores everywhere, except around the eyes and neck.<ref name=Hcare/>

The goal of treating oily skin is to remove excess surface sebum without complete removal of skin [[lipids]].<ref name=Hcare/> Severe degreasing treatment can foster an actual worsening of sebum secretion, which defeats the aim of the cleansing.<ref name=Hcare/> A method of cleansing oily skin is to wash with a solution of a mild synthetic [[detergent]]<ref name=Hcare/> (''see:'' [[surfactant]]) containing no oils, [[wax]]es or other lipid agents that could aggravate the oily condition of the skin, sometimes combined with a toning lotion. Such a product removes the oily residue and debris from the skin surface. Some cleansing products have lower concentrations of [[hydroxy acid]]s, which remove dead cells from the upper levels of the [[stratum corneum]].<ref name=Hcare/> Those products should be used on a regular basis to work adequately.<ref name=Hcare/> A light moisturizer may be included in a product to counteract any drying effects of the cleanser.<ref name=Hcare/>

== Aging ==
{{details|senescence}}
[[Image:Rash.jpg|thumb|right|250px|A typical rash]]
[[Image:Scabies-burrow.jpg|thumb|right|200px|Skin infected with [[Scabies]]]]
As skin ages, it becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself, as a person ages and skin requires a longer time to heal in later life.

Skin sagging is caused by the fall in elasticity. Ageing skin also receives less blood flow and lower gland activity.
<br/>

==Disease ==
{{details|list of skin diseases}}
In [[medicine]], the branch concerned with the skin is called [[dermatology]].  The skin is subject to constant attack from without, and so can be afflicted by numerous ailments, such as these:

'''Tumors''':
* Benign tumors of the skin such as [[Squamous cell papilloma]] 
* [[Skin cancer]]

'''Others''':
* [[Rash]]es
* [[Blister]]s
* [[Acne vulgaris|Acne]]
* [[Keratosis pilaris]]
* [[fungus|Fungal]] infections such as [[athlete's foot]]
* [[microbe|Microbial]] infections.
* [[Calcinosis cutis]]
* [[Ring worm]]
* [[Sunburn]]
* [[Keloid]]
* [[Scabies]]

There are several other skin diseases as well.

==Variability in skin tone==

Individuals with ancestors from different parts of the world can have highly visible differences in skin pigmentation.  Individuals with African ancestry (black people) tend towards darker skin, while those of Northern European descent (white people) have paler skin.  Between these extremes are individuals of Asian, South-East Asian, Native American, Middle Eastern, Polynesian and Melanesian descent. 

The skin of black people has more variation in color from one part of the body to another than does the skin of other racial groups, particularly the palms of the hands and soles of the feet. Part of this is the result of the variations in the thickness of the skin or different parts of the body.  The thicker the skin, the more layers of cell with melanin in them, and the darker the color.<ref>Smith, Wilma and Burns, Catherine. (1999) "Managing the hair and skin of African American pediatric patients." ''Journal of Pediatric Health Care'' 13(2):72-8.</ref>  In addition, these parts of the body do not have melanin-producing cells.

Darker skin hinders UV A rays from penetrating. Since vitamin B folats are degraded by UV A and vitamin D is synthesised different skin tones are more likely to produce different vitamin deficiencies.

==Animal skin products==
Skins and hides from different animals are used for clothing, bags and other consumer products, usually in the form of leather, but also furs, rawhide, snakeskin and hagfish. Skin can also be used to make products such as [[gelatin]] and glue. See also wool.

==Skin layers==
'''Skin''' is composed of three primary layers: the ''[[epidermis (skin)|epidermis]]'', which provides waterproofing and serves as a barrier to infection; the ''[[dermis]]'', which serves as a location for the [[appendages]] of skin; and the ''[[hypodermis]] (subcutaneous adipose layer)'', which is called the [[basement membrane]].

===Epidermis===
'''Epidermis''', "epi" coming from the Greek meaning "over" or "upon", is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface and is made up of stratified squamous [[epithelium]] with an underlying [[basal lamina]].

The outermost epidermis consists of [[stratified]] [[squamous]] [[epithelium]] with an underlying connective tissue section, or dermis, and a hypodermis, or [[basement membrane]]. The epidermis contains no [[blood vessel]]s, and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis. The main type of cells which make up the epidermis are [[keratinocyte]]s, with [[melanocyte]]s and [[Langerhans cell]]s also present. The epidermis can be further subdivided into the following ''strata'' (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, basale. Cells are formed through [[mitosis]] at the basale layer. The daughter cells, (see [[cell division]]) move up the strata changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and the protein [[keratin]] is inserted. They eventually reach the corneum and slough off ([[desquamation]]). This process is called ''keratinization'' and takes place within about 30 days.  This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and [[pathogens]] out, making skin a natural barrier to infection.

[[Image:HautFingerspitzeOCT nonanimated.gif|thumb |209px| [''also see: &nbsp;[[:Image:HautFingerspitzeOCT.gif|image rotating (1.1 mb)]]'' ]<br/>
Optical Coherence Tomography tomogram of fingertip, depicting [[stratum corneum]] (~500µm 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.]]

====Components====
The epidermis contains no [[blood vessel]]s, and is nourished by [[diffusion]] from the [[dermis]]. The main type of cells which make up the epidermis are [[keratinocyte]]s, [[melanocyte]]s, [[Langerhans cell]]s and [[Merkels cell]]s.

====Layers====
Epidermis is divided into several layers where cells are formed through [[mitosis]] at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with [[keratin]]. They eventually reach the top layer called stratum corneum and become sloughed off, or desquamated. This process is called ''[[keratinization]]'' and takes place within weeks. The outermost layer of Epidermis consists of 25 to 30 layers of dead cells.

====Sublayers====
Epidermis is divided into the following 5 sublayers or strata: 

*[[Stratum corneum]]
*[[Stratum lucidum]]
*[[Stratum granulosum]]
*[[Stratum spinosum]]
*[[Stratum germinativum]] (also called "stratum basale")

[[Mnemonic]]s that are good for remembering the layers of the skin (using "stratum basale" instead of "stratum germinativum"): 
* "'''C'''her '''L'''ikes '''G'''etting '''S'''kin '''B'''otoxed" (from superficial to deep)
* "'''B'''efore '''s'''igning, '''g'''et '''l'''egal '''c'''ounsel" (from deep to superficial)
* "'''B'''efore '''S'''ex '''G'''et '''L'''atex '''C'''ondoms (from deep to superficial)

Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips.

{{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.) |
  Image2       = Gray940.png |
  Width        = 100 | <!--width of both(!) images -->
  Caption2     = A diagrammatic sectional view of the skin (''click on image to magnify''). (Dermis labeled at center right.) |
  System       = |
  Precursor    = |
  MeshName     = Dermis |
  MeshNumber   = A17.815.180 |
  DorlandsPre  = d_11 |
  DorlandsSuf  = 12289496  |
}}

===Dermis===

The '''dermis''' is the layer of skin beneath the [[epidermis (skin)|epidermis]] that consists of [[connective tissue]] and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a [[basement membrane]]. It also harbors many nerve endings that provide the sense of touch and heat. It 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 to its own cells as well as the Stratum basale of the epidermis.

Structure

The dermis 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''.

====Papillary region====
The papillary region is composed of loose [[areolar connective tissue]]. It 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. 

In the palms, fingers, soles, and toes, the influence of the papillae projecting into the epidermis forms contours in the skin's surface. These are called ''friction ridges'', because they help the hand or foot to grasp by increasing friction. Friction ridges occur in patterns (''see:'' [[fingerprint]]) that are genetically determined and are therefore unique to the individual, making it possible to use fingerprints or footprints as a means of identification.

====Reticular region====
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, [[nail (anatomy)|nails]], and [[blood vessels]].

Tattoo ink is injected into the dermis.  Stretch marks from pregnancy are also located in the dermis.

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 [[fibroblast]]s, [[macrophage]]s and [[adipocyte]]s (the hypodermis contains 50% of body fat). Fat serves as padding and insulation for the body.

Microorganisms like ''[[Staphylococcus]] epidermidis'' colonize the skin surface. These microorganisms serve as ecoorgan. 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.

==See also==
Related topics (in alphabetical order):
* [[Acid mantle]]
* [[Callus]] - thick area of skin
* [[Cosmetic surgery]]
* [[Cutaneous structure development]]
* [[List of skin diseases|Diseases]] - list of skin diseases
* [[Dermatology]] - branch of medicine
* [[Fingerprint]] - skin on fingertips
* [[Hair]] - including [[hair follicles]] in skin
* [[Human skin color|Skin color]]
* [[Hyperpigmentation]] - about excess skin color
* [[Meissner's corpuscle]]
* [[Nail (anatomy)|Nails]] - fingernails or toenails
* [[Pacinian corpuscle]]
* [[Polyphenol antioxidant]]
* [[Sweat]] - description of perspiration
* [[Superficial fascia]]

==References==
<references/>
<br/>

==External links==
*[http://www.cosmeticdoctors.co.uk/medical.asp Skin care - The British Association of Cosmetic Doctors]
<br>

{{integumentary_system}}
{{human anatomical features}}

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[[Category:Sensory organs]]
[[Category:Integumentary system]]
[[Category:Dermatology]]

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[[es:Piel]]
[[eo:Haŭto]]
[[fa:پوست]]
[[fr:Peau]]
[[gl:Pel]]
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[[id:Kulit]]
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[[pam:Balat]]
[[pl:Skóra (histologia)]]
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[[tr:Cilt (Organ)]]
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[[tl:Balat]]

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