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 A chemical composition specifies the identity, arrangement, and ratio of the elements making up a compound. Chemical formulas can be used to describe the relative amounts of elements present in a compound. For example, the chemical formula for ...

. The adjective cutaneous means "of the skin" (from Latin ''cutis'' 'skin'). In

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, the skin is an

organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an electronic keyboard instrument ** Hammond ...

of the

integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin and its appendages, which act as a physical barrier between the external environment and the internal environment that it serves ...

made up of multiple layers of ectodermal tissue and guards the underlying

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

s, bones,

ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal li ...

s, and

internal organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...

. Skin of a different nature exists in

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s, and birds. 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 os penis/

os clitoris The os clitoridis (also called the os clitoris or baubellum) is a bone inside the clitoris of many mammalian taxa. It is absent from the human clitoris, but present in the clitoris of some primates, such as the ring-tailed lemurs. It is homologous ...

. All mammals have some hair on their skin, even marine mammals like whales, dolphins, and

porpoise Porpoises are a group of fully aquatic marine mammals, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). Although similar in appearance to dolphins, they are more closely related to narwhals an ...

s 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 Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of anima ...

against pathogens and excessive water loss. Its other functions are

insulation Insulation may refer to: Thermal * Thermal insulation, use of materials to reduce rates of heat transfer ** List of insulation materials ** Building insulation, thermal insulation added to buildings for comfort and energy efficiency *** Insulated ...

, 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 as 4 mm thick. The speed and quality of wound healing in skin is promoted by the reception of estrogen. Fur 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.

Reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s and most fish have hard protective scales on their skin for protection, and birds have hard

feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...

s, all made of tough

beta-keratin Beta-keratin (β-keratin), is a member of a structural protein family found in the epidermis of reptiles and birds. Beta-keratins were named so because they are components of epidermal stratum corneum rich in stacked beta sheets, in contrast to ...

Amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

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.

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 Old Norse 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

Mammalian skin is composed of two primary layers: * The ''

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

'', which provides waterproofing and serves as a barrier to infection. * The ''

dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...

'', which serves as a location for the appendages 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 pathogens from entering, and is a stratified squamous

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

, composed of proliferating basal and differentiated suprabasal keratinocytes. Keratinocytes are the major

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

, constituting 95% of the

, while Merkel cells,

melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...

s and Langerhans cells are also present. The

can be further subdivided into the following ''strata'' or layers (beginning with the outermost layer):The Ageing Skin – Structure
. pharmaxchange.info. March 3, 2011 *

Stratum corneum The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compact ...

* 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 In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

and the daughter

move up the strata changing shape and composition as they undergo multiple stages of

cell differentiation Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellula ...

to eventually become anucleated. During that process, keratinocytes will become highly organized, forming cellular junctions ( desmosomes) between each other and secreting keratin

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

and lipids which contribute to the formation of an extracellular matrix and provide mechanical

strength Strength may refer to: Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human ca ...

to the skin. Keratinocytes from the

stratum corneum The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compact ...

are eventually shed from the surface ( desquamation). The

contains no blood vessels, and

in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the

Basement membrane

The

and

are separated by a thin sheet of

fibers Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...

called the

basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between Epithelium, epithelial tissues including mesot ...

, which is made through the action of both tissues. The basement membrane controls the traffic of the

and

molecules A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

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 Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

remodeling or repair processes.

Dermis

The dermis is the layer of skin beneath the

that consists of

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

and cushions the body from stress and strain. The

provides tensile

and

elasticity Elasticity often refers to: *Elasticity (physics), continuum mechanics of bodies that deform reversibly under stress Elasticity may also refer to: Information technology * Elasticity (data store), the flexibility of the data model and the cl ...

to the skin through an extracellular matrix composed of

collagen fibrils Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

, microfibrils, and elastic fibers, embedded in

hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans ...

and

proteoglycan Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is a serine (Ser) residue to whic ...

s. Skin proteoglycans are varied and have very specific locations. For example,

, versican and decorin are present throughout the dermis and

extracellular matrix, whereas biglycan and perlecan are only found in the epidermis. It harbors many

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...

s (nerve endings) that provide the sense of touch and heat through nociceptors and

thermoreceptor A thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron, that codes absolute and relative changes in temperature, primarily within the innocuous range. In the mammalian peripheral nervous s ...

s. It also contains the

hair follicle The hair follicle is an organ found in mammalian skin. It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a complex interaction between h ...

sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial sur ...

s, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the

provide nourishment and waste removal from its own

as well as for the

. 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

through a

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

. 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

and receives its name from the dense concentration of

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

ous,

elastic Elastic is a word often used to describe or identify certain types of elastomer, elastic used in garments or stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rubber used to hold objects togeth ...

, and

reticular In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...

fibers that weave throughout it. These protein fibers give the

its properties of

extensibility Extensibility is a software engineering and systems design principle that provides for future growth. Extensibility is a measure of the ability to extend a system and the level of effort required to implement the extension. Extensions can be th ...

, and

. Also located within the reticular region are the

roots of the hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine ...

s, sebaceous glands, receptors, nails, and blood vessels.

Subcutaneous tissue

The

subcutaneous tissue The subcutaneous tissue (), also called the hypodermis, hypoderm (), subcutis, superficial fascia, is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the layer are fibroblasts, adipose cells, and macr ...

(also hypodermis) is not part of the skin, and lies below the

. Its purpose is to attach the skin to underlying bone and

as well as supplying it with blood vessels and

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

s. It consists of loose

and

elastin Elastin is a protein that in humans is encoded by the ''ELN'' gene. Elastin is a key component of the extracellular matrix in gnathostomes (jawed vertebrates). It is highly elastic and present in connective tissue allowing many tissues in the bod ...

. The main cell types are

fibroblast A fibroblast is a type of cell (biology), biological cell that synthesizes the extracellular matrix and collagen, produces the structural framework (Stroma (tissue), stroma) for animal Tissue (biology), tissues, and plays a critical role in wound ...

macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...

s and

adipocyte Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...

s (the subcutaneous tissue contains 50% of body fat). Fat serves as padding and insulation for the body.

Microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

like ''

Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...

epidermidis'' colonize the skin surface. The density of

skin flora Skin flora, also called skin microbiota, refers to microbiota (communities of microorganisms) that reside on the skin, typically human skin. Many of them are bacteria of which there are around 1,000 species upon human skin from nineteen phyla.P ...

depends on region of the skin. The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the

, gut and

urogenital The genitourinary system, or urogenital system, are the organs of the reproductive system and the urinary system. These are grouped together because of their proximity to each other, their common embryological origin and the use of common pathwa ...

openings.

Detailed cross section

Structure in fish, amphibians, birds, and reptiles

Fish

The epidermis of fish and of most

s consists entirely of live

, with only minimal quantities of keratin in the cells of the superficial layer. It is generally permeable, and in the case of many

amphibians Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbore ...

, may actually be a major respiratory organ. The

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

typically contains relatively little of the

found in tetrapods. 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

s do have scales of a different kind, as do

pangolin Pangolins, sometimes known as scaly anteaters, are mammals of the order Pholidota (, from Ancient Greek ϕολιδωτός – "clad in scales"). The one extant family, the Manidae, has three genera: '' Manis'', '' Phataginus'', and '' Smut ...

Cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue ...

have numerous tooth-like denticles embedded in their skin, in place of true scales.

Sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial sur ...

s and sebaceous glands are both unique to mammals, but other types of skin gland are found in other vertebrates. Fish typically have a numerous individual mucus-secreting skin

that aid in insulation and protection, but may also have

poison Poison is a chemical substance that has a detrimental effect to life. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figuratively, with a broa ...

glands, photophores, or

that produce a more watery, serous fluid. In

, the mucus

are gathered together to form sac-like glands. Most living

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

s, the

, and fish, the

is often relatively colorless. Instead, the color of the skin is largely due to chromatophores in the

, which, in addition to melanin, may contain guanine or

carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...

pigments. Many species, such as

chameleon Chameleons or chamaeleons (family Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 202 species described as of June 2015. The members of this family are best known for their distinct range of colors, bein ...

s and

flounder Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries. Taxonomy The name "flounder" is used for several only distantly related species, thou ...

s may be able to change the color of their skin by adjusting the relative size of their chromatophores.

Amphibians

Overview

Amphibians possess two types of

gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Structure De ...

mucous Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

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. FrogSkin

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

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

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

al 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

Mucous gland Mucous gland, also known as muciparous glands, are found in several different parts of the body, and they typically stain lighter than serous glands during standard histological preparation. Most are multicellular, but goblet cells are single-cel ...

s 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

of birds and

s is closer to that of mammals, with a layer of dead keratin-filled

at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial

such as toads. However, in all of these animals there is no clear differentiation of the

into distinct layers, as occurs in humans, with the change in cell type being relatively gradual. The mammalian

always possesses at least a

stratum germinativum The ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis, the external covering of skin in mammals. The ''stratum basale'' is a single layer of columnar or ...

and

, but the other intermediate layers found in humans are not always distinguishable.

Hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...

is a distinctive feature of mammalian skin, while

s are (at least among living species) similarly unique to birds. Birds and

s have relatively few skin

s, although there may be a few structures for specific purposes, such as pheromone-secreting

in some

s, or the uropygial gland of most birds.

Development

Cutaneous structures arise from the

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 ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis, the external covering of skin in mammals. The ''stratum basale'' is a single layer of columnar or ...

. The basal layer is a

stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...

layer and through asymmetrical divisions, becomes the source of skin cells throughout life. It is maintained as a stem cell layer through an autocrine signal, TGF alpha, and through paracrine signaling from FGF7 (

keratinocyte growth factor The keratinocyte growth factor (KGF), also known as FGF7, is a growth factor present in the epithelialization-phase of wound healing. In this phase, keratinocytes are covering the wound, forming the epithelium. KGF is a small signaling molecule t ...

) 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: # Protection: an anatomical barrier from pathogens and damage between the internal and external

environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...

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 A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

that jump to heat and cold, touch, pressure, vibration, and tissue

injury An injury is any physiological damage to living tissue caused by immediate physical stress. An injury can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, o ...

(see somatosensory system and haptic perception). # Thermoregulation:

Eccrine Merocrine (or eccrine) is a term used to classify exocrine glands and their secretions in the study of histology. A cell is classified as merocrine if the secretions of that cell are excreted via exocytosis from secretory cells into an epithelia ...

( sweat) glands and dilated blood vessels (increased superficial

perfusion Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as the rate at which blood is deliver ...

) aid heat loss, while constricted

vessels Vessel(s) or The Vessel may refer to: Biology *Blood vessel, a part of the circulatory system and function to transport blood throughout the body *Lymphatic vessel, a thin walled, valved structure that carries lymph *Vessel element, a narrow wat ...

greatly reduce cutaneous blood flow and conserve heat.

Erector pili muscle The arrector pili muscles, also known as hair erector muscles, are small muscles attached to hair follicles in mammals. Contraction of these muscles causes the hairs to stand on end, known colloquially as goose bumps (piloerection). Structure ...

s in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair or fur. # Control of

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss. # Storage and

synthesis Synthesis or synthesize may refer to: Science Chemistry and biochemistry *Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors ** Organic synthesis, the chemical synthesis of organ ...

: acts as a storage center for lipids and water # Absorption through the skin: Oxygen, nitrogen and carbon dioxide can diffuse into the

in small amounts; some animals use their skin as their sole respiration organ (in humans, the

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

. 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.

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 homeostasis generally declines with age, in part because

stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...

/progenitor cells fail to self-renew or differentiate. Skin aging is caused in part by TGF-β by blocking the conversion of

dermal fibroblasts Dermal fibroblasts are cells within the dermis layer of skin which are responsible for generating connective tissue and allowing the skin to recover from injury. Using organelles (particularly the rough endoplasmic reticulum), dermal fibroblasts ge ...

into 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. Moreover, these factors may be worsened by sun exposure in a process known as photoaging.

References

External links

* * {{Authority control Soft tissue Leathermaking Organs (anatomy) Animal anatomy Skin physiology