epithelium
   HOME

TheInfoList



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 organic material, Cellular respiration#Aerobic respiration, ...

animal
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 ...
, along with
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consum ...
,
muscle tissue Muscle tissues are soft tissue of a tendon A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton i ...

muscle tissue
and
nervous tissue Nervous tissue, also called neural tissue, is the main tissue component of the nervous system In Biology, biology, the nervous system is a Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, se ...
. 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: 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 little
intercellular 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, ...
. Epithelial tissues line the outer surfaces of
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...
and
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino ...
s throughout the body, as well as the inner surfaces of
cavitiesA cavity is a hollow in an object. Biology and healthcare *Cavity or Tooth decay, dental caries, damage to the structure of a tooth *Body cavity, a fluid filled space in many animals where organs typically develop **The gastrovascular cavity, which ...

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

epidermis
, the outermost layer of the
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biolo ...

skin
. There are three principal shapes of epithelial cell: squamous, columnar, and cuboidal. These can be arranged in a single 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
gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, ...

gland
s are made up of epithelial cells. Functions of epithelial cells include
diffusion File:DiffusionMicroMacro.gif, 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diff ...

diffusion
, filtration,
secretionSecretion is the movement of material from one point to another, such as a secreted chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up sp ...
, selective
absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...
,
germination seedlings, three days after germination Germination is the process by which an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell t ...

germination
, and
transcellular transport Transcellular transport involves the transportation of solutes by a cell ''through'' a cell. One classic example is the movement of glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the ...
. Compound epithelium has protective functions. Epithelial layers contain no blood vessels (
avascular The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, organ system that permits blood to circulate and transport nutrien ...
), so they must receive nourishment via
diffusion File:DiffusionMicroMacro.gif, 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diff ...
of substances from the underlying connective tissue, through 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 ...
.Freshney, 2002
p. 3
/ref>
Cell junction Cell junctions (or intercellular bridges) are a class of cellular structures consisting of multiprotein complexes that provide contact or adhesion between neighboring Cell (biology), cells or between a cell and the extracellular matrix in animals. ...
s are especially abundant in epithelial tissues.


Classification

In general, epithelial tissues are classified by the number of their layers and by the
shape A shape or figure is the form of an object or its external boundary, outline, or external surface File:Water droplet lying on a damask.jpg, Water droplet lying on a damask. Surface tension is high enough to prevent floating below the textile. ...
and function of the cells. The three principal shapes associated with epithelial cells are squamous, cuboidal, and columnar. * Squamous epithelium has cells that are wider than their height (flat and scale-like). This is found as the lining of the mouth, oesophagus, and including blood vessels and in the alveoli of the lungs. * Cuboidal epithelium has cells whose height and width are approximately the same (cube shaped). * Columnar epithelium has cells taller than they are wide (column-shaped). Columnar epithelium can be further classified into ciliated columnar epithelium and glandular columnar epithelium. 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 cell Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...

stratified squamous epithelium
,
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 e ...

stratified cuboidal epithelium
, and
stratified columnar epithelium Stratified columnar epithelium is a rare type of epithelial tissue Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biologica ...
,van Lommel, 2002
p. 97
/ref> 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 A pseudostratified epithelium is a type of 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 Ani ...

pseudostratified columnar epithelium
.
Transitional epithelium Transitional epithelium also known as urothelium is a type of stratified epithelium. Transitional epithelium is a type of tissue that changes shape in response to stretching (stretchable epithelium). The transitional epithelium usually appear cu ...

Transitional epithelium
has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium.


Simple epithelium

Simple epithelium is a single layer of cells with every cell in direct contact with 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 ...
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, simple epithelial tissues are classified by the shape of their cells. The four major classes of simple epithelium are (1) simple squamous, (2) simple cuboidal, (3) simple columnar, and (4) pseudostratified. :(1) : Squamous epithelial cells appear scale-like, flattened, or rounded (e.g., walls of capillaries, linings of the pericardial, pleural, and peritoneal cavities, linings of the alveoli of the lungs). :(2) : These cells may have secretory, absorptive, or excretory functions. Examples include small collecting ducts of the kidney, pancreas, and salivary gland. :(3) : Cells can be secretory, absorptive, or excretory. Simple columnar epithelium can be ciliated or non-ciliated; ciliated columnar is found in the female reproductive tract and uterus. Non-ciliated epithelium can also possess
microvilli Microvilli (singular: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption (chemistry), abs ...
. Some tissues contain goblet cells and are referred to as simple glandular columnar epithelium. These secrete mucus and are found in the stomach, colon, and rectum. :(4)
Pseudostratified columnar epithelium A pseudostratified epithelium is a type of 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 Ani ...

Pseudostratified columnar epithelium
: These can be ciliated or non-ciliated. The ciliated type is also called
respiratory epithelium The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are d ...
since it is almost exclusively confined to the larger respiratory airways of the nasal cavity, trachea, and bronchi.


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:


Cell types

The basic cell types are squamous, cuboidal, and columnar, classed by their shape.


Structure

Epithelial tissue is
scutoid A scutoid is a geometric solid between two parallel surfaces. The boundary of each of the surfaces (and of all the other parallel surfaces between them) is a polygon In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, ...
shaped, tightly packed and forms 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 Endothelium is a single layer of squamous epithelial cell, 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 (anat ...
.


Location

Epithelium lines both the outside (
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biolo ...

skin
) and the inside cavities and lumina of bodies. The outermost layer of
human skin The human skin is the outer covering of the body and is the largest organ 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, act ...

human skin
is composed of dead stratified squamous,
keratinized Keratin () is one of a family of fibrous structural proteins known as scleroproteins. '' α-Keratin'' is a type of keratin found in vertebrates. It is the key structural material making up scales Scale or scales may refer to: Mathematics * Sca ...
epithelial cells. Tissues that line the inside of the mouth, the
esophagus The esophagus (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 E ...

esophagus
, the
vagina 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, femal ...

vagina
, and part of the
rectum The rectum is the final straight portion of the large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the re ...

rectum
are composed of 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
lung The lungs are the primary organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma ...

lung
s, the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
, the reproductive and urinary tracts, and make up the
exocrine Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in ...
and
endocrine gland Endocrine glands are ductless glands of the endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating dista ...
s. The outer surface of the
cornea The cornea is the transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...

cornea
is covered with fast-growing, easily regenerated epithelial cells. A specialised form of epithelium,
endothelium Endothelium is a single layer of squamous epithelial cell, 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 (anat ...
, forms the inner lining of
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system that permits blood to circulate and transport nutrients (such as amino ...
s and the
heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste ...

heart
, and is known as vascular endothelium, and lining
lymphatic vessels 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 ...

lymphatic vessels
as lymphatic endothelium. Another type,
mesothelium The mesothelium is a membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other sma ...
, forms the walls of the
pericardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart The heart is a cardiac muscle, muscular Organ (biology), organ in most animals, which pumps blood through the blood vessels of the circulatory syst ...

pericardium
,
pleurae The pleural cavity, pleural space, or interpleural space, is the potential space Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things t ...
, and
peritoneum The peritoneum is the serous membrane In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργα ...

peritoneum
. In arthropods, the
integument In biology, integument is the natural covering of an organism or an organ, such as its skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, an ...
, or external "skin", consists of a single layer of epithelial ectoderm from which arises the
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non-homology (biology), homologous, differing in the ...
, an outer covering of
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13O5N)n ( ) is a long-chain polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical su ...

chitin
, the rigidity of which varies as per its chemical composition.


Basement membrane

The basal surface of epithelial tissue rests on 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 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 , but no
blood supply The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, ...

blood supply
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 The basal lamina is a layer of 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 ...

Basal lamina
is made up of laminin (glycoproteins) secreted by epithelial cells. The
Reticular lamina Reticular fibers, reticular fibres or reticulin is a type of fiber in 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 de ...
beneath the basal lamina is made up of collagen proteins secreted by
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consum ...
.


Cell junctions

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 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, ...
, or they build up the paracellular barrier of epithelia and control the
paracellular transport Paracellular transport refers to the transfer of substances across an epithelium by passing through the intercellular space ''between'' the cells. It is in contrast to transcellular transport, where the substances travel ''through'' the cell, passi ...
. Cell junctions are the contact points between plasma membrane and tissue cells. There are mainly 5 different types of cell junctions:
tight junction Tight junctions, also known as occluding junctions or zonulae occludentes (singular, zonula occludens) are multiprotein junctional complexes whose general function is to prevent leakage of transported solutes and water and seals the paracellular ...
s,
adherens junctionImage:Adherens Junctions structural proteins.svg, 350px, Principal interactions of structural proteins at cadherin-based adherens junction. Actin filaments are associated with adherens junctions in addition to several other actin-binding proteins suc ...
s,
desmosome A desmosome (; #Etymology, "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin language, Latin for ''adhering spot''), is a cell (biology), cell structure specialized for cell-to-cell adhesion. A type of Cell juncti ...

desmosome
s,
hemidesmosome Hemidesmosomes are very small stud-like structures found in keratinocyte Keratinocytes are the primary type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monast ...
s, and
gap junction Editing ''Gap junctions are specialized intercellular connections between a multitude of animal cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a smal ...
s. 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 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 Co ...

connexin
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 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 live ...

germ layers
: * from
ectoderm 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 ...

ectoderm
(e.g., the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
); * from
endoderm Endoderm is the innermost of the three primary germ layer A germ layer is a primary layer of 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 r ...
(e.g., the lining of the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
); * from
mesoderm The mesoderm is the middle layer of the three 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 (a ...

mesoderm
(e.g., the inner linings of
body cavities A body cavity is any space or compartment, or potential space in the 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 ...

body cavities
). However, it is important to note that
pathologists A list of people notable in the field of pathology. A * John Abercrombie (physician), John Abercrombie, Scottish physician, neuropathologist and philosopher. * Maude Abbott (1869–1940), Canadian pathologist, one of the earliest women graduat ...
do not consider
endothelium Endothelium is a single layer of squamous epithelial cell, 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 (anat ...
and
mesothelium The mesothelium is a membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other sma ...
(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 Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible Ca ...
s, whereas true epithelial cancers are called
carcinoma Carcinoma is a malignancy that develops from epithelial cells. Specifically, a carcinoma is a cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other parts of the bo ...
s. 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.


Functions

Epithelial tissues have as their primary functions: # to protect the tissues that lie beneath from
radiation upThe international symbol for types and levels of ionizing radiation (radioactivity) that are unsafe for unshielded humans. Radiation, in general, exists throughout nature, such as in light and sound. In physics Physics (from grc ...

radiation
, desiccation,
toxins A toxin is a harmful substance produced within living cells or organisms; synthetic toxicants created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived from the word toxi ...
, 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 injur ...
# 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 Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state w ...
# the secretion of hormones into the
circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a complex biological network, network which connects several biologically ...
, 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 hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules in multicellular organisms, ...

glands
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 Goblet cells are simple columnar epithelial cell 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. ...
, which secrete
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 ...
. There are two major classifications of glands:
endocrine gland Endocrine glands are ductless glands of the endocrine system The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating dista ...
s and
exocrine gland Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is any member of a class of signaling molecules i ...
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 Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all Mammal, land mammals (except for goats and rabbits). Their functions include lubricating the eyes (basal tears), removing irritants (reflex tears), and ...

tears
,
saliva Saliva (commonly referred to as spit) is an extracellular fluid Extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body w ...
,
oil An oil is any nonpolar chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can b ...

oil
(sebum),
enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecules known as . Almost all in the need in order to occu ...

enzyme
, digestive juices,
sweat Perspiration, also known as sweating, is the production of fluids secreted by the sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, fle ...

sweat
, etc.


Sensing the extracellular environment

"Some epithelial cells are
ciliated The cilium (; the plural is cilia) is an organelle found on Eukaryote, eukaryotic cells in the shape of a slender protuberance that projects from the much larger Cell (biology), cell body. There are two major types of cilia: ''motile'' and ''no ...
, especially in
respiratory epithelium The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network which connects several biologically relevant entities. Biological organization spans several scales and are d ...
, 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,
thermoceptionThermoception or thermoreception is the sensation and perception Perception (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally ...
, and
mechanosensationMechanosensation is the transduction of mechanical stimuli into neural signals. Mechanosensation provides the basis for the sense A sense is a biological system used by an organism for sensation, the process of gathering information about the wor ...
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 Motility is the ability of an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (bi ...

motile
."


Host immune response

Epithelial cells express many genes that encode and proteins involved in cell-cell communication 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
epigenomeAn epigenome consists of a record of the chemical changes to the DNA and histone proteins of an organism; these changes can be passed down to an organism's offspring via transgenerational stranded epigenetic inheritance. Changes to the epigenome c ...

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 bodiesInclusion bodies are aggregates of specific types of 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 ...

inclusion bodies
shown at (3); an uninfected cell shown at (2) and (4) showing the difference between an infected
cell nucleus In , the nucleus (pl. ''nuclei''; from or , meaning ''kernel'' or ''seed'') is a found in . Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian s, have , and a few others including s have . The main structure ...

cell nucleus
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 Cytokeratins are keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprotein, scleroproteins. ''Alpha-keratin, α-Keratin'' is a type of keratin found in vertebrates. It is the key structural material making up Sc ...
group are almost exclusively found in epithelial cells, so they are often used for this purpose. Cancers originating from the epithelium are classified as
carcinoma Carcinoma is a malignancy that develops from epithelial cells. Specifically, a carcinoma is a cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other parts of the bo ...
s. In contrast,
sarcoma A sarcoma is a malignant tumor Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible Ca ...
s develop in
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consum ...
. When epithelial cells or tissues are damaged from
cystic fibrosis Cystic fibrosis (CF) is a genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene In biology, a gene (from ''genos'' "...Wilhelm Johan ...
, 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 Lips are a visible body part at the mouth of many animals, including humans. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech. Human lips are a tactile sensory organ, and can be an ...

lip
. The word has both
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
and
count Count (feminine: countess) is a historical title of nobility Nobility is a social class normally ranked immediately below Royal family, royalty and found in some societies that have a formal aristocracy (class), aristocracy. Nobility ...
senses; the plural form is epithelia.


Additional images

File:Dogsquamos100x.jpg, Squamous epithelium 100x File:Cheekcells stained.jpg, Human cheek cells (Nonkeratinized stratified squamous epithelium) 500x 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 polarityCell polarity is a fundamental feature of many types of cells. Epithelial cells are one example of a polarized cell type, featuring distinct 'apical', 'lateral' and 'basal' plasma membrane cell membrane vs. Prokaryotes The cell membrane (also ...
*
Glycocalyx The glycocalyx, also known as the pericellular matrix, is a glycoprotein Glycoproteins are protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residue ...

Glycocalyx
* Inner and
Outer enamel epithelium The outer enamel epithelium, also known as the external enamel epithelium, is a layer of cuboidal cells located on the periphery of the enamel organ The enamel organ, also known as the dental organ, is a cellular aggregation seen in a developing t ...
* Iris pigment epithelium *
Neuroepithelial cellNeuroepithelial cells, or neuroectodermal cells, form the wall of the closed neural tube in early embryonic development ''Embryonic'' is the twelfth studio album by experimental rock Experimental rock, also called avant-rock, is a subgenre of rock ...
*
Retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Gene ...
*
Skin cancer Skin cancers are cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible Cancer s ...

Skin cancer
*
Sulcular epithelium The enamel ''(A)'' of the crown (tooth)">crown '' File:서봉총 금관 금제드리개.jpg, The Seobongchong Golden Crown of Ancient Silla, which is 339th National Treasure of South Korea. It is basically following the standard type of Sill ...


References


Bibliography

* *


Further reading

* * * *


External links


Epithelium Photomicrographs
* Simple squamous epithelium of the
glomerulus ''Glomerulus'' () is a common term used in anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργαν ...
(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)