Skin

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This article is about the outer covering of animals. Skin_sentence_0

For skin in humans, see human skin. Skin_sentence_1

For other uses, see Skin (disambiguation). Skin_sentence_2

Skin_table_infobox_0

SkinSkin_header_cell_0_0_0
DetailsSkin_header_cell_0_1_0
IdentifiersSkin_header_cell_0_2_0
LatinSkin_header_cell_0_3_0 CutisSkin_cell_0_3_1
MeSHSkin_header_cell_0_4_0 Skin_cell_0_4_1
TA98Skin_header_cell_0_5_0 Skin_cell_0_5_1
TA2Skin_header_cell_0_6_0 Skin_cell_0_6_1

Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Skin_sentence_3

Other animal coverings, such as the arthropod exoskeleton, have different developmental origin, structure and chemical composition. Skin_sentence_4

The adjective cutaneous means "of the skin" (from Latin cutis 'skin'). Skin_sentence_5

In mammals, the skin is an organ of the integumentary system made up of multiple layers of ectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin_sentence_6

Skin of a different nature exists in amphibians, reptiles, and birds. Skin_sentence_7

All mammals have some hair on their skin, even marine mammals like whales, dolphins, and porpoises which appear to be hairless. Skin_sentence_8

The skin interfaces with the environment and is the first line of defense from external factors. Skin_sentence_9

For example, the skin plays a key role in protecting the body against pathogens and excessive water loss. Skin_sentence_10

Its other functions are insulation, temperature regulation, sensation, and the production of vitamin D folates. Skin_sentence_11

Severely damaged skin may heal by forming scar tissue. Skin_sentence_12

This is sometimes discoloured and depigmented. Skin_sentence_13

The thickness of skin also varies from location to location on an organism. Skin_sentence_14

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. Skin_sentence_15

The skin on the palms and the soles of the feet is 4 mm thick and is the thickest skin on the body. Skin_sentence_16

The speed and quality of wound healing in skin is promoted by the reception of estrogen. Skin_sentence_17

Fur is dense hair. Skin_sentence_18

Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage. Skin_sentence_19

On some animals, the skin is very hard and thick, and can be processed to create leather. Skin_sentence_20

Reptiles and most fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Skin_sentence_21

Amphibian skin is not a strong barrier, especially regarding the passage of chemicals via skin and is often subject to osmosis and diffusive forces. Skin_sentence_22

For example, a frog sitting in an anesthetic solution would be sedated quickly, as the chemical diffuses through its skin. Skin_sentence_23

Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions. Skin_sentence_24

Structure in humans and other mammals Skin_section_0

See also: Human skin Skin_sentence_25

Skin_table_infobox_1

DermisSkin_header_cell_1_0_0
IdentifiersSkin_header_cell_1_1_0
MeSHSkin_header_cell_1_2_0 Skin_cell_1_2_1
TA98Skin_header_cell_1_3_0 Skin_cell_1_3_1
TA2Skin_header_cell_1_4_0 Skin_cell_1_4_1

Mammalian skin is composed of two primary layers: Skin_sentence_26

Skin_unordered_list_0

  • the epidermis, which provides waterproofing and serves as a barrier to infection; andSkin_item_0_0
  • the dermis, which serves as a location for the appendages of skin;Skin_item_0_1

Epidermis Skin_section_1

Main article: Epidermis Skin_sentence_27

The epidermis is composed of the outermost layers of the skin. Skin_sentence_28

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. Skin_sentence_29

Keratinocytes are the major cells, constituting 95% of the epidermis, while Merkel cells, melanocytes and Langerhans cells are also present. Skin_sentence_30

The epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer): Skin_sentence_31

Skin_unordered_list_1

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. Skin_sentence_32

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. Skin_sentence_33

Keratinocytes from the stratum corneum are eventually shed from the surface (desquamation). Skin_sentence_34

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. Skin_sentence_35

Basement membrane Skin_section_2

Main article: Basement membrane Skin_sentence_36

The epidermis and dermis are separated by a thin sheet of fibers called the basement membrane, which is made through the action of both tissues. Skin_sentence_37

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. Skin_sentence_38

Dermis Skin_section_3

Main article: Dermis Skin_sentence_39

The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. Skin_sentence_40

The dermis provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils, microfibrils, and elastic fibers, embedded in hyaluronan and proteoglycans. Skin_sentence_41

Skin proteoglycans are varied and have very specific locations. Skin_sentence_42

For example, hyaluronan, versican and decorin are present throughout the dermis and epidermis extracellular matrix, whereas biglycan and perlecan are only found in the epidermis. Skin_sentence_43

It harbors many mechanoreceptors (nerve endings) that provide the sense of touch and heat through nociceptors and thermoreceptors. Skin_sentence_44

It also contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. Skin_sentence_45

The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as for the epidermis. Skin_sentence_46

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. Skin_sentence_47

Papillary region Skin_section_4

The papillary region is composed of loose areolar connective tissue. Skin_sentence_48

This is named for its fingerlike projections called papillae that extend toward the epidermis. Skin_sentence_49

The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin. Skin_sentence_50

Reticular region Skin_section_5

The reticular region lies deep in the papillary region and is usually much thicker. Skin_sentence_51

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. Skin_sentence_52

These protein fibers give the dermis its properties of strength, extensibility, and elasticity. Skin_sentence_53

Also located within the reticular region are the roots of the hair, sweat glands, sebaceous glands, receptors, nails, and blood vessels. Skin_sentence_54

Subcutaneous tissue Skin_section_6

Main article: Subcutaneous tissue Skin_sentence_55

The subcutaneous tissue (also hypodermis) is not part of the skin, and lies below the dermis. Skin_sentence_56

Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. Skin_sentence_57

It consists of loose connective tissue and elastin. Skin_sentence_58

The main cell types are fibroblasts, macrophages and adipocytes (the subcutaneous tissue contains 50% of body fat). Skin_sentence_59

Fat serves as padding and insulation for the body. Skin_sentence_60

Microorganisms like Staphylococcus epidermidis colonize the skin surface. Skin_sentence_61

The density of skin flora depends on region of the skin. Skin_sentence_62

The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut and urogenital openings. Skin_sentence_63

Detailed cross section Skin_section_7

Structure in Fish, Amphibians, Birds, and Reptiles Skin_section_8

Fish Skin_section_9

See also: Fish scales Skin_sentence_64

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. Skin_sentence_65

It is generally permeable, and in the case of many amphibians, may actually be a major respiratory organ. Skin_sentence_66

The dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods. Skin_sentence_67

Instead, in most species, it is largely replaced by solid, protective bony scales. Skin_sentence_68

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. Skin_sentence_69

Cartilaginous fish have numerous tooth-like denticles embedded in their skin, in place of true scales. Skin_sentence_70

Sweat glands and sebaceous glands are both unique to mammals, but other types of skin gland are found in other vertebrates. Skin_sentence_71

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. Skin_sentence_72

In amphibians, the mucus cells are gathered together to form sac-like glands. Skin_sentence_73

Most living amphibians also possess granular glands in the skin, that secrete irritating or toxic compounds. Skin_sentence_74

Although melanin is found in the skin of many species, in the reptiles, the amphibians, and fish, the epidermis is often relatively colorless. Skin_sentence_75

Instead, the color of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Skin_sentence_76

Many species, such as chameleons and flounders may be able to change the color of their skin by adjusting the relative size of their chromatophores. Skin_sentence_77

Amphibians Skin_section_10

See also: amphibians Skin_sentence_78

Overview Skin_section_11

Amphibians possess two types of glands, mucous and granular (serous). Skin_sentence_79

Both of these glands are part of the integument and thus considered cutaneous. Skin_sentence_80

Mucous and granular glands are both divided into three different sections which all connect to structure the gland as a whole. Skin_sentence_81

The three individual parts of the gland are the duct, the intercalary region, and lastly the alveolar gland (sac). Skin_sentence_82

Structurally, the duct is derived via keratinocytes and passes through to the surface of the epidermal or outer skin layer thus allowing external secretions of the body. Skin_sentence_83

The gland alveolus is a sac shaped structure that is found on the bottom or base region of the granular gland. Skin_sentence_84

The cells in this sac specialize in secretion. Skin_sentence_85

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. Skin_sentence_86

In general, granular glands are larger in size than the mucous glands, however mucous glands hold a much greater majority in overall number. Skin_sentence_87

Granular Glands Skin_section_12

Granular glands can be identified as venomous and often differ in the type of toxin as well as the concentrations of secretions across various orders and species within the amphibians. Skin_sentence_88

They are located in clusters differing in concentration depending on amphibian taxa. Skin_sentence_89

The toxins can be fatal to most vertebrates or have no effect against others. Skin_sentence_90

These glands are alveolar meaning they structurally have little sacs in which venom is produced and held before it is secreted upon defensive behaviors. Skin_sentence_91

Structurally, the ducts of the granular gland initially maintain a cylindrical shape. Skin_sentence_92

However, when the ducts become mature and full of fluid, the base of the ducts become swollen due to the pressure from the inside. Skin_sentence_93

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. Skin_sentence_94

The intercalary region of granular glands is more developed and mature in comparison with mucous glands. Skin_sentence_95

This region resides as a ring of cells surrounding the basal portion of the duct which are argued to have an ectodermal muscular nature due to their influence over the lumen (space inside the tube) of the duct with dilation and constriction functions during secretions. Skin_sentence_96

The cells are found radially around the duct and provide a distinct attachment site for muscle fibers around the gland's body. Skin_sentence_97

The gland alveolus is a sac that is divided into three specific regions/layers. Skin_sentence_98

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. Skin_sentence_99

The nerves send signals to the muscles as well as the epithelial layers. Skin_sentence_100

Lastly, the epithelium or tunica propria encloses the gland. Skin_sentence_101

Mucous Glands Skin_section_13

Mucous glands are non-venomous and offer a different functionality for amphibians than granular. Skin_sentence_102

Mucous glands cover the entire surface area of the amphibian body and specialize in keeping the body lubricated. Skin_sentence_103

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. Skin_sentence_104

The ducts of the mucous gland appear as cylindrical vertical tubes that break through the epidermal layer to the surface of the skin. Skin_sentence_105

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. Skin_sentence_106

Intercalary cells react identically to those of granular glands but on a smaller scale. Skin_sentence_107

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. Skin_sentence_108

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. Skin_sentence_109

This gland lacks a tunica propria and appears to have delicate and intricate fibers which pass over the gland's muscle and epithelial layers. Skin_sentence_110

Birds and reptiles Skin_section_14

Main article: Reptile scales Skin_sentence_111

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. Skin_sentence_112

A similar pattern is also seen in some of the more terrestrial amphibians such as toads. Skin_sentence_113

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. Skin_sentence_114

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. Skin_sentence_115

Hair is a distinctive feature of mammalian skin, while feathers are (at least among living species) similarly unique to birds. Skin_sentence_116

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_sentence_117

Development Skin_section_15

Cutaneous structures arise from the epidermis and include a variety of features such as hair, feathers, claws and nails. Skin_sentence_118

During embryogenesis, the epidermis splits into two layers: the periderm (which is lost) and the basal layer. Skin_sentence_119

The basal layer is a stem cell layer and through asymmetrical divisions, becomes the source of skin cells throughout life. Skin_sentence_120

It is maintained as a stem cell layer through an autocrine signal, TGF alpha, and through paracrine signaling from FGF7 (keratinocyte growth factor) produced by the dermis below the basal cells. Skin_sentence_121

In mice, over-expression of these factors leads to an overproduction of granular cells and thick skin. Skin_sentence_122

Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system. Skin_sentence_123

This reaction-diffusion system combines an activator, Sonic hedgehog, with an inhibitor, BMP4 or BMP2, to form clusters of cells in a regular pattern. Skin_sentence_124

Sonic hedgehog-expressing epidermal cells induce the condensation of cells in the mesoderm. Skin_sentence_125

The clusters of mesodermal cells signal back to the epidermis to form the appropriate structure for that position. Skin_sentence_126

BMP signals from the epidermis inhibit the formation of placodes in nearby ectoderm. Skin_sentence_127

It is believed that the mesoderm defines the pattern. Skin_sentence_128

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. Skin_sentence_129

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. Skin_sentence_130

Functions Skin_section_16

Skin performs the following functions: Skin_sentence_131

Skin_ordered_list_2

  1. Protection: an anatomical barrier from pathogens and damage between the internal and external environment in bodily defense. (See Skin absorption.) Langerhans cells in the skin are part of the adaptive immune system.Skin_item_2_7
  2. : contains a variety of nerve endings that jump to heat and cold, touch, pressure, vibration, and tissue injury (see somatosensory system and haptic perception).Skin_item_2_8
  3. 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.Skin_item_2_9
  4. Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.Skin_item_2_10
  5. Storage and synthesis: acts as a storage center for lipids and waterSkin_item_2_11
  6. Absorption through the skin: 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") Some medications are absorbed through the skin.Skin_item_2_12
  7. 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.Skin_item_2_13
  8. Camouflage, 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.Skin_item_2_14

Mechanics Skin_section_17

Main article: Soft tissue Skin_sentence_132

Skin is a soft tissue and exhibits key mechanical behaviors of these tissues. Skin_sentence_133

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. Skin_sentence_134

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. Skin_sentence_135

Small circular holes punched on the skin may widen or close into ellipses, or shrink and remain circular, depending on preexisting stresses. Skin_sentence_136

Ageing Skin_section_18

Tissue homeostasis generally declines with age, in part because stem/progenitor cells fail to self-renew or differentiate. Skin_sentence_137

In the skin of mice, mitochondrial oxidative stress can promote cellular senescence and aging phenotypes. Skin_sentence_138

Ordinarily mitochondrial superoxide dismutase (SOD2) protects against oxidative stress. Skin_sentence_139

Using a mouse model of genetic SOD2 deficiency, it was shown that failure to express this important antioxidant enzyme in epidermal cells caused cellular senescence, nuclear DNA damage, and irreversible arrest of proliferation of a fraction of keratinocytes. Skin_sentence_140

Skin aging is caused in part by TGF-β, which reduces the subcutaneous fat that gives skin a pleasant appearance and texture. Skin_sentence_141

TGF-β does this by blocking the conversion of dermal fibroblasts into fat cells; with fewer fat cells underneath to provide support, the skin becomes saggy and wrinkled. Skin_sentence_142

Subcutaneous fat also produces cathelicidin, which is a peptide that fights bacterial infections. Skin_sentence_143

Society and culture Skin_section_19

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. Skin_sentence_144

The term hides or rawhide refers to the covering of a large adult animal such as a cow, buffalo, horse etc. Skin_sentence_145

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_sentence_146

Skin from sheep, goat and cattle was used to make parchment for manuscripts. Skin_sentence_147

Skin can also be cooked to make pork rind or crackling. Skin_sentence_148

See also Skin_section_20

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Skin.