Skin
Overview
 
The dermis is the layer of skin beneath the epidermis that consists of connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...

. It also harbors many Mechanoreceptor
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

s (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.
Encyclopedia

Dermis

The dermis is the layer of skin beneath the epidermis that consists of connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...

. It also harbors many Mechanoreceptor
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

s (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 from its own cells as well as from the Stratum basale of the epidermis.

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. 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.

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
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....

, and reticular fibres that weave throughout it. These protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 fibres 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
Cutaneous receptor
A cutaneous receptor is a type of sensory receptor found in the dermis or epidermis. They are a part of the somatosensory system. Cutaneous receptors include e.g...

, nails
Nail (anatomy)
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals....

, 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
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

 as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblast
Fibroblast
A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...

s, macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

s and adipocyte
Adipocyte
However, in some reports and textbooks, the number of fat cell increased in childhood and adolescence. The total number is constant in both obese and lean adult...

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
Staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

 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.

In fish and amphibians

The epidermis of fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 and of most amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s 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 tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

s. Instead, in most species, it is largely replaced by solid, protective bony scale
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

s. Apart from some particularly large dermal bones that form parts of the skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

, these scales are lost in tetrapods, although many reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s do have scales of a different kind, as do pangolin
Pangolin
A pangolin , also scaly anteater or Trenggiling, is a mammal of the order Pholidota. There is only one extant family and one genus of pangolins, comprising eight species. There are also a number of extinct taxa. Pangolins have large keratin scales covering their skin and are the only mammals with...

s. 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
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

-secreting skin cells that aid in insulation and protection, but may also have poison
Poison
In the context of biology, poisons are substances that can cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism....

 glands, photophore
Photophore
A photophore is a light-emitting organ which appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors...

s, 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 chromatophore
Chromatophore
Chromatophores are pigment-containing and light-reflecting cells found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye colour in cold-blooded animals and are generated in the neural crest during embryonic development...

s in the dermis, which, in addition to melanin, may contain guanine
Guanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...

 or carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

 pigments. Many species, such as chameleon
Chameleon
Chameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...

s and flounder
Flounder
The flounder is an ocean-dwelling flatfish species that is found in coastal lagoons and estuaries of the Northern Atlantic and Pacific Oceans.-Taxonomy:There are a number of geographical and taxonomical species to which flounder belong.*Western Atlantic...

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 of bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s and reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s 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 toad
Toad
A toad is any of a number of species of amphibians in the order Anura characterized by dry, leathery skin , short legs, and snoat-like parotoid glands...

s. 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 feather
Feather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...

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
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

-secreting cells in some reptiles, or the uropygial gland
Uropygial gland
The uropygial gland, informally known as the preen gland, is a gland found in the large majority of birds that secretes an oil that birds use for preening. The chief compounds of preen oil are diester waxes called uropygiols....

 of most birds.

Mechanics

Skin has a soft tissue mechanical behavior when stretched. The intact skin is prestreched (i.e. has residual stress
Residual stress
Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...

) like neoprene
Neoprene
Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...

 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
Goatskin (material)
Goatskin is the skin of a goat.Non tanned goatskin is used for parchment or for drumheads or sounding boards of some musical instruments, e.g., mišnice in medieval Europe, bodhrán in Ireland, esraj in India and for instrumental drum skin named bedug in Indonesia.Tanned leather from goatskin is...

), pig, snake (snakeskin
Material
Material is anything made of matter, constituted of one or more substances. Wood, cement, hydrogen, air and water are all examples of materials. Sometimes the term "material" is used more narrowly to refer to substances or components with certain physical properties that are used as inputs to...

) etc. or the young of a large animal.

The term hides
Hides
A hide is an animal skin treated for human use. Hides include leather from cattle and other livestock animals, alligator skins, snake skins for shoes and fashion accessories and furs from wild cats, mink and bears. In some areas, leather is produced on a domestic or small industrial scale, but most...

 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
Leather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...

, but also fur
Fur
Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensives body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to animal...

s.

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

Skin can also be cooked to make pork rind
Pork rind
Pork rind , is the fried or roasted skin of a pig. Frying melts most of the fat from the pork rind...

 or crackling.

Dutch artist Jalila Essaïdi created actual bulletproof skin.

Detailed cross section

See also

  • List of cutaneous conditions
  • Acid mantle
    Acid mantle
    The acid mantle is a very fine, slightly acidic film on the surface of the skin acting as a barrier to bacteria, viruses and other potential contaminants that might penetrate the skin. The pH of the skin is between 4.5 and 6.2, so it is acidic...

  • Callus
    Callus
    A callus is an especially toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses...

     - thick area of skin
  • Cutaneous structure development
    Cutaneous structure development
    Cutaneous structures arise from the epidermis and include a variety of features such as hair, feathers, claws and nails.During embryogenesis, the epidermis splits into two layers: the periderm and the basal layer. The basal layer is a stem cell layer and through asymmetrical divisions, becomes...

  • Hair
    Hair
    Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

     - including hair follicles in skin
  • Intertriginous
    Intertriginous
    Intertriginous is a medical term used to define an area where two skin areas may touch or rub together. Examples of intertriginous areas are the axilla of the arm, the anogenital region, nares, skin folds of the breasts, and between digits...

  • Moult
    Moult
    In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...

  • Meissner's corpuscle
    Meissner's corpuscle
    Meissner's corpuscles are a type of mechanoreceptor. They are a type of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they have highest sensitivity when sensing vibrations lower than 50 Hertz...

  • Pacinian corpuscle
    Pacinian corpuscle
    Lamellar corpuscles or Pacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to vibration and pressure. Vibrational role may be used to detect surface, e.g., rough vs...

  • Rawhide
  • Role of skin in locomotion
    Role of Skin in Locomotion
    Role of skin in locomotion describes how the integumentary system is involved in locomotion. Typically the integumentary system can be thought of as skin, however the integumentary system also includes the segmented exoskeleton in arthropods and feathers of birds. The primary role of the...

  • Superficial fascia
    Superficial fascia
    Superficial fascia is found in the subcutis in most regions of the body, blending with the reticular layer of the dermis. It is present on the face, over the upper portion of the sternocleidomastoid, at the nape of the neck, and overlying the sternum. It is mainly loose areolar connective tissue...

  • Potential of embryonic stem cells to reconstitute a skin.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK