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Epithelium or epithelial tissue is one of the four basic types of
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
tissue, along with
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
, muscle tissue and
nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system. The nervous system regulates and controls body functions and activity. It consists of two parts: the central nervous system (CNS) comprising the bra ...
. It is a thin, continuous, protective layer of compactly packed
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
with a little intercellular matrix. Epithelial tissues line the outer surfaces of organs 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 awa ...
s throughout the body, as well as the inner surfaces of cavities in many internal organs. An example is the epidermis, the outermost layer 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, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...
. There are three principal shapes of epithelial cell: squamous (scaly), columnar, and cuboidal. These can be arranged in a singular layer of cells as simple epithelium, either squamous, columnar, or cuboidal, or in layers of two or more cells deep as stratified (layered), or ''compound'', either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei. This sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
, filtration, secretion, selective
absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which s ...
, germination, and
transcellular transport Transcellular transport involves the transportation of solutes by a cell ''through'' a cell. Transcellular transport can occur in three different ways active transport, passive transport, and transcytosis. Active Transport Main article: Active ...
. Compound epithelium has protective functions. Epithelial layers contain no blood vessels ( avascular), so they must receive nourishment via
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
of substances from the underlying connective tissue, through the basement membrane.
Cell junction Cell junctions (or intercellular bridges) are a class of cellular structures consisting of multiprotein complexes that provide contact or adhesion between neighboring cells or between a cell and the extracellular matrix in animals. They also main ...
s are especially abundant in epithelial tissues.


Classification


Simple epithelium

Simple epithelium is a single layer of cells with every cell in direct contact with the basement membrane that separates it from the underlying connective tissue. In general, it is found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes. In general, epithelial tissues are classified by the number of their layers and by the
shape A shape or figure is a graphical representation of an object or its external boundary, outline, or external surface, as opposed to other properties such as color, texture, or material type. A plane shape or plane figure is constrained to lie ...
and function of the cells. The basic cell types are squamous, cuboidal, and columnar, classed by their shape. By layer, epithelium is classed as either simple epithelium, only one cell thick (unilayered), or stratified epithelium having two or more cells in thickness, or multi-layered – as
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 ...
,
stratified cuboidal epithelium Stratified cuboidal epithelium is a type of epithelial tissue composed of multiple layers of cube-shaped cells. Only the most superficial layer is made up of cuboidal cells, and the other layers can be cells of other types. Topmost layer of skin ...
, and stratified columnar epithelium, and both types of layering can be made up of any of the cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as pseudostratified columnar epithelium. Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium.


Stratified epithelium

Stratified or compound epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers, the cells can be squamous, cuboidal, or columnar. Stratified epithelia (of columnar, cuboidal, or squamous type) can have the following specializations:


Structure

Epithelial tissue cells can adopt shapes of varying complexity (from polyhedral to scutoidal t
punakoidal
, are tightly packed and form a continuous sheet. It has almost no intercellular spaces. All epithelia is usually separated from underlying tissues by an extracellular fibrous basement membrane. The lining of the mouth, lung alveoli and kidney tubules are all made of epithelial tissue. The lining of the blood and lymphatic vessels are of a specialised form of epithelium called
endothelium The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the ve ...
.


Location

Epithelium lines both the outside (
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 ...
) and the inside cavities and lumina of bodies. The outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells. Tissues that line the inside of the mouth, the
esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
, the vagina, and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and
endocrine gland Endocrine glands are ductless glands of the endocrine system that secrete their products, hormones, directly into the blood. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thy ...
s. The outer surface of the
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...
is covered with fast-growing, easily regenerated epithelial cells. A specialised form of epithelium,
endothelium The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the ve ...
, forms the inner lining of
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 awa ...
s and the
heart The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
, and is known as vascular endothelium, and lining lymphatic vessels as lymphatic endothelium. Another type, mesothelium, forms the walls of the
pericardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made ...
, pleurae, and
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...
. In arthropods, the integument, or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle, an outer covering of chitin, the rigidity of which varies as per its chemical composition.


Basement membrane

The basal surface of epithelial tissue rests on a basement membrane and the free/apical surface faces body fluid or outside. The basement membrane acts as a scaffolding on which epithelium can grow and regenerate after injuries. Epithelial tissue has a nerve supply, but no
blood supply The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...
and must be nourished by substances diffusing from the blood vessels in the underlying tissue. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium. The Basal lamina is made up of laminin (glycoproteins) secreted by epithelial cells. The Reticular lamina beneath the basal lamina is made up of collagen proteins secreted by
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
.


Cell junctions

Cell junctions Cell junctions (or intercellular bridges) are a class of cellular structures consisting of multiprotein complexes that provide contact or adhesion between neighboring cells or between a cell and the extracellular matrix in animals. They also maint ...
are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the
extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
, or they build up the paracellular barrier of epithelia and control the paracellular transport. Cell junctions are the contact points between plasma membrane and tissue cells. There are mainly 5 different types of cell junctions: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions. Tight junctions are a pair of trans-membrane protein fused on outer plasma membrane. Adherens junctions are a plaque (protein layer on the inside plasma membrane) which attaches both cells' microfilaments. Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. Hemidesmosomes resemble desmosomes on a section. They are made up of the integrin (a transmembrane protein) instead of cadherin. They attach the epithelial cell to the basement membrane. Gap junctions connect the cytoplasm of two cells and are made up of proteins called
connexin Connexins (Cx)TC# 1.A.24, or gap junction proteins, are structurally related transmembrane proteins that assemble to form vertebrate gap junctions. An entirely different family of proteins, the innexins, form gap junctions in invertebrates. Eac ...
s (six of which come together to make a connexion).


Development

Epithelial tissues are derived from all of the embryological
germ layers A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three prim ...
: * from ectoderm (e.g., the epidermis); * from endoderm (e.g., the lining of the gastrointestinal tract); * from mesoderm (e.g., the inner linings of body cavities). However, it is important to note that pathologists do not consider
endothelium The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the ve ...
and mesothelium (both derived from mesoderm) to be true epithelium. This is because such tissues present very different pathology. For that reason, pathologists label cancers in endothelium and mesothelium
sarcoma A sarcoma is a malignant tumor, a type of cancer that arises from transformed cells of mesenchymal ( connective tissue) origin. Connective tissue is a broad term that includes bone, cartilage, fat, vascular, or hematopoietic tissues, and sar ...
s, whereas true epithelial cancers are called carcinomas. Additionally, the filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is generally accepted that the epithelium arises from all three germ layers.


Cell turnover

Epithelia turn over at some of the fastest rates in the body. For epithelial layers to maintain constant cell numbers essential to their functions, the number of cells that divide must match those that die. They do this mechanically. If there are too few the cells the stretch that they experience rapidly activates cell division. Alternatively, when too many cells accumulate, crowding triggers their death by activation epithelial
cell extrusion Cell extrusion, discovered in 2001, is a process conserved in epithelial from humans to sea sponge to seamlessly remove unwanted or dying cells while maintaining the integrity of the epithelial barrier. If cells were to die without extrusion, gap ...
. Here, cells fated for elimination are seamlessly squeezed out by contracting a band of actin and myosin around and below the cell, preventing any gaps from forming that could disrupt their barriers. Failure to do so can result in aggressive tumors and their invasion by aberrant basal cell extrusion.


Functions

Epithelial tissues have as their primary functions: # to protect the tissues that lie beneath from radiation, desiccation, toxins, invasion by pathogens, and physical
trauma Trauma most often refers to: *Major trauma, in physical medicine, severe physical injury caused by an external source *Psychological trauma, a type of damage to the psyche that occurs as a result of a severely distressing event *Traumatic inju ...
# the regulation and exchange of chemicals between the underlying tissues and a
body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, and ...
# the secretion of hormones into the
circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
, as well as the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts # to provide sensation #Absorb water and digested food in the lining of digestive canal.


Glandular tissue

Glandular tissue is the type of epithelium that forms the
glands In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormones) for release into the bloodstream ( endocrine gland) or into cavities inside the body or its outer surface ( exocrine gland). Structure ...
from the infolding of epithelium and subsequent growth in the underlying connective tissue. They may be specialized columnar or cuboidal tissues consisting of goblet cells, which secrete
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
. There are two major classifications of glands:
endocrine gland Endocrine glands are ductless glands of the endocrine system that secrete their products, hormones, directly into the blood. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thy ...
s and
exocrine gland Exocrine glands are glands that secrete substances on to an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one ...
s: * Endocrine glands secrete their product into the extracellular space where it is rapidly taken up by the circulatory system. * Exocrine glands secrete their products into a duct that then delivers the product to the lumen of an organ or onto the free surface of the epithelium. Their secretions include tears, saliva, oil (sebum),
enzyme Enzymes () are proteins that act as biological catalysts 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 products ...
, digestive juices, sweat, etc.


Sensing the extracellular environment

Some epithelial cells are
ciliated The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proj ...
, especially in respiratory epithelium, and they commonly exist as a sheet of polarised cells forming a tube or tubule with cilia projecting into the lumen." Primary cilia on epithelial cells provide chemosensation, thermoception, and mechanosensation of the extracellular environment by playing "a sensory role mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of the fluid flow, and mediation of fluid flow if the cilia are motile.


Host immune response

Epithelial cells express many genes that encode immune mediators and proteins involved in
cell-cell communication In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellular ...
with hematopoietic immune cells. The resulting immune functions of these non-hematopoietic, structural cells contribute to the mammalian immune system ("structural immunity"). Relevant aspects of the epithelial cell response to infections are encoded in the epigenome of these cells, which enables a rapid response to immunological challenges.


Clinical significance

The slide shows at (1) an epithelial cell infected by ''Chlamydia pneumoniae''; their inclusion bodies shown at (3); an uninfected cell shown at (2) and (4) showing the difference between an infected
cell nucleus The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
and an uninfected cell nucleus. Epithelium grown in culture can be identified by examining its morphological characteristics. Epithelial cells tend to cluster together, and have a "characteristic tight pavement-like appearance". But this is not always the case, such as when the cells are derived from a tumor. In these cases, it is often necessary to use certain biochemical markers to make a positive identification. The intermediate filament proteins in the cytokeratin group are almost exclusively found in epithelial cells, so they are often used for this purpose. Cancers originating from the epithelium are classified as carcinomas. In contrast,
sarcoma A sarcoma is a malignant tumor, a type of cancer that arises from transformed cells of mesenchymal ( connective tissue) origin. Connective tissue is a broad term that includes bone, cartilage, fat, vascular, or hematopoietic tissues, and sar ...
s develop in
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
. When epithelial cells or tissues are damaged from cystic fibrosis, sweat glands are also damaged, causing a frosty coating of the skin.


Etymology and pronunciation

The word ''epithelium'' uses the Greek roots ἐπί (''epi''), "on" or "upon", and θηλή (''thēlē''), "nipple". Epithelium is so called because the name was originally used to describe the translucent covering of small "nipples" of tissue on the lip. The word has both
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...
and
count Count (feminine: countess) is a historical title of nobility in certain European countries, varying in relative status, generally of middling rank in the hierarchy of nobility. Pine, L. G. ''Titles: How the King Became His Majesty''. New York ...
senses; the plural form is epithelia.


Additional images

File:Dogsquamos100x.jpg, Squamous epithelium 100× File:Cheekcells stained.jpg, Human cheek cells (Nonkeratinized stratified squamous epithelium) 500× File:Female urethra histology.jpg, Histology of female urethra showing transitional epithelium File:Sweat gland histology 2014.jpg, Histology of sweat gland showing stratified cuboidal epithelium


See also

* Dark cell * Epithelial-mesenchymal transition * Epithelial polarity *
Glycocalyx The glycocalyx, also known as the pericellular matrix, is a glycoprotein and glycolipid covering that surrounds the cell membranes of bacteria, epithelial cells, and other cells. In 1970, Martinez-Palomo discovered the cell coating in animal c ...
*
Inner Interior may refer to: Arts and media * ''Interior'' (Degas) (also known as ''The Rape''), painting by Edgar Degas * ''Interior'' (play), 1895 play by Belgian playwright Maurice Maeterlinck * ''The Interior'' (novel), by Lisa See * Interior de ...
and Outer enamel epithelium *
Iris pigment epithelium The iris pigment epithelium (IPE) is a one cell thick layer of cuboidal cells lying behind the iris. The epithelial cells are highly pigmented due to the numerous large melanosomes which pack the cytoplasm of each cell. Towards the central axis, ...
* Neuroepithelial cell * Retinal pigment epithelium * Skin cancer * Sulcular epithelium


References


Further reading

* * * *


External links


Epithelium Photomicrographs
* Simple squamous epithelium of the glomerulus (kidney)
Diagrams of simple squamous epithelium
* Stratified squamous epithelium of the vagina * Stratified squamous epithelium of the skin (thin skin) * Stratified squamous epithelium of the skin (thick skin)



{{Authority control Tissues (biology)