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Skin is the layer of usually soft, flexible outer tissue covering the body of a
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the
arthropod exoskeleton Arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is ...
, have different developmental origin, structure and
chemical composition {{Unreferenced, date=December 2017 Chemical composition refers to identity and number of the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an el ...
. The adjective cutaneous means "of the skin" (from Latin ''cutis'' 'skin'). In
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s, the skin is an
organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cate ...
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, acting as a physical barrier between the external environment and the internal environment that it serves to p ...

integumentary system
made up of multiple layers of
ectodermal The ectoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrate Vertebrates () comprise all species of animal Animals (also ...
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
and guards the underlying
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s,
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
s,
ligament A ligament is the Connective tissue#Types, 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 incl ...

ligament
s, and
internal organs An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma, the tissue peculiar to (or ...

internal organs
. Skin of a different nature exists in
amphibian Amphibians are ectothermic, tetrapod 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 terrestrial animal, ter ...
s,
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s, and
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s. 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 The baculum (also penis bone, penile bone, or ''os penis'', ''os genitale'' or ''os priapi'') is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the vari ...
/ os clitoris. All mammals have some hair on their skin, even
marine mammal Marine mammals are aquatic mammals Aquatic and semiaquatic mammals are a diverse group of mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and ...
s like
whale Whales are a widely distributed and diverse group of fully s. They are an informal grouping within the infraorder , which usually excludes s and s. Whales, dolphins and porpoises belong to the order , which consists of s. Their closest n ...

whale
s,
dolphin Dolphin is the common name of aquatic mammal Aquatic and semiaquatic mammals are a diverse group of mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mam ...

dolphin
s, and
porpoise Porpoises are a group of fully aquatic marine mammals, similar in appearance to a dolphin Dolphin is the common name of aquatic mammals within the infraorder Cetacea. The term dolphin usually refers to the extant families Delphinidae (the ...

porpoise
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 animals ...

body
against
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
s and excessive water loss. Its other functions are
insulation
insulation
,
temperature regulation Thermoregulation is the ability of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a sy ...
, sensation, and the production of
vitamin D Vitamin D is a group of fat-soluble secosteroid 250px, The parent steroid skeleton. The B-ring of the parent steroid is broken between C9 and C10 to yield D vitamins. A secosteroid () is a type of steroid , hypothetical a steroid with ...
folates. Severely damaged skin may heal by forming
scar tissue "Scar Tissue" is the first single from the American rock band Red Hot Chili Peppers' seventh studio album '' Californication'', released in 1999. It is one of their most successful songs, spending a then-record 16 consecutive weeks atop the US ...
. 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 Estrogens or oestrogens, are a class of natural or synthetic s responsible for the development and regulation of the female and s. There are three major estrogens that have estrogenic hormonal activity: (E1), (E2), and (E3). Estradiol, an ...

estrogen
.
Fur Fur is a thick growth of 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 ...

Fur
is dense hair. Primarily, fur augments the insulation the skin provides but can also serve as a
secondary sexual characteristic Secondary sex characteristics are features that appear during puberty in human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedality, bipedalism and large, complex brains. This has ena ...
or as
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard The leopard (''Pan ...

camouflage
. On some animals, the skin is very hard and thick and can be processed to create
leather Leather is a strong, flexible and durable material obtained from the tanning Tanning may refer to: *Tanning (leather), treating animal skins to produce leather *Sun tanning, using the sun to darken pale skin **Indoor tanning, the use of arti ...

leather
.
Reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

Reptile
s and most
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
have hard protective scales on their skin for protection, and
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s have hard
feather Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both Bird, avian (bird) and some non-avian (non-bird) and possibly other archosauromorpha, archosauromorphs. They are conside ...

feather
s, all made of tough
beta-keratin β-keratin or beta-keratin is a member of a structural protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of func ...
s.
Amphibian Amphibians are ectothermic, tetrapod 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 terrestrial animal, ter ...
skin is not a strong barrier, especially regarding the passage of chemicals via skin, and is often subject to
osmosis Osmosis (, ) is the spontaneous net movement or diffusion File:DiffusionMicroMacro.gif, 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) an ...

osmosis
and diffusive forces. For example, a
frog A frog is any member of a diverse and largely carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization all ...

frog
sitting in an
anesthetic An anesthetic (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Engl ...
solution would be sedated quickly as the chemical diffuses through its skin.
Amphibian Amphibians are ectothermic, tetrapod 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 terrestrial animal, ter ...
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 Old Norse, Old Nordic, or Old Scandinavian is a stage of development of North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia Scandinavia; : ''Skades ...
skinn "animal hide, fur", ultimately from the
Proto-Indo-European Proto-Indo-European (PIE) is the theorized common ancestor of the Indo-European language family The Indo-European languages are a language family A language is a structured system of communication used by humans, including speech ( ...
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 Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...

epidermis
'', which provides waterproofing and serves as a barrier to infection; and * the ''
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
'', which serves as a location for the
appendages An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are co ...
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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
from entering, and is a stratified squamous
epithelium Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as ...
, composed of
proliferating
proliferating
basal and differentiated suprabasal
keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''st ...
s.
Keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''st ...
s are the major
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
, constituting 95% of the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
, while
Merkel cell Merkel cells, also known as Merkel-Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of the fi ...
s,
melanocyte Melanocytes are melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, wher ...
s and
Langerhans cell Langerhans cells (LC) are tissue-resident macrophages Macrophages (abbreviated as Mφ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell White blood cel ...
s are also present. The
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
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 Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be ...
*
Stratum lucidum The stratum lucidum (Latin for "clear layer") is a thin, clear layer of dead skin cells in the epidermis named for its translucent appearance under a microscope A microscope (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to ...
(only in and soles) *
Stratum granulosum The stratum granulosum (or granular layer) is a thin layer of cells in the epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a v ...
*
Stratum spinosum The stratum spinosum (or spinous layer/prickle cell layer) is a layer of the epidermis (skin), epidermis found between the stratum granulosum and Stratum germinativum, stratum basale. This layer is composed of polyhedral keratinocytes. These are jo ...
*
Stratum basale The ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of ...
(also called the ''stratum germinativum'')
Keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''st ...
s in the
stratum basale The ''stratum basale'' (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of ...
proliferate through
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
and the daughter
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
move up the strata changing shape and composition as they undergo multiple stages of
cell differentiation Cellular differentiation is the process in which a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or re ...
to eventually become anucleated. During that process,
keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''st ...
s will become highly organized, forming cellular junctions (
desmosomes A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally s ...
) between each other and secreting
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprot ...

keratin
proteins Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

proteins
and
lipids In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

lipids
which contribute to the formation of an
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
and provide mechanical
strength 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 capability *A common attrib ...
to the skin.
Keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''st ...
s from the
stratum corneum The stratum corneum (Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be ...
are eventually shed from the surface (
desquamation Desquamation, commonly called skin peeling, is the shedding of the outermost membrane or layer of a tissue, such as the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main func ...

desquamation
). The
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
contains no
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...
s, and
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
in the deepest layers are nourished by diffusion from
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

blood
capillaries A capillary is a small blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biologic ...

capillaries
extending to the upper layers of the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
.


Basement membrane

The
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...

epidermis
and
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
are separated by a thin sheet of
fibers Fiber or fibre (from la, fibra, links=no) is a natural Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is all of space and time and their contents, including ...
called the
basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processe ...
, which is made through the action of both tissues. The basement membrane controls the traffic of the
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
and
molecules A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecules
between the dermis and epidermis but also serves, through the binding of a variety of
cytokine Cytokines are a broad and loose category of small protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and ...

cytokine
s and
growth factors A growth factor is a naturally occurring substance capable of stimulating cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential gr ...
, as a reservoir for their controlled release during
physiological Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
remodeling or repair processes.


Dermis

The dermis is the layer of skin beneath the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
that consists of
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
and cushions the body from stress and strain. The
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
provides tensile
strength 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 capability *A common attrib ...
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 clu ...
to the skin through an
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
composed of
collagen fibrils upright=1.5, Tropocollagen molecule: three left-handed procollagens (red, green, blue) join to form a right-handed triple helical tropocollagen. Collagen () is the main structural protein Proteins are large biomolecules or macromolecules that ...
, microfibrils, and
elastic fibers Elastic fibers (or yellow fibers) are an essential component of the extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemi ...
, embedded in
hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base A conjugate acid, within the , is a chemical compound formed when an acid () to a —in other words, it is a base with a ion added to it, as in the reverse reaction it loses a hydrogen ion ...

hyaluronan
and
proteoglycan Proteoglycans are protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958 ...
s. Skin proteoglycans are varied and have very specific locations. For example,
hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base A conjugate acid, within the , is a chemical compound formed when an acid () to a —in other words, it is a base with a ion added to it, as in the reverse reaction it loses a hydrogen ion ...

hyaluronan
,
versican Versican is a large extracellular matrix proteoglycan Proteoglycans are protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallogra ...
and
decorin Decorin is a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme ca ...
are present throughout the dermis and
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...

epidermis
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
, whereas
biglycan Biglycan is a small leucine-rich repeat A leucine-rich repeat (LRR) is a protein structural motif that forms an α/β horseshoe tertiary structure, fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the h ...
and
perlecan Perlecan (PLC) also known as basement membrane-specific Heparan sulfate#Proteoglycans, heparan sulfate proteoglycan core protein (HSPG) or heparan sulfate proteoglycan 2 (HSPG2), is a protein that in humans is encoded by the ''HSPG2'' gene. Perle ...
are only found in the epidermis. It harbors many
mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an electrically excitable cell Cell most often refers to: * Cell (biology), the fu ...
s (nerve endings) that provide the sense of
touch The somatosensory system is a part of the sensory nervous system The sensory nervous system is a part of the nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly comple ...

touch
and heat through
nociceptor A nociceptor ("pain receptor") is a sensory neuron Sensory neurons, also known as afferent neurons, are neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates w ...
s and
thermoreceptorA thermoreceptor is a non-specialised sense receptor, or more accurately the receptive portion of a sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system ...
s. It also contains the
hair follicle The hair follicle is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given org ...

hair follicle
s,
sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of an ...
s,
sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is a ...
s,
apocrine glands Apocrine () is a term used to classify exocrine glands Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in m ...

apocrine glands
,
lymphatic vessel The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes) structured like blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels t ...
s and
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...
s. The
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...
s in the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
provide nourishment and waste removal from its own
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
as well as for the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
. Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals. The
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
is tightly connected to the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
through a
basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processe ...
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 Loose connective tissue (also called areolar connective tissue) is a category of connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tis ...
. This is named for its fingerlike projections called ''papillae'' that extend toward the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
. The papillae provide the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
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
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
and receives its name from the dense concentration of
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
ous, elastic, and reticular fibers that weave throughout it. These
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
fibers give the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
its properties of
strength 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 capability *A common attrib ...
,
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 A system is a group of Interaction, interacting or interrelated elements that ...

extensibility
, 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 clu ...
. Also located within the reticular region are the roots of the hair,
sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of an ...
s,
sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is a ...
s,
receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ne ...
, nails, and
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...
s.


Subcutaneous tissue

The
subcutaneous tissue The subcutaneous tissue (), also called the hypodermis, hypoderm (), subcutis, superficial fascia, is the lowermost layer of the integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body ...
(also hypodermis) is not part of the skin, and lies below the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
. Its purpose is to attach the skin to underlying
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
and
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
as well as supplying it with
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...
s and
nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term diale ...

nerve
s. It consists of loose
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
and
elastin Elastin is a key protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, f ...
. The main cell types are
fibroblast A fibroblast is a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referre ...

fibroblast
s,
macrophage Macrophages (abbreviated as Mφ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the ...

macrophage
s and
adipocyte Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue Connective tissue is one of the many basic types of animal An ...

adipocyte
s (the subcutaneous tissue contains 50% of
body fat Adipose tissue, body fat, or simply fat is a loose connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biolo ...
).
Fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

Fat
serves as padding and insulation for the body.
Microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...

Microorganisms
like ''
Staphylococcus ''Staphylococcus'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy) ...
epidermidis'' colonize the skin surface. The density of
skin flora The term skin flora, also commonly called skin microbiota, refers to ( of s) that reside on the , typically . Many of them are of which there are around 1,000 species upon human skin from nineteen .Pappas S. (2009)Your Body Is a Wonderland ... ...
depends on region of the skin. The disinfected skin surface gets recolonized from
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
residing in the deeper areas of the
hair follicle The hair follicle is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given org ...

hair follicle
, gut and
urogenital The genitourinary system, or urogenital system, are the organs of the reproductive system and the urinary system The urinary system, also known as the renal system or urinary tract, consists of the kidneys, ureters, bladder The urinary bl ...
openings.


Detailed cross section


Structure in fish, amphibians, birds, and reptiles


Fish

The epidermis of
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
and of most
amphibian Amphibians are ectothermic, tetrapod 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 terrestrial animal, ter ...
s consists entirely of live
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
, with only minimal quantities of
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprot ...

keratin
in the cells of the superficial layer. It is generally permeable, and in the case of many
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an o ...

amphibians
, may actually be a major respiratory organ. The
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
of
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may al ...
typically contains relatively little of the
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
found in
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s. Instead, in most species, it is largely replaced by solid, protective bony
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathematics), set of point (mathematics), points in some Poli ...
. Apart from some particularly large dermal bones that form parts of the
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
, these
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathematics), set of point (mathematics), points in some Poli ...
are lost in
tetrapods Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...

tetrapods
, although many
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s do have
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathematics), set of point (mathematics), points in some Poli ...
of a different kind, as do
pangolin Pangolins, sometimes known as scaly anteaters, are mammals Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in ...

pangolin
s.
Cartilaginous fish Chondrichthyes (; ) is a class (biology), 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 ...
have numerous tooth-like denticles embedded in their skin, in place of true
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathematics), set of point (mathematics), points in some Poli ...
.
Sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of an ...
s and
sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is a ...
s are both unique to
mammals Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the R ...
, but other types of skin gland are found in other
vertebrates Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...

vertebrates
.
Fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

Fish
typically have a numerous individual
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...
-secreting skin
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
that aid in insulation and protection, but may also have
poison In biology, poisons are Chemical substance, substances that can cause death, injury or harm to organs, Tissue (biology), tissues, Cell (biology), cells, and DNA usually by chemical reactions or other activity (chemistry), activity on the molecul ...

poison
glands In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

glands
,
photophore A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, od ...
s, or
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
that produce a more watery,
serous In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physica ...
fluid. In
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an o ...

amphibians
, the
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
are gathered together to form sac-like
glands In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

glands
. Most living
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an o ...

amphibians
also possess ''granular
glands In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

glands
'' in the skin, that secrete irritating or toxic compounds. Although
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

melanin
is found in the skin of many species, in the
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s, the
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an o ...

amphibians
, and
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
, the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
is often relatively colorless. Instead, the color of the skin is largely due to
chromatophore Chromatophores are cells that produce color, of which many types are pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Genera ...
s in the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
, which, in addition to melanin, may contain
guanine Guanine () (symbol A symbol is a mark, sign, or word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolation with semantic, objective or pragmatics, practical me ...

guanine
or
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organ ...
pigments A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compound , CH4; is among the simplest organic compou ...

pigments
. Many species, such as
chameleon Chameleons or chamaeleons (family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and ...

chameleon
s and
flounder Flounders are a group of flatfish A flatfish is a member of the ray-finned Actinopterygii (New Latin ('having rays') + Greek ( 'wing, fins')), members of which are known as ray-finned fishes, is a clade (traditionally Class (biology), ...
s may be able to change the color of their skin by adjusting the relative size of their
chromatophores Chromatophores are cells that produce color, of which many types are Biological pigment, pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and ...

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 hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

gland
s,
mucous Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists ...
and granular (serous). Both of these glands are part of the
integument In biology, integument is the natural covering of an organism or an organ, such as its skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, an ...
and thus considered
cutaneous Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism ...
. 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
keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''st ...
s and passes through to the surface of the 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

Granular glands can be identified as
venom Venom is a type of poison In biology, poisons are Chemical substance, substances that can cause death, injury or harm to organs, Tissue (biology), tissues, Cell (biology), cells, and DNA usually by chemical reactions or other activity (chemi ...

venom
ous 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 Amphibians are ectothermic, tetrapod 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 terrestrial animal, ter ...
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 layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( ...

ectoderm
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 A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. The ...
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
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
of
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s and
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s is closer to that of
mammals Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the R ...
, with a layer of dead keratin-filled
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an o ...

amphibians
such as
toad Toad is a common name for certain frog A frog is any member of a diverse and largely carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since preh ...

toad
s. However, in all of these animals there is no clear
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
of the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
into distinct layers, as occurs in
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

human
s, with the change in cell type being relatively gradual. The mammalian
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
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 epidermis is the outermost of the three layers that make up the skin, the inner layers being the ...
and
stratum corneum The stratum corneum (Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be ...
, but the other intermediate layers found in
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

human
s are not always distinguishable.
Hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Ph ...

Hair
is a distinctive feature of mammalian skin, while
feather Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both Bird, avian (bird) and some non-avian (non-bird) and possibly other archosauromorpha, archosauromorphs. They are conside ...

feather
s are (at least among living species) similarly unique to
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s.
Bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

Bird
s and
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s have relatively few skin
gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

gland
s, although there may be a few structures for specific purposes, such as
pheromone A pheromone (from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ...

pheromone
-secreting
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
in some
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s, or the
uropygial gland The uropygial gland, informally known as the preen gland or the oil gland, is a bilobed sebaceous gland A sebaceous gland is a microscopic exocrine gland A gland is a group of cells in an animal's body that synthesizes substances (such as hor ...
of most birds.


Development


Functions

Skin performs the following functions: # Protection: an anatomical barrier from
pathogens In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
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 ...

environment
in bodily defense. (See Skin absorption.)
Langerhans cell Langerhans cells (LC) are tissue-resident macrophages Macrophages (abbreviated as Mφ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell White blood cel ...
s in the skin are part of the
adaptive immune system The adaptive immune system, also referred as the acquired immune system, is a subsystem of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, ...
. #
SensationSensation refers to the processing of sense Sense relates to any of the systems and corresponding organs involved in sensation, i.e. the physical process of responding to Stimulus (physiology), stimuli and providing data for perception. During sensa ...
: contains a variety of
nerve endings A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences), is a ...
that jump to heat and cold,
touch The somatosensory system is a part of the sensory nervous system The sensory nervous system is a part of the nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly comple ...

touch
,
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

pressure
,
vibration Vibration is a mechanical phenomenon whereby oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparentl ...

vibration
, and
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
injury Injury, also known as physical trauma, is damage to the body caused by an external force. This may be caused by accidents, falls, hits, weapons, and other causes. Major trauma is injury that has the potential to cause prolonged disability or ...

injury
(see
somatosensory system The somatosensory system is a part of the sensory nervous system The sensory nervous system is a part of the nervous system In biology, the classical doctrine of the nervous system determines that it is a Complex system, highly comple ...
and
haptic perception Haptic perception ( gr, haptόs, script=latn, italics=yes "palpable", ''haptikόs'' "suitable for touch") means literally the ability "to grasp something". Perception Perception (from the Latin ''perceptio'', meaning gathering or receivin ...
). # Thermoregulation: Eccrine (
sweat Perspiration, also known as sweating, is the production of fluids secreted by the sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, fle ...

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 (anatomy), organ or a tissue (biology), tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as ...
) aid heat loss, while constricted vessels greatly reduce cutaneous
blood flow Hemodynamics American and British English spelling differences#ae and oe, or haemodynamics are the Fluid dynamics, dynamics of blood flow. The circulatory system is controlled by homeostasis, homeostatic mechanisms of autoregulation, just as hydraul ...

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 Fur is a thick growth of hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular bi ...

fur
. # Control of
evaporation Evaporation is a type of vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phe ...

evaporation
: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss. # Storage and
synthesis Synthesis or synthesize may also 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 o ...

synthesis
: acts as a storage center for
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
s and water # Absorption through the skin:
Oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

Oxygen
,
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
and
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
can diffuse into the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
in small amounts; some animals use their skin as their sole
respiration organ The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans sev ...
(in
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

human
s, the
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
comprising the outermost 0.25–0.40 mm of the skin are "almost exclusively supplied by external oxygen", although the "contribution to total
respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellular ...
is negligible") Some medications are absorbed through the skin. # Water resistance: The skin acts as a water resistant barrier so essential
nutrients A nutrient is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition * Matter, anything that has mass and ta ...
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 The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
. This is helped in part by the sebaceous glands that release
sebum A sebaceous gland is a microscopic exocrine Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is a ...

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 Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard The leopard (''Pan ...

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.


Ageing

Tissue
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
generally declines with age, in part because
stem Stem or STEM may refer to: Biology * Plant stem, the aboveground structures that have vascular tissue and that support leaves and flowers ** Stipe (botany), a stalk that supports some other structure ** Stipe (mycology), the stem supporting the c ...
/progenitor cells fail to self-renew or differentiate. In the skin of mice,
mitochondrial A mitochondrion (, plural mitochondria) is a double membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the ide ...

mitochondrial
oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species Reactive oxygen species (ROS) are highly chemicals formed from O2. Examples of ROS include s, , , , and . The reduction of molecular oxygen ...
can promote
cellular senescence File:SABG MEFs.jpg, (Upper) Primary mouse embryonic fibroblast cells (MEFs) before senescence. Spindle-shaped.(Lower) MEFs became senescent after passages. Cells grow larger, flatten shape and expressed senescence-associated Beta-galactosidase, β- ...

cellular senescence
and
aging Ageing or aging (see spelling differences Despite the various English dialects Dialect The term dialect (from Latin , , from the Ancient Greek word , , "discourse", from , , "through" and , , "I speak") is used in two distinct ways ...
phenotypes. Ordinarily mitochondrial superoxide dismutase (
SOD2 Superoxide dismutase 2, mitochondrial (SOD2), also known as manganese-dependent superoxide dismutase A superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1&min ...
) 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 DNA repair is a collection of processes by which a identifies and corrects damage to the molecules that encode its . In human cells, both normal activities and environmental factors such as can cause DNA damage, resulting in tens of thousan ...
, and irreversible arrest of proliferation of a fraction of
keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''st ...
s. 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 Adipocytes, also known as lipocytes and fat cells, are the cell (biology), 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 ad ...
; with fewer fat cells underneath to provide support, the skin becomes saggy and wrinkled. Subcutaneous fat also produces
cathelicidin Cathelicidin antimicrobial peptide (CAMP) are polypeptide that is primarily stored in the lysosomes of macrophages and polymorphonuclear leukocytes (PMNs); in humans, the ''CAMP'' gene In biology, a gene (from ''genos'' "...Wilhelm Johanns ...
, which is a
peptide Peptides (from Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the dialects of the Greek language spoken ...
that fights bacterial infections.


Society and culture

The term "skin" may also refer to the covering of a small animal, such as a
sheep Sheep (''Ovis aries'') are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order (biology), order Artiodactyla, the even-toed ungulates. Although the name ''sheep'' applies to many species ...

sheep
,
goat The domestic goat or simply goat (''Capra hircus'') is a domesticated species of typically kept as . It was from the (''C. aegagrus'') of and . The goat is a member of the animal family and the subfamily , meaning it is closely related ...

goat
( goatskin),
pig The pig (''Sus domesticus''), often called swine, hog, or domestic pig when distinguishing from other members of the genus '' Sus'', is an omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and ani ...

pig
, snake (
snakeskin Snakeskin may either refer to the skin of a live snake, the shed skin of a snake after molting In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chem ...
) 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 Cow Cattle, or cows (female) and bulls (male), are the most common type of large domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the ...

cow
, ,
horse The horse (''Equus ferus caballus'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to ...

horse
etc. Skins and hides from the different animals are used for
clothing Clothing (also known as clothes, apparel, and attire) are items worn on the body. Typically, clothing is made of fabrics or textile A textile is a flexible material made by creating an interlocking bundle of yarn Yarn is a long ...

clothing
, bags and other
consumer products A final good or consumer good is a final product In Production (economics), production, a final product, or finished product is a product (business), product that is ready for sale.Wouters, Mark; Selto, Frank H.; Hilton, Ronald W.; Maher, Michae ...
, usually in the form of
leather Leather is a strong, flexible and durable material obtained from the tanning Tanning may refer to: *Tanning (leather), treating animal skins to produce leather *Sun tanning, using the sun to darken pale skin **Indoor tanning, the use of arti ...

leather
, but also as
fur Fur is a thick growth of hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular bi ...

fur
s. Skin from
sheep Sheep (''Ovis aries'') are quadrupedal, ruminant mammals typically kept as livestock. Like all ruminants, sheep are members of the order (biology), order Artiodactyla, the even-toed ungulates. Although the name ''sheep'' applies to many species ...

sheep
,
goat The domestic goat or simply goat (''Capra hircus'') is a domesticated species of typically kept as . It was from the (''C. aegagrus'') of and . The goat is a member of the animal family and the subfamily , meaning it is closely related ...

goat
and
cattle Cattle, taurine cattle, Eurasian cattle, or European cattle (''Bos taurus'' or ''Bos primigenius taurus'') are large domestication, domesticated Cloven hoof, cloven-hooved herbivores. They are a prominent modern member of the subfamily Bovinae ...

cattle
was used to make
parchment Parchment is a writing material Writing material refers to the materials that provide the surfaces on which humans use writing instruments A writing implement or writing instrument is an object used to produce writing Writing is a mediu ...

parchment
for
manuscript A manuscript (abbreviated MS for singular and MSS for plural) was, traditionally, any document written by hand – or, once practical typewriter A typewriter is a or machine for characters. Typically, a typewriter has an array ...

manuscript
s. Skin can also be cooked to make
pork rind Pork rind is the culinary term for the skin of a pig. It can be used in many different ways. It can be Rendering (animal products), rendered, fried in fat, or roasted to produce a kind of pork cracklings, cracklings (US) or scratchings (UK); t ...

pork rind
or .


See also

* Cutaneous reflex in human locomotion * Cutaneous respiration – gas exchange conducted through skin * Moult * Role of skin in locomotion * Skinning


References


External links

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