Epithelium

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Epithelium

In biology and medicine, epithelium is a tissue

Biological tissue

Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function....
composed of cell

Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
s that line the cavities and surfaces of structures throughout the body. Many gland

Gland

A gland is an Organ in an animal's body that synthesizes a substance for release such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface ....
s are also formed from epithelial tissue. It lies on top of connective tissue

Connective tissue

Connective tissue is a form of fibrous biological tissue.It is one of the four types of tissue in traditional classifications .Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% of the total protein content....
, and the two layers are separated by a basement membrane

Basement membrane

The basement membrane is a sheet of cells and fibers that covers two other kinds of cells -- the epithelium, which lines the cavities and surfaces of organs, and the endothelium, which lines the interior surface of blood vessels....
.

In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue

Connective tissue

Nervous tissue

Nervous tissue is one of four major classes of vertebrate Biological tissue. The function of the nervous tissue is in communication between parts of the body....
.

Epithelium is often defined by the expression of the adhesion molecule e-cadherin, as opposed to n-cadherin, which is used by cells of the connective tissue

Connective tissue

Skin

The skin is the outer covering of the body, also known as the epidermis. It is the largest organ of the integumentary system made up of multiple layers of epithelial biological tissue, and guards the underlying muscles, bones, ligaments and organ s....
) and the inside cavities and lumen

Lumen (anatomy)

A lumen in biology is the inside space of a tubular structure, such as an artery or intestine. By extension, a lumen can also be the inside space of a cellular component or structure, such as the endoplasmic reticulum....
of bodies.

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Encyclopedia

In biology and medicine, epithelium is a tissue

Biological tissue

Cell (biology)

Gland

Connective tissue

Basement membrane

Connective tissue

Nervous tissue

Connective tissue

Location

Epithelium lines both the outside (skin

Skin

Lumen (anatomy)

Skin

Tissue

Tissue may refer to:-*/-*/-NAJEKA-*/*/BFFE-*/-*/* Aerial tissue, an acrobatics art form and one of the circus arts* Tissue , a group of biological cells that perform a similar function...
that line the inside of the mouth, the esophagus, 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 lung

Lung

The lung is the essential respiration organ in air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located in the chest on either side of the heart....
s, the gastrointestinal tract

Gastrointestinal tract

The digestive tract is the system of Organ s within multicellular animals that takes in food, digestion it to extract energy and nutrients, and expels the remaining waste....
, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the cornea

Cornea

The cornea is the transparency front part of the eye that covers the Iris , pupil, and anterior chamber. Together with the cilliary muscles, the cornea reflects light, and as a result helps the eye to dilate, accounting for approximately two-thirds of the eye's total optical power....
is covered with fast-growing, easily-regenerated epithelial cells.

Endothelium

Endothelium

The endothelium is the thin layer of cell that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall....
(the inner lining of blood vessel

Blood vessel

The blood vessels are the part of the circulatory system that transport blood throughout the body. There are three major types of blood vessels: the artery, which carry the blood away from the heart, the capillary, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from...
s, the heart

Heart

The heart is a muscle organ in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods....
, and lymphatic vessels) is a specialized form of epithelium. Another type, mesothelium

Mesothelium

Pericardium

The pericardium is a double-walled sac that contains the heart and the roots of the great vessels....
, pleurae, and peritoneum

Peritoneum

In higher vertebrates, the peritoneum is the serous membrane that forms the lining of the abdomen — it covers most of the intra-abdominal organs....
.

Structure

Epithelial tissue covers the whole surface of the body. It is made up of cells closely packed and ranged in one or more layers. This tissue is specialised to form the covering or lining of all internal and external body surfaces. Epithelial tissue that occurs on surfaces on the interior of the body is known as endothelium. Epithelial cells are packed tightly together, with almost no intercellular spaces and only a small amount of intercellular substance. Epithelial tissue, regardless of the type, is usually separated from the underlying tissue by a thin sheet of connective tissue; basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.

Types of Epithelial Tissue Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium.

Simple epithelium can be subdivided according to the shape and function of its cells.

Simple squamous epithelium

Squamous (pavement) epithelium. Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells, for example, tend to have horizontally flattened, elliptical nuclei because of the thin flattened form of the cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.

Simple cuboidal epithelium

As their name implies, cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes.

Simple columnar epithelium

Columnar epithelial cells occur in one or more layers. The cells are elongated and column-shaped. The nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells are specialised for sensory reception such as in the nose, ears and the taste buds of the tongue. Goblet cells (unicellular glands) are found between the columnar epithelial cells of the duodenum. They secrete mucus or slime, a lubricating substance which keeps the surface smooth.

Ciliated columnar epithelium

These are simple columnar epithelial cells, but in addition, they possess fine hair-like outgrowths, cilia on their free surfaces. These cilia are capable of rapid, rhythmic, wavelike beatings in a certain direction. This movement of the cilia in a certain direction causes the mucus, which is secreted by the goblet cells, to move (flow or stream) in that direction. Ciliated epithelium is usually found in the air passages like the nose. It is also found in the uterus and Fallopian tubes of females. The movement of the cilia propel the ovum to the uterus.

Glandular Epithelium

Columnar epithelium with goblet cells is called glandular epithelium. Some parts of the glandular epithelium consist of such a large number of goblet cells that there are only a few normal epithelial cells left. Columnar and cuboidal epithelial cells often become specialised as gland cells which are capable of synthesising and secreting certain substances such as enzymes, hormones, milk, mucus, sweat, wax and saliva. Unicellular glands consist of single, isolated glandular cells such as the goblet cells. Sometimes a portion of the epithelial tissue becomes invaginated and a multicellular gland is formed. Multicellular glands are composed of clusters of cells. Most glands are multicellular including the the salivary glands.

Stratified Epithelium

Where body linings have to withstand wear and tear, the epithelia are composed of several layers of cells and are then called compound or stratified epithelium. The top cells are flat and scaly and it may or may not be keratinised (i.e. containing a tough, resistant protein called keratin). The mammalian skin is an example of dry, keratinised, stratified epithelium. The lining of the mouth cavity is an example of an unkeratinisied, stratified epithelium.

Functions of Epithelial Tissue

Protection Epithelial cells from the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria and from excessive loss of water.

Sensation Sensory stimuli penetrate specialised epithelial cells. Specialised epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears, nose and on the tongue.

Secretion In glands, epithelial tissue is specialised to secrete specific chemical substances such as enzymes, hormones and lubricating fluids.

Absorption Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food.

Excretion Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands.

Diffusion Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (eg. walls of capillaries and lungs).

Cleaning Ciliated epithelium assists in removing dust particles and foreign bodies which have entered the air passages.

Reduces Friction The smooth, tightly-interlocking, epithelial cells that line the entire circulatory system reduce friction between the blood and the walls of the blood vessels.

Function

Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability.

Classification

Epithelial cells are classified by the following three factors:

Shape (of most superficial cells)

Type	Description
Squamous	Squamous epithelium covers many surfaces in the human body, including the cheeks, inner surfaces of blood vessels, filtration tubules and the major cavities found within the body. It also form the walls of the alveoli in the lungs. The individual cells are smooth, flat and very thin, as well as typically soft. They fit closely together in tissues; providing a smooth, low-friction surface over which fluids can move easily. In the alveoli, the thinness of the cells allows for rapid diffusion of gases between the alveoli and the blood. They are not regarded as being relatively active in terms of metabolism, but are highly key in the diffusion of water and specific substances through certain tissues.
Cuboidal	As the name suggests, these cells have a shape similar to a cube, meaning their width is the same size as their length and height. The Cell nucleus nuclei of these cells are usually located in the center. The cuboidal epithelium forms are the smallest.
Columnar Columnar In biology, columnar refers to the shape of epithelial cells that are taller than they are wide. Form follows function in biology, and columnar morphorphology hints at the functions of the cell....	These cells are taller than they are wide like a cylindrical structure. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. The nucleus is also closer to the base of the cell. The small intestine Small intestine In vertebrates, including mammals, reptiles, birds, and bony fish, the small intestine is the part of the gastrointestinal tract following the stomach, and is where the vast majority of digestion takes place.... is a tubular organ Organ (anatomy) In biology, an organ is a biological tissue that performs a specific function or group of functions. Usually there is a main tissue and sporadic tissues.... lined with this type of tissue. Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete mucus Mucus In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. It is a viscous colloid containing antiseptic enzymes and immunoglobulins that serves to protect Epithelium in the respiratory,... . The free surface of the columnar cell has tiny projections called microvilli.

Stratification

Type	Description
Simple	There is a single layer of cells.
Stratified	More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina Basal lamina The basal lamina is a layer of extracellular matrix on which epithelium sits and which is secreted by the epithelial cells. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below.... . Stratified cells can usually withstand large amounts of stress.
Pseudostratified	This term is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, with each cell touching the basal lamina Basal lamina The basal lamina is a layer of extracellular matrix on which epithelium sits and which is secreted by the epithelial cells. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below.... directly. However, without careful observation, the location of the nuclei within the cells may give the impression that the epithelium is stratified. For example, the epithelium lining the bronchi in mammals is pseudostratified epithelium composed of at least three cells types (basal cells with nuclei located close to the basal lamina; ciliated cells with nuclei halfway between the basal lamina and the cellular layer; and mucous cells so full of mucous granules that the nuclei are often difficult to clearly discern but are almost always near the basal lamina).

Specializations

Type	Description
Keratin Keratin Keratins are a family of fibrous protein; tough and insoluble, they form the hard but mineral structures found in reptiles, birds, amphibians and mammals.... ized cells	Contain keratin Keratin Keratins are a family of fibrous protein; tough and insoluble, they form the hard but mineral structures found in reptiles, birds, amphibians and mammals.... (a cytoskeletal Cytoskeleton The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm. The cytoskeleton is present in all cells; it was once thought this structure was unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton.... protein). While keratinized epithelium occurs mainly in the skin, it is also found in the mouth Mouth The mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva.... and nose Nose Anatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for Respiration in conjunction with the mouth.... , providing a tough, impermeable barrier.
Ciliated cells	Have apical plasma membrane extensions composed of microtubule Microtubule Microtubules are one of the components of the cytoskeleton. They have a diameter of 25 Nanometre and length varying from 200 nanometers to 25 micrometers.... s capable of beating rhythmically to move mucus Mucus In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. It is a viscous colloid containing antiseptic enzymes and immunoglobulins that serves to protect Epithelium in the respiratory,... or other substances through a duct. Cilia are common in the respiratory system Respiratory system A respiratory system?s function is to allow gas exchange. The space between the alveoli and the capillaries, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on the organism.... and the lining of the oviduct Oviduct In oviparous animals , the passage from the ovary to the outside of the body is known as the oviduct. The eggs travel along the oviduct. These eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body.... . Additionally, the structure of the cilia found on ciliated epithelial cells is identical to that covering the single-cell organisms collectively known as ciliated prozoans Ciliate The ciliates are a group of protists characterized by the presence of hair-like organelles called cilium, which are identical in structure to flagellum but typically shorter and present in much larger numbers with a different undulating pattern than flagella.... . this cell secretes out of pores and hairs . The have ordor fission. Absorption is also a main key in this.

Examples

System	Tissue	Epithelium	Subtype >- \| circulatory	blood vessels	Simple squamous	endothelium Endothelium The endothelium is the thin layer of cell that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall.... \|- \| digestive	ducts Duct (anatomy) In anatomy and physiology, a duct is a circumscribed channel leading from an exocrine gland or Organ .Types of ductsExamples include:... of submandibular gland Submandibular gland The paired submandibular glands are salivary glands located beneath the floor of the mouth. In humans, they account for 70% of the salivary volume and weigh about 15 grams.... s	Stratified columnar	- \| digestive	attached gingiva Gingiva The gingiva , or gums, consists of the mucosal tissue that lies over the alveolar bone....	Stratified squamous, keratinized	- \| digestive	dorsum of tongue Tongue The tongue is skeletal muscle on the floor of the mouth that manipulates food for chewing . It is the primary organ of taste. Much of the upper surface of the tongue is covered in papillae and taste buds....	Stratified squamous, keratinized	- \| digestive	hard palate Hard palate The hard palate is a thin horizontal bone plate of the skull, located in the roof of the mouth. It spans the arch formed by the upper teeth.It is formed by the palatine process of the maxilla and horizontal plate of palatine bone....	Stratified squamous, keratinized	- o \| digestive	oesophagus	Stratified squamous, non-keratinised	- \| digestive	stomach Stomach In most mammals, the stomach is a hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication....	Simple columnar, non-ciliated	- \| digestive	small intestine Small intestine In vertebrates, including mammals, reptiles, birds, and bony fish, the small intestine is the part of the gastrointestinal tract following the stomach, and is where the vast majority of digestion takes place....	Simple columnar, non-ciliated	intestinal epithelium Intestinal epithelium The intestinal epithelium is the epithelium that covers the small intestine and large intestine. It is simple columnar and Cilia.They primarily take part in the digestive system.... \|- \| digestive	large intestine Large intestine The large intestine is the last part of the digestive system?the final stage of the alimentary canal?in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass this useless feces from the body....	Simple columnar, non-ciliated	intestinal epithelium Intestinal epithelium The intestinal epithelium is the epithelium that covers the small intestine and large intestine. It is simple columnar and Cilia.They primarily take part in the digestive system.... \|- \| digestive	rectum Rectum The rectum is the final straight portion of the large intestine in some mammals, and the Gastrointestinal tract in others, terminating in the anus....	Simple columnar, non-ciliated	- \| digestive	anus Anus The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as coprolite ; food material after all the nutrients have b...	Stratified squamous, non-keratinized superior to Hilton's white line Hilton's white line Hilton's white line is a boundary in the anal canal.Below it, lymphatic drainage is to the superficial inguinal nodes.A landmark for the intermuscular border between internal and external anal sphincter muscles.... Stratified squamous, keratinized inferior to Hilton's white line Hilton's white line Hilton's white line is a boundary in the anal canal.Below it, lymphatic drainage is to the superficial inguinal nodes.A landmark for the intermuscular border between internal and external anal sphincter muscles....	- \| digestive	gallbladder Gallbladder The gallbladder is a small non-vital Organ which aids in the digestive process and concentrates bile produced in the liver....	Simple columnar, non-ciliated	- \| endocrine	thyroid Thyroid The thyroid is one of the largest endocrine glands in the body. This gland is found in the neck inferior to the thyroid cartilage and at approximately the same level as the cricoid cartilage.... follicles	Simple cuboidal	- \| nervous	ependyma Ependyma Ependyma is the thin Epithelium lining the ventricular system of the brain and the spinal cord. Ependyma is one of the four types of neuroglia in the central nervous system....	Simple cuboidal	- \| lymphatic	lymph vessel Lymph vessel In anatomy, lymph vessels are thin walled, valved structures that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the vascular system....	Simple squamous	endothelium Endothelium The endothelium is the thin layer of cell that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall.... \|- \| integumentary	skin Skin The skin is the outer covering of the body, also known as the epidermis. It is the largest organ of the integumentary system made up of multiple layers of epithelial biological tissue, and guards the underlying muscles, bones, ligaments and organ s.... - dead superficial layer	Stratified squamous, keratinized	- \| integumentary	sweat gland Sweat gland The skin contains two different groups of sweat glands: apocrine sweat glands and merocrine sweat glands. Both gland types contain myoepithelial cells , specialized epithelial cells located between the gland cells and the underlying basal lamina.... ducts	Stratified cuboidal	- \| integumentary	mesothelium Mesothelium The mesothelium is a membrane that forms the lining of several body cavities: the pleura , peritoneum and pericardium . Mesothelial tissue also surrounds the male internal reproductive organs and covers the internal reproductive organs of women .... of body cavities	Simple squamous	mesothelium Mesothelium The mesothelium is a membrane that forms the lining of several body cavities: the pleura , peritoneum and pericardium . Mesothelial tissue also surrounds the male internal reproductive organs and covers the internal reproductive organs of women .... \|- \| reproductive - female	ovaries	Simple cuboidal	germinal epithelium (female) Germinal epithelium (female) The surface of the ovary is covered by a layer of simple cuboidal cells which constitutes the germinal epithelium of Waldeyer.These cells are derived from the mesoderm during embryonic development and are closely related to the mesothelium of the peritoneum.... \|- \| reproductive - female	Fallopian tubes	Simple columnar, ciliated	- \| reproductive - female	uterus Uterus The uterus is a major female hormone-responsive reproductive sex organ of most mammals, including humans. It is within the uterus that the fetus develops during gestation....	Simple columnar, ciliated	- \| reproductive - female	endometrium Endometrium The endometrium is the inner membrane of the mammalian uterus....	Simple columnar	- \| reproductive - female	cervix Cervix The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall.... (endocervix)	Simple columnar	- \| reproductive - female	cervix Cervix The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall.... (ectocervix)	Stratified squamous, non-keratinised	- \| reproductive - female	vagina Vagina The vagina is a fibromuscular cylinder tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles....	Stratified squamous, non-keratinised	- \| reproductive - female	labia majora Labia majora The labia majora are two prominent longitudinal cutaneous folds which extend downward and backward from the mons pubis to the perineum and form the lateral boundaries of the cleft of venus, which contains the labia minora, interlabial sulci, clitoral hood, clitoral glans, frenulum clitoridis, the Hart's Line, and the vulval vestibule, which...	Stratified squamous, keratinised	- \| reproductive - male	tubuli recti	Simple cuboidal	germinal epithelium (male) Germinal epithelium (male) The germinal epithelium is the outermost layer of the testicle.... \|- \| reproductive - male	rete testis Rete testis Rete testis is an anastomosis network of delicate tubules located in the hilum of the testicle that carries spermatozoon from the seminiferous tubules to the vasa efferentia....	Simple cuboidal	- \| reproductive - male	ductuli efferentes	Pseudostratified columnar	- \| reproductive - male	epididymis Epididymis The epididymis is part of the male reproductive system and is present in all male mammals. It is a narrow, tightly-coiled tube connecting the efferent ducts from the rear of each testicle to its vas deferens....	Pseudostratified columnar, with stereocilia Stereocilia Stereocilia are mechanosensing organelles of hair cells, which respond to fluid motion or fluid pressure changes in numerous types of animals for various functions, primarily hearing....	- \| reproductive - male	vas deferens Vas deferens The vas deferens , also called ductus deferens, is part of the male anatomy of some species; they transport sperm from the epididymis in anticipation of ejaculation....	Pseudostratified columnar	- \| reproductive - male	ejaculatory duct Ejaculatory duct The Ejaculatory ducts are paired structures in male anatomy, about 2 cm in length.Each ejaculatory duct is formed by the union of the vas deferens with the Excretory duct of seminal gland....	Simple columnar	- \| reproductive - male (gland)	bulbourethral glands	Simple columnar	- \| reproductive - male (gland)	seminal vesicle Seminal vesicle The seminal vesicles are a pair of simple tubular glands posteroinferior to the urinary bladder of males....	Pseudostratified columnar	- \| respiratory	oropharynx Oropharynx The Oropharynx reaches from the Uvula to the level of the hyoid bone.It opens anteriorly, through the isthmus faucium, into the mouth, while in its lateral wall, between the two palatine arches, is the palatine tonsil....	Stratified squamous, non-keratinised	- \| respiratory	larynx Larynx The larynx , colloquially known as the voicebox, is an organ in the neck of mammals involved in protection of the vertebrate trachea and sound production....	Pseudostratified columnar, ciliated	respiratory epithelium Respiratory epithelium Respiratory epithelium is a type of epithelium found lining the respiratory tract, where it serves to moisten and protect the airways. It also functions as a barrier to potential pathogens and foreign particles, preventing infection and tissue injury by action of the #mucociliary escalator.... \|- \| respiratory	larynx Larynx The larynx , colloquially known as the voicebox, is an organ in the neck of mammals involved in protection of the vertebrate trachea and sound production.... - True vocal cords	Stratified squamous, non-keratinised	- \| respiratory	trachea Vertebrate trachea The traceartes, or windpipe, is a tube that has an inner diameter of about 20-25 mm and a length of about 10-16 cm in humans. It commences at the larynx and bifurcates into the primary bronchus in mammals, and from the pharynx to the syrinx in birds, allowing the passage of air to the lungs....	Pseudostratified columnar, ciliated	respiratory epithelium Respiratory epithelium Respiratory epithelium is a type of epithelium found lining the respiratory tract, where it serves to moisten and protect the airways. It also functions as a barrier to potential pathogens and foreign particles, preventing infection and tissue injury by action of the #mucociliary escalator.... \|- \| respiratory	respiratory bronchioles	Simple cuboidal	- \| sensory	cornea Cornea The cornea is the transparency front part of the eye that covers the Iris , pupil, and anterior chamber. Together with the cilliary muscles, the cornea reflects light, and as a result helps the eye to dilate, accounting for approximately two-thirds of the eye's total optical power....	Stratified squamous, non-keratinised	corneal epithelium Corneal epithelium The corneal epithelium is made up of epithelial tissue and covers the front of the cornea. It consists of several layers of cells.The cells of the deepest layer are columnar; then follow two or three layers of polyhedral cells, the majority of which are prickle cells similar to those found in the stratum mucosum of the cuticle.... \|- \| sensory	nose Nose Anatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for Respiration in conjunction with the mouth....	Pseudostratified columnar	olfactory epithelium Olfactory epithelium The olfactory epithelium is a specialized epithelium tissue inside the nasal cavity that is involved in olfaction. In humans, it measures about 2 cm by 5 cm long and lies on the roof of the nasal cavity about 3 inches above and behind the nostrils.... \|- \| urinary	kidney Kidney The kidneys are Organ that have numerous biological roles. Their primary role is to maintain the homeostasis balance of bodily fluids by filtering and secreting Metabolomics#Metabolitess and minerals from the blood and excreting them, along with water , as urine.... - proximal convoluted tubule	Simple cuboidal, ciliated	- \| urinary	kidney Kidney The kidneys are Organ that have numerous biological roles. Their primary role is to maintain the homeostasis balance of bodily fluids by filtering and secreting Metabolomics#Metabolitess and minerals from the blood and excreting them, along with water , as urine.... - ascending thin limb	Simple squamous	- \| urinary	kidney Kidney The kidneys are Organ that have numerous biological roles. Their primary role is to maintain the homeostasis balance of bodily fluids by filtering and secreting Metabolomics#Metabolitess and minerals from the blood and excreting them, along with water , as urine.... - distal convoluted tubule Distal convoluted tubule The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system....	Simple cuboidal, non-ciliated	- \| urinary	kidney Kidney The kidneys are Organ that have numerous biological roles. Their primary role is to maintain the homeostasis balance of bodily fluids by filtering and secreting Metabolomics#Metabolitess and minerals from the blood and excreting them, along with water , as urine.... - collecting duct	Simple cuboidal	- \| urinary	renal pelvis Renal pelvis The renal pelvis is the funnel-like dilated proximal part of the ureter in the kidney.It is the point of convergence of two or three major calyces....	Transitional	urothelium Urothelium The urothelium is the Biological tissue layer that lines much of the urinary tract, including the renal pelvis, the ureters, the Urinary bladder, and parts of the urethra.... \|- \| urinary	ureter Ureter In human anatomy, the ureters are muscular ducts that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long....	Transitional	urothelium Urothelium The urothelium is the Biological tissue layer that lines much of the urinary tract, including the renal pelvis, the ureters, the Urinary bladder, and parts of the urethra.... \|- \| urinary	urinary bladder Urinary bladder In anatomy, the urinary bladder is a solid, muscle, and distensible organ that sits on the pelvic floor in mammals. It is the organ that collects urine excreted by the kidneys prior to disposal by urination....	Transitional	urothelium Urothelium The urothelium is the Biological tissue layer that lines much of the urinary tract, including the renal pelvis, the ureters, the Urinary bladder, and parts of the urethra.... \|- \| urinary	prostatic urethra Prostatic urethra The prostatic urethra, the widest and most dilatable part of the urethra canal, is about 3 cm. long.It runs almost vertically through the prostate from its base to its apex, lying nearer its anterior than its posterior surface; the form of the canal is spindle-shaped, being wider in the middle than at either extremity, and narrowest below,...	Transitional	urothelium Urothelium The urothelium is the Biological tissue layer that lines much of the urinary tract, including the renal pelvis, the ureters, the Urinary bladder, and parts of the urethra.... \|- \| urinary	membranous urethra Membranous urethra The intermediate part of male urethra is the shortest, least dilatable, and, with the exception of the external orifice, the narrowest part of the canal....	Pseudostratified columnar, non-ciliated	- \| urinary	penile urethra	Pseudostratified columnar, non-ciliated	- \| urinary	external urethral orifice External urethral orifice External urethral orifice can refer to:* External urethral orifice * External urethral orifice ...	Stratified squamous	-

Cell junctions

A cell junction is a structure within a tissue of a multicellular organism. 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, or they built up the paracellular barrier of epithelia and control the paracellular transport

Paracellular transport

Paracellular transport refers to the transfer of substances between cells of an epithelium.It is in contrast to "transcellular transport", where the substances travel through the cell, passing through both the apical membrane and basolateral membrane....
.

Secretory epithelia

As stated above, secretion is one major function of epithelial cells. Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classification of glands: endocrine glands

Endocrine glands

Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into the blood rather than through a duct....
and exocrine glands. Endocrine glands are glands that secrete their product directly onto a surface rather than through a duct. This group contains the glands of the Endocrine system

Embryology

In general, there are epithelial tissues deriving from all of the embryological germ layers:

from ectoderm
Ectoderm

The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.Generally speaking, the ectoderm differentiates to form the nervous system, Epidermis , and the outer part of integumentary system....
(e.g., the epidermis
Epidermis (skin)

The epidermis is the outermost layer of the skin, composed of terminally differentiated stratified squamous epithelium, acting as the body's major barrier against an inhospitable environment....
);
from endoderm
Endoderm

Endoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron from the inner layer of the gastrula, which develops into the endoderm....
(e.g., the lining of the gastrointestinal tract
Gastrointestinal tract

The digestive tract is the system of Organ s within multicellular animals that takes in food, digestion it to extract energy and nutrients, and expels the remaining waste....
);
from mesoderm
Mesoderm

One of the three germ layers found in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms in the embryo during gastrulation when some of the cells migrating inward to form the endoderm, produce an additional layer that lies between the endoderm and the ectoderm....
(e.g., the inner linings of body cavities).

However, it is important to note that pathologists do not consider endothelium

Endothelium

Mesothelium

The mesothelium is a membrane that forms the lining of several body cavities: the pleura , peritoneum and pericardium . Mesothelial tissue also surrounds the male internal reproductive organs and covers the internal reproductive organs of women ....
(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

Sarcoma

A sarcoma is a cancer of the connective tissue resulting in mesoderm proliferation.This is in contrast to Carcinoma, which are of Epithelium origin ....
s, whereas true epithelial cancers are called carcinoma

Carcinoma

A carcinoma is any malignant cancer that arises from Epithelium. Carcinomas invade surrounding tissues and organs and may Metastasis, or spread, to lymph nodes and other sites....
s. Also, the filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is, in general, accepted that the epithelium arises from all three germ layers.

Epithelium

Location

Structure

Simple squamous epithelium

Simple cuboidal epithelium

Simple columnar epithelium

Ciliated columnar epithelium

Glandular Epithelium

Stratified Epithelium

Functions of Epithelial Tissue

Function

Classification

Shape (of most superficial cells)

Stratification

Specializations

Examples

Cell junctions

Secretory epithelia

Embryology

Additional images