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

Other , such as the arthropod exoskeleton, have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latin cutis 'skin'). In , the skin is an organ of the integumentary system made up of multiple layers of tissue and guards the underlying , , , and . Skin of a different nature exists in , , and . Skin (including cutaneous and subcutaneous tissues) plays crucial roles in formation, structure, and function of extraskeletal apparatus such as horns of bovids (e.g., cattle) and rhinos, cervids' antlers, giraffids' ossicones, armadillos' osteoderm, and /.

All mammals have some hair on their skin, even like , , and that 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 and excessive water loss. Its other functions are insulation, , 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 on 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. The skin on the palms and the soles of the feet is the thickest skin on the body as 4 mm thick. The speed and quality of wound healing in skin is promoted by the reception of .

is dense hair. Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as . On some animals, the skin is very hard and thick and can be processed to create . and most have hard protective scales on their skin for protection, and have hard , all made of tough . 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. skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.


Etymology
The word skin originally only referred to dressed and tanned animal hide and the usual word for human skin was hide. Skin is a borrowing from skinn "animal hide, fur", ultimately from the Proto-Indo-European root *sek-, meaning "to cut" (probably a reference to the fact that in those times animal hide was commonly cut off to be used as garment).


Structure in humans and other mammals
'')
Optical coherence tomogram of fingertip, depicting (~500 µm thick) with stratum disjunctum on top and (connection to stratum spinosum) in the middle. At the bottom superficial parts of the dermis are visible. Ducts of are visible.]] Mammalian skin is composed of two primary layers:
  • The , which provides waterproofing and serves as a barrier to infection.
  • The , which serves as a location for the 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 from entering, and is a stratified squamous ,
(2023). 9780632064298, Blackwell Publishing.
composed of basal and differentiated suprabasal .

are the major cells, constituting 95% of the epidermis, while , and are also present. The epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer):

in the proliferate through and the daughter cells move up the strata changing shape and composition as they undergo multiple stages of cell differentiation to eventually become anucleated. During that process, will become highly organized, forming () between each other and secreting and which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin. from the are eventually shed from the surface ().

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


Basement membrane
The and are separated by a thin sheet of called the basement membrane, which is made through the action of both tissues. The basement membrane controls the traffic of the cells and between the dermis and epidermis but also serves, through the binding of a variety of and , as a reservoir for their controlled release during remodeling or repair processes.


Dermis
The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The provides tensile strength and elasticity to the skin through an extracellular matrix composed of , , and , embedded in and . Skin proteoglycans are varied and have very specific locations. For example, , and are present throughout the dermis and extracellular matrix, whereas and are only found in the epidermis.

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

Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals.

The 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.


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 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 , , and fibers that weave throughout it. These fibers give the its properties of strength, , and elasticity. Also located within the reticular region are the roots of the hair, , , receptors, nails, and .


Subcutaneous tissue
The subcutaneous tissue (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 and . It consists of loose connective tissue and . The main cell types are , and (the subcutaneous tissue contains 50% of ). serves as padding and insulation for the body.

like epidermis 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 , gut and openings.


Detailed cross section

Structure in fish, amphibians, birds, and reptiles

Fish
The epidermis of and of most consists entirely of live cells, with only minimal quantities of in the cells 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 connective tissue found in . Instead, in most species, it is largely replaced by solid, protective bony scales. Apart from some particularly large dermal bones that form parts of the , these scales are lost in , although many do have scales of a different kind, as do . Cartilaginous fish have numerous tooth-like embedded in their skin, in place of true scales.

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

Although is found in the skin of many species, in the , the , and , the epidermis is often relatively colorless. Instead, the color of the skin is largely due to in the , which, in addition to melanin, may contain or . Many species, such as and may be able to change the color of their skin by adjusting the relative size of their .


Amphibians

Overview
Amphibians possess two types of , 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 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 that 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.


Granular glands
Granular glands can be identified as 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.

Structurally, the ducts of the granular gland initially maintain a cylindrical shape. However, when the ducts become mature and full of 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.

The intercalary region of granular glands is 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 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.

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.


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.

The ducts of the mucous gland appear as cylindrical vertical tubes that 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.

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.

The alveolar or 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.


Birds and reptiles
The epidermis of and is closer to that of , 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 such as . However, in all of these animals there is no clear differentiation of the epidermis into distinct layers, as occurs in , with the change in cell type being relatively gradual. The mammalian epidermis always possesses at least a stratum germinativum and , but the other intermediate layers found in are not always distinguishable. is a distinctive feature of mammalian skin, while are (at least among living species) similarly unique to .
(1977). 9780039102845, Holt-Saunders International.

and have relatively few skin , although there may be a few structures for specific purposes, such as -secreting cells in some , or the of most birds.


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 (which is lost) and the basal layer. 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 signaling from FGF7 (keratinocyte growth factor) produced by the dermis below the basal cells. In mice, over-expression of these factors leads to an overproduction of granular cells and thick skin.

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.


Functions
Skin performs the following functions:

  1. Protection: an anatomical barrier from and damage between the internal and external environment in bodily defense. (See .) in the skin are part of the adaptive immune system.
  2. : contains a variety of that jump to , , , , and tissue (see somatosensory system and haptic perception).
  3. Thermoregulation: () glands and dilated blood vessels (increased superficial ) aid heat loss, while constricted greatly reduce cutaneous and conserve heat. Erector pili muscles in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair or .
  4. Control of : the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.
  5. Storage and : acts as a storage center for and water
  6. : , and can diffuse into the epidermis in small amounts; some animals use their skin as their sole respiration organ (in , 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.
  7. Water resistance: The skin acts as a water 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 epidermis. 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.
    (2023). 9781586630973, Metro Books.
  8. , 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.


Mechanics
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. 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.


Aging
Tissue generally declines with age, in part because /progenitor cells fail to self-renew or differentiate. Skin aging is caused in part by TGF-β by blocking the conversion of dermal fibroblasts into which provide support. Common changes in the skin as a result of aging range from , discoloration, and skin laxity, but can manifest in more severe forms such as skin malignancies. Moreover, these factors may be worsened by sun exposure in a process known as .


See also
  • Cutaneous reflex in human locomotion
  • Cutaneous respiration – gas exchange conducted through skin
  • Role of skin in locomotion


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