Skin is the layer of usually soft, flexible outertissue covering the body of avertebrate animal, with three main functions: protection, regulation, and sensation.
All mammals have some hair on their skin, evenmarine mammals likewhales,dolphins, andporpoises 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 thebody againstpathogens[3] and excessive water loss.[4] Its other functions areinsulation,temperature regulation, sensation, and the production ofvitamin D folates. Severely damaged skin may heal by formingscar tissue. 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 at 4 mm thick. The speed and quality of wound healing in skin is promoted byestrogen.[5][6][7]
Fur is dense hair.[8] Primarily, fur augments the insulation the skin provides but can also serve as asecondary sexual characteristic or ascamouflage. On some animals, the skin is very hard and thick and can be processed to createleather.Reptiles and mostfish have hard protective scales on their skin for protection, andbirds have hardfeathers, all made of toughbeta-keratins.Amphibian skin is not a strong barrier, especially regarding the passage of chemicals via skin, and is often subject toosmosis and diffusive forces. For example, afrog sitting in ananesthetic solution would be sedated quickly as the chemical diffuses through its skin.Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.[9]
On 11 January 2024, biologists reported the discovery of the oldest known skin, fossilized about 289 million years ago, and possibly the skin from an ancient reptile.[10][11]
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 fromOld Norseskinn "animal hide, fur", ultimately from theProto-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).[12]
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 preventingpathogens from entering, and is a stratified squamousepithelium,[13] composed ofproliferating basal anddifferentiated suprabasalkeratinocytes.
Theepidermis anddermis are separated by a thin sheet offibers called thebasement membrane, which is made through the action of bothtissues.The basement membrane controls the traffic of thecells andmolecules between the dermis and epidermis but also serves, through the binding of a variety ofcytokines andgrowth factors, as a reservoir for their controlled release duringphysiological remodeling or repair processes.[16]
Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals.[2]
Thedermis is tightly connected to theepidermis through abasement membrane and is structurally divided into two areas: a superficial area adjacent to the epidermis, called thepapillary region, and a deep thicker area known as thereticular region.
The papillary region is composed of looseareolar connective tissue. This is named for its fingerlike projections calledpapillae that extend toward theepidermis. The papillae provide thedermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.
The epidermis offish and of mostamphibians consists entirely of livecells, with only minimal quantities ofkeratin in the cells of the superficial layer.[18] It is generally permeable, and in the case of manyamphibians, may actually be a major respiratory organ.[19] Thedermis ofbony fish typically contains relatively little of theconnective tissue found intetrapods.[18] Instead, in most species, it is largely replaced by solid, protective bonyscales.[20] Apart from some particularly large dermal bones that form parts of theskull, thesescales are lost intetrapods, although manyreptiles do havescales of a different kind, as dopangolins.[21]Cartilaginous fish have numerous tooth-likedenticles embedded in their skin, in place of truescales.[22]
Amphibians possess two types ofglands,mucous and granular (serous). Both of these glands are part of theintegument and thus consideredcutaneous. 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 viakeratinocytes and passes through to the surface of theepidermal 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, which are greater in number.[24]
Granular glands can be identified asvenomous 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 onamphibian 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.[24]
Structurally, the ducts of the granular gland initially maintain a cylindrical shape. When the ducts mature and fill with 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.[25]
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 anectodermal 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.[25]
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.[25]
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.[24]
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.[25]
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.[25]
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.[25]
Theepidermis ofbirds andreptiles is closer to that ofmammals, with a layer of dead keratin-filledcells at the surface, to help reduce water loss.[26] A similar pattern is also seen in some of the more terrestrialamphibians such astoads. In these animals, there is no cleardifferentiation of theepidermis into distinct layers initially, as occurs inhumans, with the change incell type being relatively gradual.[27] The mammalianepidermis always possesses at least astratum germinativum andstratum corneum, but the other intermediate layers found inhumans are not always distinguishable.
Hair is a distinctive feature of mammalian skin, whilefeathers are (at least among living species) similarly unique tobirds.[23]
Cutaneous structures arise from theepidermis 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 thebasal layer. The basal layer is astem cell layer and through asymmetrical divisions, becomes the source of skin cells throughout life. It is maintained as a stem cell layer through anautocrine signal,TGF alpha, and throughparacrine 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 ofgranular cells and thick skin.[28][29]
Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system. Thisreaction-diffusion system combines an activator,Sonic hedgehog, with an inhibitor, BMP4 or BMP2, to form clusters of cells in a regular pattern. Sonic hedgehog-expressing epidermal cells induce the condensation of cells in themesoderm. The clusters of mesodermal cells signal back to the epidermis to form the appropriate structure for that position. BMP signals from the epidermis inhibit the formation of placodes in nearby ectoderm.[30]
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.[31]
Thermoregulation:Eccrine (sweat) glands and dilated blood vessels (increased superficialperfusion) aid heat loss, while constrictedvessels greatly reduce cutaneousblood flow and conserve heat.Erector pili muscles in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair orfur.
Control ofevaporation: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.[4]
Storage andsynthesis: acts as a storage center forlipids and water
Water resistance: The skin acts as a water resistant barrier so essentialnutrients aren't washed out of the body. The nutrients and oils that help hydrate the skin are covered by the most outer skin layer, theepidermis. This is helped in part by the sebaceous glands that releasesebum, 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.[33]
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.[34]
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.[35] 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.[36]
Tissuehomeostasis generally declines with age, in part becausestem/progenitor cells fail to self-renew ordifferentiate. Skin aging is caused in part byTGF-β by blocking the conversion ofdermal fibroblasts intofat cells which provide support. Common changes in the skin as a result of aging range fromwrinkles, discoloration, and skin laxity, but can manifest in more severe forms such as skin malignancies.[37][38] Moreover, these factors may be worsened by sun exposure in a process known asphotoaging.[38]
^Alibardi, Lorenzo (15 August 2003). "Adaptation to the land: The skin of reptiles in comparison to that of amphibians and endotherm amniotes".Journal of Experimental Zoology.298B (1):12–41.Bibcode:2003JEZB..298...12A.doi:10.1002/jez.b.24.PMID12949767.
^abNasoori, Alireza (August 2020). "Formation, structure, and function of extra-skeletal bones in mammals".Biological Reviews.95 (4):986–1019.doi:10.1111/brv.12597.PMID32338826.S2CID216556342.
^"fur".Archived from the original on 3 March 2017. Retrieved4 March 2017 – via The Free Dictionary.
^Clarke, B. T. (August 1997). "The natural history of amphibian skin secretions, their normal functioning and potential medical applications".Biological Reviews of the Cambridge Philosophical Society.72 (3):365–379.doi:10.1111/j.1469-185X.1997.tb00018.x.PMID9336100.
^abBreitkreutz, D; Mirancea, N; Nischt, R (2009). "Basement membranes in skin: Unique matrix structures with diverse functions?".Histochemistry and Cell Biology.132 (1):1–10.doi:10.1007/s00418-009-0586-0.PMID19333614.S2CID21633122.
^Iozzo, Renato V. (August 2005). "Basement membrane proteoglycans: from cellar to ceiling".Nature Reviews Molecular Cell Biology.6 (8):646–656.doi:10.1038/nrm1702.PMID16064139.S2CID22151754.
^abcdRomer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia: Holt-Saunders International. pp. 129–145.ISBN978-0-03-910284-5.
^abcToledo, R.C.; Jared, C. (May 1995). "Cutaneous granular glands and amphibian venoms".Comparative Biochemistry and Physiology Part A: Physiology.111 (1):1–29.doi:10.1016/0300-9629(95)98515-I.
^Bush, James A.; Ferguson, Mark W.J.; Mason, Tracey; McGrouther, D. Angus (May 2008). "Skin tension or skin compression? Small circular wounds are likely to shrink, not gape".Journal of Plastic, Reconstructive & Aesthetic Surgery.61 (5):529–534.doi:10.1016/j.bjps.2007.06.004.PMID17652049.
^abRabe, Jessica H.; Mamelak, Adam J.; McElgunn, Patrick J.S.; Morison, Warwick L.; Sauder, Daniel N. (July 2006). "Photoaging: Mechanisms and repair".Journal of the American Academy of Dermatology.55 (1):1–19.doi:10.1016/j.jaad.2005.05.010.PMID16781287.
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