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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 nine number of the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, a ...
. The adjective cutaneous means "of the skin" (from Latin ''cutis'' 'skin'). In
mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), class Mammalia (), and characterized by the presence of mammary glands which in Female#Mammalian female, females produce milk ...
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 Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform ...

muscle
s,
bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, store minerals, provide str ...
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 includ ...

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 Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
s,
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

reptile
s, and
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a four-c ...
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 that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their ...
s like
whale Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. They are an informal grouping within the infraorder Cetacea, usually excluding dolphins and porpoises. Whales, dolphins and porpoises belong ...

whale
s,
dolphin Dolphin is a common name of aquatic mammals within the infraorder Cetacea. The term dolphin usually refers to the extant families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river ...
s, and
porpoise Porpoises are a group of fully aquatic marine mammals, similar in appearance to a dolphin, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). They are, however, more closely related to narwhals and ...

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 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, ...
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 Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...
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 Estrogen, or oestrogen, is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ...

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 humans, and at sexual maturity in other animals. These characteristics are particularly evident in the sexual dimorphism, sexually dimorphic phenotypic traits that distinguish ...
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's spotted coat, the bat ...

camouflage
. On some animals, the skin is very hard and thick and can be processed to create
leather Leather is a durable and flexible material created by tanning animal rawhide and skins. The most common raw material is cattle hide. It can be produced at manufacturing scales ranging from artisan to modern industrial scale. Leather makin ...

leather
.
Reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

Reptile
s and most
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

fish
have hard protective scales on their skin for protection, and
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a four-c ...
s have hard
feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both avian (bird) and some non-avian (non-bird) and possibly other archosauromorphs. They are considered the most complex integumentary struc ...

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 Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
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 of solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liq ...

osmosis
and diffusive forces. For example, a
frog A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobat ...

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 Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
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 languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia and th ...
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. Its proposed features have been derived by linguistic reconstruction from documented Indo-European languages. No direct record of Proto-Indo-Europ ...
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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...

epidermis
'', which provides waterproofing and serves as a barrier to infection; and * the ''
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
'', 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's body. In invertebrate biology, an appendage refers to any of the homology (biology), homologous body parts that may extend from a b ...
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 eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume o ...
, composed of proliferating basal and differentiated suprabasal
keratinocyte Keratinocytes are the primary type of cell found in the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a ...
s.
Keratinocyte Keratinocytes are the primary type of cell found in the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a ...
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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
, 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 fin ...
s,
melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis ...
s and
Langerhans cell Langerhans cells (LC) are tissue-resident macrophages of the skin, and contain organelles called Birbeck granules. They are present in all layers of the epidermis and are most prominent in the stratum spinosum. They also occur in the papillar ...
s are also present. The
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
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 language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. There has been a long-standing belief in dermatology that the stratum corneum consisted of dead cell (biology), cells (co ...
*
Stratum lucidum (Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country located mostly in the southern half of South America. Sharing the bulk of the Southern Cone with Chile to the west, the country is ...
(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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis l ...
*
Stratum spinosum The stratum spinosum (or spinous layer/prickle cell layer) is a layer of the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis ...
*
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 make up the skin, the inner layers being the ...
(also called the ''stratum germinativum'')
Keratinocyte Keratinocytes are the primary type of cell found in the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a ...
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 make up the skin, the inner layers being the ...
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 proces ...

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 found in the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a ...
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 (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...

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 Nobe ...

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 mechanisms ...

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 found in the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a ...
s from the
stratum corneum The ''stratum corneum'' (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. There has been a long-standing belief in dermatology that the stratum corneum consisted of dead cell (biology), cells (co ...
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 ...
). The
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
contains no
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away fr ...
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 fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear s ...
capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter, and having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules. These microvessel ...
extending to the upper layers of the
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
.


Basement membrane

The
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...

epidermis
and
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
are separated by a thin sheet of
fibers Fiber or fibre (from la, fibra, links=no) is a #Natural fibers, natural or #Man-made fibers, man-made substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineeri ...
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 processes ...
, 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 scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atom An atom is the smallest unit of ordinary matter In ...

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 biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of function ...
s and
growth factors A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein Proteins are large biomolecules or macromolecules that a ...
, as a reservoir for their controlled release during
physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, B ...
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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
that consists 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 tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in ...
and cushions the body from stress and strain. The
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
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 composed of bundles of protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (bi ...
, embedded in
hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminogly ...

hyaluronan
and
proteoglycan Proteoglycans are 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 E ...
s. Skin proteoglycans are varied and have very specific locations. For example,
hyaluronan Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminogly ...

hyaluronan
,
versican Versican is a large extracellular matrix proteoglycan that is present in a variety of human tissues. It is encoded by the ''VCAN'' gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_ ...
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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...

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 proteoglycan Heparan sulfate (HS) is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan (HSPG, i.e. Heparan Sulfate ProteoGlycan) in which two o ...
are only found in the epidermis. It harbors many
mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory cell that responds to mechanical pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epi ...
s (nerve endings) that provide the sense of
touch The somatosensory system is a part of the sensory nervous system. The somatosensory system is a complex system of sensory neurons and neural pathways that responds to changes at the surface or inside the body. The axons (as afferent nerve fibers) ...

touch
and heat through
nociceptor A nociceptor ("pain receptor") is a sensory neuron that responds to damaging or potentially damaging stimuli by sending “possible threat” signals to the spinal cord and the brain. If the brain perceives the threat as credible, it creates the s ...
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 (anatomy), organ found in mammalian skin. It resides in the Dermis, dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a comple ...

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 animal, with three main functions: protection ...
s,
sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are glands that secrete substances onto an Epithelium, epithelial surface by way of a Duct (anatomy), duct. Examples of exocrine glands include sweat gland, sweat, salivary, mammary, cer ...
s,
apocrine glands Apocrine () is a term used to classify exocrine glands in the study of histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology Biology is the natural science that studies life and living organism ...
,
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 that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away fr ...
s. The
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away fr ...
s in the
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
. 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 Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
is tightly connected to the
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
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 processes ...
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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
. The papillae provide the
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
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 four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in ...
and receives its name from the dense concentration of
collagen 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 and macromolecules th ...
ous, elastic, and reticular fibers that weave throughout it. These
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabol ...

protein
fibers give the
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
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 animal, with three main functions: protection ...
s,
sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are glands that secrete substances onto an Epithelium, epithelial surface by way of a Duct (anatomy), duct. Examples of exocrine glands include sweat gland, sweat, salivary, mammary, cer ...
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 that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away fr ...
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 Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
. Its purpose is to attach the skin to underlying
bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, store minerals, provide str ...
and
muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform ...

muscle
as well as supplying it with
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away fr ...
s and
nerve 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 ...

nerve
s. It consists of loose
connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in ...
and
elastin Elastin is a key 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 Enzy ...
. The main
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 religious recluse lives * Prison cell, a room used to hold peopl ...
types are
fibroblast A fibroblast is a type of biological cell that synthesizes the extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...

fibroblast
s,
macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests anything that does not have o ...

macrophage
s and
adipocyte Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cell Mesenchymal stem cells (MSCs) also known as mesenchy ...

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 four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesod ...
).
Fat In nutrition Nutrition is the biochemical and physiological process by which an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies ...

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

Microorganisms
like ''
Staphylococcus ''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...
epidermidis'' colonize the skin surface. The density of
skin flora The term skin flora (also commonly referred to as skin microbiota) refers to the microorganisms which reside on the skin, typically human skin. Many of them are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of C ...
depends on region of the skin. The disinfected skin surface gets recolonized from
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a Bacte ...
residing in the deeper areas of the
hair follicle The hair follicle is an Organ (anatomy), organ found in mammalian skin. It resides in the Dermis, dermal layer of the skin and is made up of 20 different cell types, each with distinct functions. The hair follicle regulates hair growth via a comple ...

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 animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

fish
and of most
amphibian Amphibians are ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
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 (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...

keratin
in the cells of the superficial layer. It is generally permeable, and in the case of many
amphibians Amphibians are ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
, may actually be a major respiratory organ. The
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
of
bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primaril ...
typically contains relatively little of the
connective tissue Connective tissue is one of the four basic types of animal tissue (biology), tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in ...
found in
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes Neontology#Extant taxa vs. extinct taxa, extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including mam ...
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 Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, sto ...

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 (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient ...

tetrapods
, although many
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

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 of the order Pholidota (, from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from aroun ...

pangolin
s.
Cartilaginous fish Chondrichthyes (; from Greek χονδρ- 'cartilage', ἰχθύς 'fish') is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'' ...
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 animal, with three main functions: protection ...
s and
sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are glands that secrete substances onto an Epithelium, epithelial surface by way of a Duct (anatomy), duct. Examples of exocrine glands include sweat gland, sweat, salivary, mammary, cer ...
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 eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...

vertebrates
.
Fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

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 membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers ...
-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 substances that can cause death, injury or harm to organs, tissues, cells, and DNA usually by chemical reactions or other activity on the molecular scales, when an organism is exposed to a sufficient quantity. Th ...

poison
glands, photophores, 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 fluid. In
amphibians Amphibians are ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
, the
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers ...
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. Most living
amphibians Amphibians are ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
also possess ''granular glands'' in the skin, that secrete irritating or toxic compounds. Although melanin is found in the skin of many species, in the
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

reptile
s, the
amphibians Amphibians are ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
, and
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

fish
, the
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
is often relatively colorless. Instead, the color of the skin is largely due to chromatophores in the
dermis The dermis or corium is a layer of skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such a ...
, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as chameleons and flounders may be able to change the color of their skin by adjusting the relative size of their chromatophores.


Amphibians


Overview

Amphibians possess two types of glands, Mucous gland, mucous and granular (serous). Both of these glands are part of the integument and thus considered cutaneous. Mucous and granular glands are both divided into three different sections which all connect to structure the gland as a whole. The three individual parts of the gland are the duct, the intercalary region, and lastly the alveolar gland (sac). Structurally, the duct is derived via
keratinocyte Keratinocytes are the primary type of cell found in the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a ...
s and passes through to the surface of the Epidermis, epidermal or outer skin layer thus allowing external secretions of the body. The gland alveolus is a sac shaped structure that is found on the bottom or base region of the granular gland. The cells in this sac specialize in secretion. Between the alveolar gland and the duct is the intercalary system which can be summed up as a transitional region connecting the duct to the grand alveolar beneath the epidermal skin layer. In general, granular glands are larger in size than the mucous glands, however mucous glands hold a much greater majority in overall number.


Granular glands

Granular glands can be identified as venomous and often differ in the type of toxin as well as the concentrations of secretions across various orders and species within the amphibians. They are located in clusters differing in concentration depending on
amphibian Amphibians are ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
taxa. The toxins can be fatal to most vertebrates or have no effect against others. These glands are alveolar meaning they structurally have little sacs in which venom is produced and held before it is secreted upon defensive behaviors. Structurally, the ducts of the granular gland initially maintain a cylindrical shape. However, when the ducts become mature and full of fluid, the base of the ducts become swollen due to the pressure from the inside. This causes the epidermal layer to form a pit like opening on the surface of the duct in which the inner fluid will be secreted in an upwards fashion. The intercalary region of granular glands is more developed and mature in comparison with mucous glands. This region resides as a ring of cells surrounding the basal portion of the duct which are argued to have an ectodermal muscular nature due to their influence over the lumen (space inside the tube) of the duct with dilation and constriction functions during secretions. The cells are found radially around the duct and provide a distinct attachment site for muscle fibers around the gland's body. The gland alveolus is a sac that is divided into three specific regions/layers. The outer layer or tunica fibrosa is composed of densely packed connective-tissue which connects with fibers from the spongy intermediate layer where elastic fibers, as well as nerves, reside. The nerves send signals to the muscles as well as the epithelial layers. Lastly, the epithelium or tunica propria encloses the gland.


Mucous glands

Mucous glands are non-venomous and offer a different functionality for amphibians than granular. Mucous glands cover the entire surface area of the amphibian body and specialize in keeping the body lubricated. There are many other functions of the mucous glands such as controlling the pH, thermoregulation, adhesive properties to the environment, anti-predator behaviors (slimy to the grasp), chemical communication, even anti-bacterial/viral properties for protection against pathogens. The ducts of the mucous gland appear as cylindrical vertical tubes that break through the epidermal layer to the surface of the skin. The cells lining the inside of the ducts are oriented with their longitudinal axis forming 90-degree angles surrounding the duct in a helical fashion. Intercalary cells react identically to those of granular glands but on a smaller scale. Among the amphibians, there are taxa which contain a modified intercalary region (depending on the function of the glands), yet the majority share the same structure. The 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 make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
of
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a four-c ...
s and
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

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 ectothermic, tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore bi ...
such as toads. However, in all of these animals there is no clear cell differentiation, differentiation of the
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
into distinct layers, as occurs in humans, with the change in
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 religious recluse lives * Prison cell, a room used to hold peopl ...
type being relatively gradual. The mammalian
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
always possesses at least a stratum germinativum and
stratum corneum The ''stratum corneum'' (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. There has been a long-standing belief in dermatology that the stratum corneum consisted of dead cell (biology), cells (co ...
, but the other intermediate layers found in humans are not always distinguishable. Hair is a distinctive feature of mammalian skin, while
feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both avian (bird) and some non-avian (non-bird) and possibly other archosauromorphs. They are considered the most complex integumentary struc ...

feather
s are (at least among living species) similarly unique to
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a four-c ...
s. Birds and
reptile Reptiles, as most commonly defined, are the animals in the Class (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

reptile
s have relatively few skin glands, although there may be a few structures for specific purposes, such as 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 (biology), class Reptilia , a paraphyletic grouping comprising all amniotes except synapsids (mammals and their extinct relatives) and Aves (birds). Living reptiles comprise turtle ...

reptile
s, or the uropygial gland 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 natural environment, environment in bodily defense. (See Skin absorption.)
Langerhans cell Langerhans cells (LC) are tissue-resident macrophages of the skin, and contain organelles called Birbeck granules. They are present in all layers of the epidermis and are most prominent in the stratum spinosum. They also occur in the papillar ...
s in the skin are part of the adaptive immune system. # Wikt:sensation, Sensation: contains a variety of nerve endings that jump to thermoreceptor, heat and cold,
touch The somatosensory system is a part of the sensory nervous system. The somatosensory system is a complex system of sensory neurons and neural pathways that responds to changes at the surface or inside the body. The axons (as afferent nerve fibers) ...

touch
, pressure, 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 (see somatosensory system and haptic perception). # Thermoregulation: Eccrine (sweat) glands and dilated blood vessels (increased superficial perfusion) aid heat loss, while constricted blood vessel, vessels greatly reduce cutaneous blood flow and conserve heat. Erector pili muscles in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair or fur. # Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss. # Storage and biosynthesis, synthesis: acts as a storage center for lipids and water # Skin absorption, Absorption through the skin: Oxygen, nitrogen and carbon dioxide can diffuse into the
epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the am ...
in small amounts; some animals use their skin as their sole respiration organ (in humans, 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 (physiology), respiration is negligible") Some Topical medication, medications are absorbed through the skin. # Water resistance: The skin acts as a water resistant barrier so essential nutrients aren't washed out of the body. The nutrients and oils that help hydrate the skin are covered by the most outer skin layer, the Epidermis (skin), epidermis. This is helped in part by the sebaceous glands that release sebum, an oily liquid. Water itself will not cause the elimination of oils on the skin, because the oils residing in our dermis flow and would be affected by water without the epidermis. # Camouflage, whether the skin is naked or covered in fur, scales, or feathers, skin structures provide protective coloration and patterns that help to conceal animals from predators or prey.


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 generally declines with age, in part because stem cell, stem/progenitor cells fail to self-renew or cellular differentiation, differentiate. In the skin of mice, mitochondrion, mitochondrial oxidative stress can promote cellular senescence and ageing, aging phenotypes. Ordinarily mitochondrial superoxide dismutase (SOD2) protects against oxidative stress. Using a mouse model of genetic SOD2 deficiency, it was shown that failure to express this important antioxidant enzyme in epidermal cells caused cellular senescence, nuclear DNA damage (naturally occurring), DNA damage, and irreversible arrest of proliferation of a fraction of
keratinocyte Keratinocytes are the primary type of cell found in the epidermis The epidermis is the outermost of the three layers that make up the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a ...
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; with fewer fat cells underneath to provide support, the skin becomes saggy and wrinkled. Subcutaneous fat also produces cathelicidin, which is a peptide that fights bacterial infections.


Society and culture

The term "skin" may also refer to the covering of a small animal, such as a sheep, goat (goatskin (material), goatskin), pig, snake (snakeskin) etc. or the young of a large animal. The term Hide (skin), hides or Rawhide (material), rawhide refers to the covering of a large adult animal such as a cow, American bison, buffalo, horse etc. Skins and Hide (skin), hides from the different animals are used for clothing, bags and other consumer products, usually in the form of
leather Leather is a durable and flexible material created by tanning animal rawhide and skins. The most common raw material is cattle hide. It can be produced at manufacturing scales ranging from artisan to modern industrial scale. Leather makin ...

leather
, but also as furs. Skin from sheep, goat and cattle was used to make parchment for manuscripts. Skin can also be cooked to make pork rind or pork rind, crackling.


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