This article is about the outer covering of animals.
For skin in humans, see human skin.
For other uses, see Skin (disambiguation).
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.
The adjective cutaneous means "of the skin" (from Latin cutis 'skin').
The skin interfaces with the environment and is the first line of defense from external factors.
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 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.
The skin on the palms and the soles of the feet is 4 mm thick and is the thickest skin on the body.
The speed and quality of wound healing in skin is promoted by the reception of estrogen.
Fur is dense hair.
On some animals, the skin is very hard and thick, and can be processed to create leather.
Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.
Structure in humans and other mammals
See also: Human skin
Mammalian skin is composed of two primary layers:
- the epidermis, which provides waterproofing and serves as a barrier to infection; and
- the dermis, which serves as a location for the appendages of skin;
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 pathogens from entering, and is a stratified squamous epithelium, composed of proliferating basal and differentiated suprabasal keratinocytes.
The epidermis can be further subdivided into the following strata or layers (beginning with the outermost layer):
- Stratum corneum
- Stratum lucidum (only in palms and soles)
- Stratum granulosum
- Stratum spinosum
- Stratum basale (also called the stratum germinativum)
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.
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.
Main article: Basement membrane
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.
Main article: Dermis
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 proteoglycans are varied and have very specific locations.
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.
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.
The reticular region lies deep in the papillary region and is usually much thicker.
Main article: Subcutaneous tissue
Fat serves as padding and insulation for the body.
The density of skin flora depends on region of the skin.
Detailed cross section
Structure in Fish, Amphibians, Birds, and Reptiles
See also: Fish scales
It is generally permeable, and in the case of many amphibians, may actually be a major respiratory organ.
Instead, in most species, it is largely replaced by solid, protective bony scales.
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.
See also: amphibians
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).
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 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.
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 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.
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 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.
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
Main article: Reptile scales
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.
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 stem cell layer and through asymmetrical divisions, becomes the source of skin cells throughout life.
In mice, over-expression of these factors leads to an overproduction of granular cells and thick skin.
Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system.
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.
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.
Skin performs the following functions:
- 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.
- : contains a variety of nerve endings that jump to heat and cold, touch, pressure, vibration, and tissue injury (see somatosensory system and haptic perception).
- 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.
- Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.
- Storage and synthesis: acts as a storage center for lipids and water
- 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.
- 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.
- 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.
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.
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.
Ordinarily mitochondrial superoxide dismutase (SOD2) protects against oxidative stress.
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 aging is caused in part by TGF-β, which reduces the subcutaneous fat that gives skin a pleasant appearance and texture.
Society and culture
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Skin.