Skin

Soft outer covering organ of vertebrates

This article is about the skin of animals in general. For skin in humans, see Human skin.

For other uses, see Skin (disambiguation).

Skin
Elephant skin
Details
Identifiers
Latin	cutis
MeSH	D012867
TA98	A16.0.00.002
TA2	7041
Anatomical terminology [edit on Wikidata]

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.

Other

os clitoris.^[2]

All mammals have some hair on their skin, even marine mammals like whales, dolphins, and porpoises 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 body against pathogens^[3] and excessive water loss.^[4] Its other functions are insulation, temperature regulation, sensation, and the production of vitamin D folates. Severely damaged skin may heal by forming scar 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 by estrogen.^[5][6][7]

Fur is dense hair.^[8] Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick and can be processed to create leather. Reptiles and most fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough beta-keratins. Amphibian skin is not a strong barrier, especially regarding the passage of chemicals via skin, and is often subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic 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]

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

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).^[12]

Structure in mammals

Dermis
The distribution of the blood vessels in the skin of the sole of the foot. (Corium – TA alternate term for dermis – is labeled at upper right.)
A diagrammatic sectional view of the skin (click on image to magnify). (Dermis labeled at center right.)
Identifiers
MeSH	D012867
TA98	A16.0.00.002
TA2	7041
Anatomical terminology [edit on Wikidata]

Mammalian skin is composed of two primary layers:

• The epidermis, which provides waterproofing and serves as a barrier to infection.
• The
  appendages
  of skin.

Epidermis

Main article: 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

differentiated suprabasal keratinocytes

.

epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer):^[14]

• Stratum corneum
• Stratum lucidum (only in palms and soles)
• Stratum granulosum
• Stratum spinosum
• Stratum basale (also called the stratum germinativum)

lipids which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin.^[15] Keratinocytes from the stratum corneum are eventually shed from the surface (desquamation

).

The

capillaries extending to the upper layers of the dermis

.

Basement membrane

Main article: Basement membrane

The

fibers called the basement membrane, which is made through the action of both tissues

. The basement membrane controls the traffic of the

physiological remodeling or repair processes.^[16]

Dermis

Main article: Dermis

The dermis is the layer of skin beneath the

hyaluronan, versican and decorin are present throughout the dermis and epidermis extracellular matrix, whereas biglycan and perlecan

are only found in the epidermis.

It harbors many

epidermis

.

Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals.[2]

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

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

elastic, and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength, extensibility, and elasticity

. Also located within the reticular region are the

roots of the hair, sweat glands, sebaceous glands, receptors, nails, and blood vessels

.

Subcutaneous tissue

Main article: Subcutaneous tissue

The

body fat). Fat

serves as padding and insulation for the body.

urogenital

openings.

Detailed cross section

Skin layers, of both the hairy and hairless skin

Structure in fish, amphibians, birds, and reptiles

Fish

This section relies largely or entirely upon a single source. Relevant discussion may be found on the talk page. Please help improve this article by introducing citations to additional sources at this section. (August 2021) (Learn how and when to remove this template message)

Further information: Fish scale

Overview

Amphibians possess two types of

Granular glands

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. They are located in clusters differing in concentration depending on amphibian 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.^[25]

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.^[26]

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 ectodermal 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.^[26]

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.^[26]

Mucous glands

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.^[25]

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.^[26]

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.^[26]

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.^[26]

Birds and reptiles

This section relies largely or entirely upon a Reptile scales The 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 feathers are (at least among living species) similarly unique to birds.[24] 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.[24] Development This section relies largely or entirely upon a single source. Relevant discussion may be found on the talk page. Please help improve this article by introducing citations to additional sources at this section. (August 2021) (Learn how and when to remove this template message ) Cutaneous structures arise from the paracrine 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.[27][28] Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system. This 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 the mesoderm. 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.[citation needed ] 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.[29] Functions Skin performs the following functions: Protection: an anatomical barrier from Skin absorption.) Langerhans cells in the skin are part of the adaptive immune system.[3][4] touch, pressure, vibration, and tissue injury (see somatosensory system and haptic perception ). Thermoregulation: Erector pili muscles in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair or fur . Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.[4] Storage and synthesis: acts as a storage center for lipids and water 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")[30] Some medications are absorbed through the skin . Water resistance: The skin acts as a water resistant barrier so essential 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.[31] 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.[32] Mechanics Main article: Soft tissue 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.[33] 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.[34] Aging Tissue fat cells which provide support. Common changes in the skin as a result of aging range from wrinkles, discoloration, and skin laxity, but can manifest in more severe forms such as skin malignancies.[35][36] Moreover, these factors may be worsened by sun exposure in a process known as photoaging.[36] See also Cutaneous reflex in human locomotion Cutaneous respiration – gas exchange conducted through skin Moult Role of skin in locomotion Skinning References PMID 12949767 . ^ S2CID 216556342 . ^ S2CID 31353914 . ^ PMID 12880413 . PMID 12473056 . PMID 10514397 . ^ Desiree May Oh, MD, Tania J. Phillips, MD (2006). "Sex Hormones and Wound Healing". Wounds. Archived from the original on 2013-01-07. Retrieved 2013-09-23.{{cite journal}}: CS1 maint: multiple names: authors list (link) ^ "fur". Archived from the original on 3 March 2017. Retrieved 4 March 2017 – via The Free Dictionary. PMID 9336100 . ^ Golembiewski, Kate (11 January 2024). "Scaly Fossil Is the Oldest-Known Piece of Skin - The specimen came from a 289 million-year-old fossil deposit and might offer clues to how skin evolved". The New York Times. Archived from the original on 11 January 2024. Retrieved 12 January 2024. PMID 38215745. Archived from the original on 12 January 2024. Retrieved 12 January 2024. ^ "Skin | Origin and meaning of skin by Online Etymology Dictionary". ^ ISBN 978-0-632-06429-8 . ISBN 978-1-711494-06-7 . ^ S2CID 21633122 . S2CID 22151754 . PMID 25785238 . ^ PMID 30692997 . PMID 25296396 . ^ Fisheries, NOAA (2022-05-03). "Fun Facts About Shocking Sharks | NOAA Fisheries". NOAA. Retrieved 2022-05-11. ^ "Pangolin Fact Sheet | Blog | Nature | PBS". Nature. 25 March 2020. Retrieved 2022-05-11. ^ "Shark and Ray Workbook 3-5 update 8-31" (PDF). Florida Oceanographic Society. ^ "Sweat Gland – an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2022-05-11. ^ ISBN 978-0-03-910284-5 . ^ doi:10.1016/0300-9629(95)98515-I . ^ S2CID 83534922 . ^ Vassar, Robert; Fuchs, Elaine (February 28, 1991). "Transgenic mice provide new insights into the role of TGF- during epidermal development and differentiation" (PDF). Genes & Development. 1991 (5): 714–727. PMID 1709129 . ^ Gilbert, Scott F. (2000). "Induction and Competence". Developmental Biology. 6th Edition. PMID 11826181 . ISBN 978-1-58663-097-3 . ^ "Camouflage". National Geographic. 2011-08-25. Archived from the original on 27 February 2017. Retrieved 27 February 2017. PMID 26144973 . PMID 17652049 . S2CID 44823023 . ^ PMID 16781287 . External links The dictionary definition of skin at Wiktionary Retrieved from "https://en.wikipedia.org/w/index.php?title=Skin&oldid=1219686464"