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 animal coverings
, 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 mammal
s, the skin is an organ
of the integumentary system
made up of multiple layers of ectodermal tissue
, and guards the underlying muscle
s and internal organs
. Skin of a different nature exists in amphibian
s, and bird
s. All mammals have some hair on their skin, even marine mammal
s like whale
s, and porpoise
s which 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
and excessive water loss.
Its other functions are insulation
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 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
is dense hair. 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
s and most fish
have hard protective scales on their skin for protection, and bird
s have hard feather
s, all made of tough beta-keratin
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.
Structure in humans and other mammals
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
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
s are the major cells
, constituting 95% of the epidermis
while Merkel cell
s and Langerhans cell
s are also present. The epidermis
can be further subdivided into the following ''strata'' or layers (beginning with the outermost layer):
[The Ageing Skin – Structure](_blank)
. pharmaxchange.info. March 3, 2011
* Stratum corneum
* Stratum lucidum
(only in palms
* Stratum granulosum
* Stratum spinosum
* Stratum basale
(also called the ''stratum germinativum'')
s 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, keratinocyte
s will become highly organized, forming cellular junctions
) between each other and secreting keratin proteins
which contribute to the formation of an extracellular matrix
and provide mechanical strength
to the skin.
s from the stratum corneum
are eventually shed from the surface (desquamation
contains no blood vessel
s, and cells
in the deepest layers are nourished by diffusion from blood capillaries
extending to the upper layers of the dermis
are separated by a thin sheet of fibers
called the basement membrane
, which is made through the action of both tissues
The basement membrane controls the traffic of the cells
between the dermis and epidermis but also serves, through the binding of a variety of cytokine
s and growth factors
, as a reservoir for their controlled release during physiological
remodeling or repair processes.
The dermis is the layer of skin beneath the epidermis
that consists of connective tissue
and cushions the body from stress
provides tensile strength
to the skin through an extracellular matrix
composed of collagen fibrils
, and elastic fibers
, embedded in hyaluronan
Skin proteoglycans are varied and have very specific locations. For example, hyaluronan
are present throughout the dermis and epidermis extracellular matrix
, whereas biglycan
are only found in the epidermis.
It harbors many mechanoreceptor
s (nerve endings) that provide the sense of touch
and heat through nociceptor
s and thermoreceptor
s. It also contains the hair follicle
s, sweat gland
s, sebaceous gland
s, apocrine glands
, lymphatic vessel
s and blood vessel
s. The blood vessel
s in the dermis
provide nourishment and waste removal from its own cells
as well as for the epidermis
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. It is composed of dense irregular connective tissue
and receives its name from the dense concentration of collagen
, and reticular
fibers that weave throughout it. These protein
fibers give the dermis
its properties of strength
, and elasticity
Also located within the reticular region are the roots of the hair
, sweat gland
s, sebaceous gland
, and blood vessel
The subcutaneous tissue
(also hypodermis) is not part of the skin, and lies below the dermis
. Its purpose is to attach the skin to underlying bone
as well as supplying it with blood vessel
s and nerve
s. It consists of loose connective tissue
. The main cell
types are fibroblast
s and adipocyte
s (the subcutaneous tissue contains 50% of body fat
serves as padding and insulation for the body.
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
Detailed cross section
Structure in fish, amphibians, birds, and reptiles
The epidermis of fish
and of most amphibian
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
s. 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 skull
, these scales
are lost in tetrapods
, although many reptile
s do have scales
of a different kind, as do pangolin
s. Cartilaginous fish
have numerous tooth-like denticles
embedded in their skin, in place of true scales
s and sebaceous gland
s are both unique to mammals
, but other types of skin gland are found in other vertebrates
typically have a numerous individual mucus
-secreting skin cells
that aid in insulation and protection, but may also have poison glands
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 the reptiles, the amphibians, and fish, the epidermis is often relatively colorless. Instead, the color of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as chameleons and flounders may be able to change the color of their skin by adjusting the relative size of their chromatophores.] [
Amphibians possess two types of glands, mucous and granular (serous). Both of these glands are part of the integument and thus considered cutaneous. 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 keratinocytes and passes through to the surface of the epidermal 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 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
The epidermis of birds and reptiles 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 toads. 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 feathers 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-secreting cells in some reptiles, or the uropygial gland of most birds. [
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.
# Sensation: 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.
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.
Tissue homeostasis generally declines with age, in part because stem/progenitor cells fail to self-renew or differentiate. In the skin of mice, mitochondrial oxidative stress can promote cellular senescence and aging phenotypes. 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. TGF-β does this by blocking the conversion of dermal fibroblasts into fat cells; with fewer fat cells underneath to provide support, the skin becomes saggy and wrinkled. Subcutaneous fat also produces cathelicidin, which is a peptide that fights bacterial infections.
Society and culture
The term "skin" may also refer to the covering of a small animal, such as a sheep, goat (goatskin), pig, snake (snakeskin) etc. or the young of a large animal.
The term hides or rawhide refers to the covering of a large adult animal such as a cow, buffalo, horse etc.
Skins and hides from the different animals are used for clothing, bags and other consumer products, usually in the form of leather, but also as furs.
Skin from sheep, goat and cattle was used to make parchment for manuscripts.
Skin can also be cooked to make pork rind or crackling.
* Cutaneous reflex in human locomotion
* Cutaneous respiration – gas exchange conducted through skin
* Role of skin in locomotion