Skin

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Skin is the soft outer covering of vertebrates. Other cuticle|animal coverings such as the arthropod exoskeleton or the seashell have different Cellular differentiation|developmental origin , structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin cutis , skin). In mammal s, the skin is the largest organ (anatomy)|organ of the integumentary system made up of multiple layers of ectodermal tissue (biology)|tissue , and guards the underlying muscle s, bone s, ligament s and organ (anatomy)|internal organ s. Skin of a different nature exists in amphibian s, reptile s, and bird s.Alibardi L. (2003). Adaptation to the land: The skin of reptiles in comparison to that of amphibians and endotherm amniotes. J Exp Zoolog B Mol Dev Evol. 298(1):12–41. PMID 12949767 All mammals have some hair on their skin, even marine mammal s 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 pathogen sProksch E, Brandner JM, Jensen JM. (2008).The skin: an indispensable barrier. Exp Dermatol. 17(12):1063–72. PMID 19043850 and excessive water loss.Madison KC. (2003). . J Invest Dermatol. 121(2):231-41. PMID 12880413 Its other functions are Thermal insulation|insulation , temperature regulation, sensation, and the production of vitamin D folates. Severely damaged skin may heal by forming scar|scar tissue . This is often 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 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 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 . Reptile s and fish have hard protective scales on their skin for protection, and bird s have hard feathers, all made of tough ß- keratin s. 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.

Functions

Skin performs the following functions:

#Protection: an anatomical barrier from pathogens and damage between the internal and external natural environment|environment in bodily defense; Langerhans cell s in the skin are part of the adaptive immune system .
# Wikt:sensation|Sensation : contains a variety of nerve endings that jump to thermoreceptor|heat and cold , touch , pressure , vibration , and tissue (biology)|tissue injury (see somatosensory system and haptic perception ).
# Heat regulation: increase perfusion and heatloss, while constricted blood vessel|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 semi-impermeable barrier to fluid loss.
#Storage and biosynthesis|synthesis : acts as a storage center for lipids and water
#Absorption: oxygen , nitrogen and carbon dioxide can diffuse into the epidermis (skin)|epidermis in small amounts; some animals use their skin as their sole respiration organ (in humans , the cell (biology)|cells comprising the outermost 0.25–0.40& nbsp;mm of the skin are "almost exclusively supplied by external oxygen ", although the "contribution to total respiration (physiology)|respiration is negligible")Stücker, M., A. Struk, P. Altmeyer, M. Herde, H. Baumgärtl & D.W. Lübbers (2002). Journal of Physiology 538 (3): 985–994.
# 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 (skin)|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

Optical coherence tomography|Optical coherence tomogram of fingertip, depicting stratum corneum (~500& nbsp;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.

Mammalian skin is composed of two primary layers:

the epidermis (skin)|epidermis , which provides waterproofing and serves as a barrier to infection; and

the dermis , which serves as a location for the appendages of skin;

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 pathogens from entering, and is a stratified squamous epithelium , composed of cell division|proliferating basal and cell differentiation|differentiated suprabasal keratinocytes . The epidermis (skin)|epidermis also helps the skin regulate body temperature .
Keratinocytes are the major cell (biology)|cells , constituting 95% of the epidermis (skin)|epidermis , while Merkel cell s, melanocyte s and Langerhans cell s are also present. The epidermis (skin)|epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer). pharmaxchange.info. March 3, 2011:

Stratum corneum

Stratum lucidum (only in Hand#Human anatomy|palms and sole (foot)|soles )

Stratum granulosum

Stratum spinosum

Stratum germinativum (also called the stratum basale )

Keratinocytes in the stratum basale proliferate through mitosis and the daughter cell (biology)|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 cell junction|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 of materials|strength to the skin. Keratinocytes from the stratum corneum are eventually shed from the surface ( desquamation ).

The epidermis (skin)|epidermis contains no blood vessel s, and cell (biology)|cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis .

Basement membrane

The epidermis (skin)|epidermis and dermis are separated by a thin sheet of fibers called the basement membrane , and is made through the action of both tissue (biology)|tissues .
The basement membrane controls the traffic of cell (biology)|cells and molecules between the dermis and epidermis (skin)|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.

Dermis

The dermis is the layer of skin beneath the epidermis (skin)|epidermis that consists of connective tissue and cushions the body from stress (biology)|stress and strain.
The dermis provides tensile strength of materials|strength and elasticity (physics)|elasticity to the skin through an extracellular matrix composed of collagen fibrils , microfibrils , and elastic fibers , embedded in proteoglycans .

It harbors many Mechanoreceptor s (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 cell (biology)|cells as well as for the epidermis (skin)|epidermis .

The dermis is tightly connected to the epidermis (skin)|epidermis through a basement membrane and is structurally divided into two areas: a superficial area adjacent to the epidermis (skin)|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 . This is named for its fingerlike projections called papillae , that extend toward the epidermis (skin)|epidermis . The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis (skin)|epidermis , 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 connective tissue , and receives its name from the dense concentration of collagenous , Elastic fibers|elastic , and Reticular fiber|reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength of materials|strength , extensibility , and elasticity (physics)|elasticity .
Also located within the reticular region are the roots of the hair , sebaceous glands , sweat glands , Cutaneous receptor|receptors , nail (anatomy)|nails , and blood vessels .

Hypodermis

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 (biology)|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 . 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 , digestive system|gut and urogenital openings.

In fish and amphibians

The epidermis of fish and of most amphibian s consists entirely of live cell (biology)|cells , with only minimal quantities of keratin in the cell (biology)|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 tetrapod s. Instead, in most species, it is largely replaced by solid, protective bony scale (zoology)|scale s. Apart from some particularly large dermal bones that form parts of the skull , these scale (zoology)|scale s are lost in tetrapods , although many reptile s do have scale (zoology)|scale s of a different kind, as do pangolin s. Cartilaginous fish have numerous tooth-like Dermal denticle|denticle s embedded in their skin, in place of true scale (zoology)|scale s.

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 cell (biology)|cells that aid in insulation and protection, but may also have poison glands , photophore s, or cell (biology)|cells that produce a more watery, serous fluid. In amphibians , the mucus cell (biology)|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 (skin)|epidermis is often relatively colourless. Instead, the colour of the skin is largely due to chromatophore s in the dermis , which, in addition to melanin , may contain guanine or carotenoid pigments . Many species, such as chameleon s and flounder s may be able to change the colour of their skin by adjusting the relative size of their chromatophores .

In birds and reptiles

The epidermis (skin)|epidermis of bird s and reptile s is closer to that of mammals , with a layer of dead keratin-filled cell (biology)|cells at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial amphibians , such as toad s. However, in all of these animals there is no clear cell differentiation|differentiation of the epidermis (skin)|epidermis into distinct layers, as occurs in humans , with the change in cell (biology)|cell type being relatively gradual. The mammalian epidermis (skin)|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 feather s 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 cell (biology)|cells in some reptiles , or the uropygial gland of most birds .

Mechanics

Skin has a soft tissue (biology)|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.

Human uses and culture

The term "skin" may also refer to the covering of a small animal , such as a sheep , goat ( goatskin (material)|goatskin ), pig , snake ( Material|snakeskin ) etc. or the young of a large animal .

The term hides or Rawhide (textile)|rawhide refers to the covering of a large adult animal such as a cow , American bison|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 fur s.

Skin from sheep , goat and cattle was used to make parchment for manuscripts .

Skin can also be cooked to make pork rind or pork rind|crackling .

Dutch artist Jalila Essaïdi created actual bulletproof skin.. Reuters (2011-09-20)

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