Epidermis (skin)
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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 animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...
, the inner layers being 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 ...
and hypodermis. The epidermal layer provides a barrier to
infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...
from environmental pathogens and regulates the amount of water released from the body into the
atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
through transepidermal water loss. The epidermis is composed of multiple layers of flattened cells that overlie a base layer ( stratum basale) composed of columnar cells arranged perpendicularly. The layers of cells develop from stem cells in the basal layer. The thickness of the epidermis varies from 31.2μm for the
penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate. The term ''pen ...
to 596.6μm for the sole of the foot with most being roughly 90μm. Thickness does not vary between the sexes but becomes thinner with age. The human epidermis is an example of
epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
, particularly a
stratified squamous epithelium A stratified squamous epithelium consists of squamous (flattened) epithelial cells arranged in layers upon a basal membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural ...
. The word epidermis is derived through Latin , itself and . Something related to or part of the epidermis is termed epidermal.


Structure


Cellular components

The epidermis primarily consists of
keratinocytes Keratinocytes are the primary type of cell found in the epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the basal layer (''stratum basale'') of the skin are sometimes refer ...
( proliferating basal and differentiated suprabasal), which comprise 90% of its cells, but also contains
melanocyte Melanocytes are melanin-producing neural-crest, neural crest-derived cell (biology), cells located in the bottom layer (the stratum basale) of the skin's epidermis (skin), epidermis, the middle layer of the eye (the uvea), the inner ear, vagina ...
s, Langerhans cells, Merkel cells, and inflammatory cells. Epidermal thickenings called Rete ridges (or rete pegs) extend downward between dermal papillae. Blood capillaries are found beneath the epidermis, and are linked to an
arteriole An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillary, capillaries. Arterioles have vascular smooth muscle, muscular walls (usually only one to two layers of smoo ...
and a
venule A venule is a very small vein in the microcirculation that allows blood to return from the capillary beds to drain into the venous system via increasingly larger veins. Post-capillary venules are the smallest of the veins with a diameter of ...
. The epidermis itself has no blood supply and is nourished almost exclusively by diffused oxygen from the surrounding air. Cellular mechanisms for regulating
water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
and
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
levels ( ENaCs) are found in all layers of the epidermis.


Cell boundaries

Epidermal cells are tightly interconnected to serve as a tight barrier against the exterior environment. The junctions between the epidermal cells are of the adherens junction type, formed by transmembrane proteins called cadherins. Inside the cell, the cadherins are linked to
actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...
filaments. In immunofluorescence microscopy, the actin filament network appears as a thick border surrounding the cells, although the actin filaments are actually located inside the cell and run parallel to the cell membrane. Because of the proximity of the neighboring cells and tightness of the junctions, the actin immunofluorescence appears as a border between cells.


Layers

The epidermis is composed of four or five layers, depending on the skin region. Those layers from outermost to innermost are: ; cornified layer (''
stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...
''): Composed of 10 to 30 layers of polyhedral, anucleated corneocytes (final step of keratinocyte differentiation), with the palms and soles having the most layers. Corneocytes contain a
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 metab ...
envelope (cornified envelope proteins) underneath the plasma membrane, are filled with water-retaining keratin proteins, attached together through corneodesmosomes and surrounded in the
extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
space by stacked layers of
lipids Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...
. Most of the barrier functions of the epidermis localize to this layer. ; clear/translucent layer (''
stratum lucidum The stratum lucidum (Latin, 'clear layer') is a thin, clear layer of dead skin cells in the epidermis named for its translucent appearance under a microscope. It is readily visible by light microscopy only in areas of thick skin, which are foun ...
'', only in palms and soles): This narrow layer is found only on the palms and soles. The epidermis of these two areas is known as "thick skin" because with this extra layer, the skin has 5 epidermal layers instead of 4. ; granular layer (''
stratum granulosum The stratum granulosum (or granular layer) is a thin layer of cells in the epidermis lying above the stratum spinosum and below the stratum corneum ( stratum lucidum on the soles and palms).James, William; Berger, Timothy; Elston, Dirk (2005) ''A ...
''): Keratinocytes lose their nuclei and their
cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
appears granular. Lipids, contained into those keratinocytes within lamellar bodies, are released into the extracellular space through exocytosis to form a lipid barrier that prevents water loss from the body as well as entry of foreign substances. Those polar lipids are then converted into non-polar lipids and arranged parallel to the cell surface. For example glycosphingolipids become ceramides and phospholipids become free fatty acids. ; spinous layer (''
stratum spinosum The stratum spinosum (or spinous layer/prickle cell layer) is a layer of the epidermis found between the stratum granulosum and stratum basale. This layer is composed of polyhedral keratinocytes. These are joined with desmosomes. Their spiny ( ...
''): Keratinocytes become connected through desmosomes and produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free
sterols A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
, phospholipids and catabolic enzymes. Langerhans cells, immunologically active cells, are located in the middle of this layer. ; basal/germinal layer ('' stratum basale/germinativum''): Composed mainly of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes.
Melanocyte Melanocytes are melanin-producing neural-crest, neural crest-derived cell (biology), cells located in the bottom layer (the stratum basale) of the skin's epidermis (skin), epidermis, the middle layer of the eye (the uvea), the inner ear, vagina ...
s are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are also found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and
lip The lips are a horizontal pair of soft appendages attached to the jaws and are the most visible part of the mouth of many animals, including humans. Mammal lips are soft, movable and serve to facilitate the ingestion of food (e.g. sucklin ...
s. They are closely associated with cutaneous nerves and seem to be involved in light touch sensation. ; Malpighian layer (''stratum malpighii''): This is usually defined as both the stratum basale and
stratum spinosum The stratum spinosum (or spinous layer/prickle cell layer) is a layer of the epidermis found between the stratum granulosum and stratum basale. This layer is composed of polyhedral keratinocytes. These are joined with desmosomes. Their spiny ( ...
. The epidermis is separated from the dermis, its underlying tissue, by a basement membrane.


Cellular kinetics


Cell division

As a
stratified squamous epithelium A stratified squamous epithelium consists of squamous (flattened) epithelial cells arranged in layers upon a basal membrane. Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural ...
, the epidermis is maintained by cell division within the stratum basale. Differentiating cells delaminate from the basement membrane and are displaced outward through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are eventually shed from the surface ( desquamation). Differentiated keratinocytes secrete keratin proteins, which contribute to the formation of an extracellular matrix that is an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss, taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum. The entire epidermis is replaced by new cell growth over a period of about 48 days.


Calcium concentration

Keratinocyte differentiation throughout the epidermis is in part mediated by a
calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, and decreasing in the stratum corneum. Calcium concentration in the stratum corneum is very low in part because those relatively dry cells are not able to dissolve the ions. This calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers. Elevation of extracellular calcium concentrations induces an increase in
intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
free calcium concentrations. Part of that intracellular increase comes from calcium released from intracellular stores and another part comes from transmembrane calcium influx, through both calcium-sensitive chloride channels and voltage-independent cation channels permeable to calcium. Moreover, it has been suggested that an extracellular calcium-sensing receptor (CaSR) also contributes to the rise in intracellular calcium concentration.


Development

Epidermal organogenesis, the formation of the epidermis, begins in the cells covering the
embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
after neurulation, the formation of the
central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...
. In most
vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
, this original one-layered structure quickly transforms into a two-layered tissue; a temporary outer layer, the embryonic ''periderm'', which is disposed once the inner basal layer or ''
stratum germinativum The stratum basale (basal layer, sometimes referred to as ''stratum germinativum'') is the deepest layer of the five layers of the epidermis, the external covering of skin in mammals. The stratum basale is a single layer of columnar or cuboida ...
'' has formed. This inner layer is a germinal epithelium that gives rise to all epidermal cells. It divides to form the outer spinous layer (''
stratum spinosum The stratum spinosum (or spinous layer/prickle cell layer) is a layer of the epidermis found between the stratum granulosum and stratum basale. This layer is composed of polyhedral keratinocytes. These are joined with desmosomes. Their spiny ( ...
''). The cells of these two layers, together called the Malpighian layer(s) after
Marcello Malpighi Marcello Malpighi (10 March 1628 – 30 November 1694) was an Italians, Italian biologist and physician, who is referred to as the "founder of microscopical anatomy, histology and father of physiology and embryology". Malpighi's name is borne by ...
, divide to form the superficial
granular Granularity (also called graininess) is the degree to which a material or system is composed of distinction (philosophy), distinguishable pieces, granular material, "granules" or grain, "grains" (metaphorically). It can either refer to the exten ...
layer (''Stratum granulosum'') of the epidermis. The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the granules of keratin. These skin cells finally become the cornified layer (''stratum corneum''), the outermost epidermal layer, where the cells become flattened sacks with their nuclei located at one end of the cell. After
birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...
these outermost cells are replaced by new cells from the stratum granulosum and throughout
life Life, also known as biota, refers to matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes. It is defined descriptively by the capacity for homeostasis, Structure#Biological, organisation, met ...
they are shed at a rate of 30 – 90 milligrams of skin flakes every hour, or 0.720 - 2.16 grams per day. Epidermal development is a product of several growth factors, two of which are: * Transforming growth factor Alpha ( TGFα) is an autocrine growth factor by which basal cells stimulate their own division. * Keratinocyte growth factor (KGF or FGF7) is a
paracrine In cellular biology, paracrine signaling is a form of cell signaling, a type of cellular communication (biology), cellular communication in which a Cell (biology), cell produces a signal to induce changes in nearby cells, altering the behaviour of ...
growth factor produced by the underlying dermal fibroblasts in which the proliferation of basal cells is regulated.


Function


Barrier

The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress ( UV light), and
chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...
compounds, and provides mechanical resistance to minor injury. Most of this barrier role is played by the stratum corneum. ;Characteristics * Physical barrier: Epidermal keratinocytes are tightly linked by cell–cell junctions associated to
cytoskeletal The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all Cell (biology), cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane ...
proteins, giving the epidermis its mechanical strength. * Chemical barrier: Highly organized lipids, acids, hydrolytic
enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...
, and
antimicrobial peptides Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between Prokaryote, prokaryotic and eukaryota, eukaryotic cells that may ...
inhibit passage of external chemicals and pathogens into the body. * Immunologically active barrier: The humoral and cellular constituents of the
immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
found in the epidermis actively combat infection. * Water content of the
stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...
drops towards the surface, creating hostile conditions for pathogenic
microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
growth. * An acidic pH (around 5.0) and low amounts of water make the epidermis hostile to many microorganic pathogens. * Non-pathogenic microorganisms on the surface of the epidermis help defend against pathogens by competing for
food Food is any substance consumed by an organism for Nutrient, nutritional support. Food is usually of plant, animal, or Fungus, fungal origin and contains essential nutrients such as carbohydrates, fats, protein (nutrient), proteins, vitamins, ...
, limiting its availability, and producing chemical secretions that inhibit the growth of pathogenic microbiota. ;Permeability *
Psychological Psychology is the scientific study of mind and behavior. Its subject matter includes the behavior of humans and nonhumans, both consciousness, conscious and Unconscious mind, unconscious phenomena, and mental processes such as thoughts, feel ...
stress, through an increase in glucocorticoids, compromises the stratum corneum and thus the barrier function. * Sudden and large shifts in
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
alter stratum corneum hydration in a way that could allow entry of pathogenic microorganisms.


Skin hydration

The ability of the skin to hold water is primarily due to the
stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...
and is critical for maintaining healthy skin. Skin hydration is quantified using corneometry. Lipids arranged through a
gradient In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...
and in an organized manner between the cells of the stratum corneum form a barrier to transepidermal water loss.


Skin color

The amount and distribution of melanin
pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
in the epidermis is the main reason for variation in skin color in ''
Homo sapiens Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
''. Melanin is found in the small melanosomes, particles formed in melanocytes from where they are transferred to the surrounding keratinocytes. The size, number, and arrangement of the melanosomes vary between racial groups, but while the number of melanocytes can vary between different body regions, their numbers remain the same in individual body regions in all human beings. In white and Asian skin the melanosomes are packed in "aggregates", but in black skin they are larger and distributed more evenly. The number of melanosomes in the keratinocytes increases with UV radiation exposure, while their distribution remain largely unaffected.


Touch

The skin contains specialized epidermal touch receptor cells called Merkel cells. Historically, the role of Merkel cells in sensing touch has been thought to be indirect, due their close association with nerve endings. However, recent work in mice and other model organisms demonstrates that Merkel cells intrinsically transform touch into electrical signals that are transmitted to the nervous system.


Clinical significance

Laboratory A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratories are found in a variety of settings such as schools ...
culture of keratinocytes to form a 3D structure ( artificial skin) recapitulating most of the properties of the epidermis is routinely used as a tool for
drug A drug is any chemical substance other than a nutrient or an essential dietary ingredient, which, when administered to a living organism, produces a biological effect. Consumption of drugs can be via insufflation (medicine), inhalation, drug i ...
development and testing.


Hyperplasia

Epidermal
hyperplasia Hyperplasia (from ancient Greek ὑπέρ ''huper'' 'over' + πλάσις ''plasis'' 'formation'), or hypergenesis, is an enlargement of an organ or tissue caused by an increase in the amount of Tissue (biology), organic tissue that results from ...
(thickening resulting from cell proliferation) has various forms: * Acanthosis is diffuse epidermal
hyperplasia Hyperplasia (from ancient Greek ὑπέρ ''huper'' 'over' + πλάσις ''plasis'' 'formation'), or hypergenesis, is an enlargement of an organ or tissue caused by an increase in the amount of Tissue (biology), organic tissue that results from ...
(thickening of the skin, and not to be confused with acanthocytes). It implies increased thickness of the Malpighian layer ( stratum basale and
stratum spinosum The stratum spinosum (or spinous layer/prickle cell layer) is a layer of the epidermis found between the stratum granulosum and stratum basale. This layer is composed of polyhedral keratinocytes. These are joined with desmosomes. Their spiny ( ...
). Acanthosis nigricans is a black, poorly defined, velvety hyperpigmented acanthosis, usually observed in the back of neck, axilla, and other folded regions of the skin. * Focal epithelial hyperplasia (Heck's disease) is an asymptomatic, benign neoplastic condition characterized by multiple white to pinkish
papule A papule is a small, well-defined bump in the skin lesion, skin. It may have a rounded, pointed or flat top, and may have a umbilication, dip. It can appear with a Peduncle (anatomy), stalk, be thread-like or look warty. It can be soft or firm a ...
s that occur diffusely in the oral cavity. * Pseudoepitheliomatous hyperplasia (PEH) is a benign condition characterized by hyperplasia of the epidermis and epithelium of skin appendages, with irregular squamous strands extending down into the dermis, and closely simulating squamous cell carcinoma (SCC). File:Acanthosis-nigricans4.jpg, Acanthosis nigricans File:Hecks disease.jpg, Heck's disease File:Histopathology of pseudoepitheliomatous hyperplasia, low magnification.jpg, Pseudoepitheliomatous hyperplasia (PEH), low magnification, with acanthotic squamous epithelium with irregular thick finger-like downgrowths into the underlying dermis. File:Histopathology of pseudoepitheliomatous hyperplasia, high magnification.jpg, PEH, high magnification, with reactive-appearing squamous downgrowths with no significant cytologic atypia. In contrast,
hyperkeratosis Hyperkeratosis is thickening of the stratum corneum (the outermost layer of the epidermis, or skin), often associated with the presence of an abnormal quantity of keratin,Kumar, Vinay; Fausto, Nelso; Abbas, Abul (2004) ''Robbins & Cotran Pathol ...
is a thickening of the
stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...
, and is not necessarily due to hyperplasia.


Additional images


See also

* List of distinct cell types in the adult human body * Skin repair


References

{{DEFAULTSORT:Epidermis (Skin) Skin anatomy