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Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport and detection of sensation. In Greek ἐπί (epi) means "on" or "upon", and θηλή (thēlē) means "nipple".^[1]

Epithelial layers are avascular, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.^[2]^[3] Epithelia can also be organised into clusters of cells that function as exocrine and endocrine glands.

Structure

Cells in epithelium are very densely packed together like bricks in a wall, leaving very little intercellular space. The cells can form continuous sheets attached to each other at many locations by adherens junctions, tight junctions, and desmosomes.^[4]

Basement membrane

All epithelial hey hay cells rest on a basement membrane, which acts as a scaffolding on which epithelium can grow and regenerate after injuries.^[5] Epithelial tissue is innervated, but avascular. This epithelial tissue must be nourished by substances diffusing from the blood vessels in the underlying tissue, but they don't have their own blood supply. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium.^[2]^[3]

Cell junctions

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 build up the paracellular barrier of epithelia and control the paracellular transport.^{[citation needed]}

Cell junctions are the contact points between plasma membrane and tissue cells. There are mainly 5 different types of cell junctions. They are 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 transmembraner 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 connexins (six of which come together to make a connexon).

Classification

In general, tissues are classified by the morphology of their cells, and the number of layers they are composed of.^[2]^[4]^[6] Epithelial tissue that is only one cell thick is known as simple epithelium.^[7] If it is two or more cells thick, it is known as stratified epithelium.^[8] However, when taller simple epithelial cells (see columnar, below) are viewed in cross section with several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as "pseudostratified" epithelium.^[9]

There are three principal morphologies associated with epithelial cells. Squamous epithelium has cells that are wider than they are tall (flat and scale-like). 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). In addition, the morphology of the cells in transitional epithelium may vary from squamous to cuboidal, depending on the amount of tension on the epithelium.^[10]

Simple epithelium

Simple epithelium is one cell thick; that is, every cell is in direct contact with the underlying basement membrane. In general, it is found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes.^[4]

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; (4) pseudostratified.^[4]

Simple squamous epithelium is found lining areas where passive diffusion of gases occur, including the walls of capillaries, the linings of the alveoli of the lungs, and the linings of the pericardial, pleural, and peritoneal cavities.

Functions

The primary functions of epithelial tissues are: (1) to protect the tissues that lie beneath it from radiation, desiccation, toxins, invasion by pathogens, and physical trauma; (2) the regulation and exchange of chemicals between the underlying tissues and a body cavity; (3) the secretion of hormones into the blood vascular system, and/or the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts glandular epithelium;^[11] (4) to provide sensation.

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 classifications of glands: endocrine glands and exocrine glands. Endocrine glands secrete their product into the extracellular space where it is rapidly taken up by the blood vascular system. The 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.

In arthropods, the integument, or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle,^[12] an outer covering of chitin the rigidity of which varies as per its chemical composition.

Sensing the extracellular environment

"Some epithelial cells are ciliated, 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, thermosensation, 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."^[13]

Embryological development

In general, there are epithelial tissues deriving from all of the embryological germ layers:^{[citation needed]}

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 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 sarcomas, whereas true epithelial cancers are called carcinomas. 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.^{[citation needed]}

Growing in culture

When growing epithelium in culture, one can determine whether or not a particular cell is epithelial by examining its morphological characteristics. Epithelial cells tend to cluster together, and have a "characteristic tight pavementlike 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, and so are often used for this purpose.^[14]

Location

Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.^{[citation needed]}

Tissues 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 lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the cornea is covered with fast-growing, easily regenerated epithelial cells. Endothelium (the inner lining of blood vessels, the heart, and lymphatic vessels) is a specialized form of epithelium. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.^{[citation needed]}

This table lists the epithelia of different organs of the human body

System	Tissue	Epithelium	Subtype
circulatory	blood vessels	Simple squamous	endothelium
digestive	ducts of submandibular glands	simple columnar	-
digestive	attached gingiva	Stratified squamous, keratinized	-
digestive	dorsum of tongue	Stratified squamous, keratinized	-
digestive	hard palate	Stratified squamous, keratinized	-
digestive	oesophagus	Stratified squamous, non-keratinized	-
digestive	stomach	Simple columnar, non-ciliated	gastric epithelium
digestive	small intestine	Simple columnar, non-ciliated	intestinal epithelium
digestive	large intestine	Simple columnar, non-ciliated	intestinal epithelium
digestive	rectum	Simple columnar, non-ciliated	-
digestive	anus	Stratified squamous, non-keratinized superior to Hilton's white line Stratified squamous, keratinized inferior to Hilton's white line	-
digestive	gallbladder	Simple columnar, non-ciliated	-
endocrine	thyroid follicles	Simple cuboidal	-
nervous	ependyma	Simple cuboidal	-
lymphatic	lymph vessel	Simple squamous	endothelium
integumentary	skin - dead superficial layer	Stratified squamous, keratinized	-
integumentary	sweat gland ducts	Stratified cuboidal	-
integumentary	mesothelium of body cavities	Simple squamous	mesothelium
reproductive - female	ovaries	Simple cuboidal	germinal epithelium (female)
reproductive - female	fallopian tubes	Simple columnar, ciliated	-
reproductive - female	endometrium (uterus)	Simple columnar, ciliated	-
reproductive - female	cervix (endocervix)	Simple columnar	-
reproductive - female	cervix (ectocervix)	Stratified squamous, non-keratinized	-
reproductive - female	vaginal epithelium	Stratified squamous, non-keratinized	-
reproductive - female	labia majora	Stratified squamous, keratinized	-
reproductive - male	tubuli recti	Simple cuboidal	germinal epithelium (male)
reproductive - male	rete testis	Simple cuboidal	-
reproductive - male	efferent ducts	Pseudostratified columnar	-
reproductive - male	epididymis	Pseudostratified columnar, with stereocilia	-
reproductive - male	vas deferens	Pseudostratified columnar	-
reproductive - male	ejaculatory duct	Simple columnar	-
reproductive - male (gland)	bulbourethral glands	Simple columnar	-
reproductive - male (gland)	seminal vesicle	Pseudostratified columnar	-
respiratory	oropharynx	Stratified squamous, non-keratinized	-
respiratory	larynx	Pseudostratified columnar, ciliated	respiratory epithelium
respiratory	larynx - true vocal cords	Stratified squamous, non-keratinized	-
respiratory	trachea	Pseudostratified columnar, ciliated	respiratory epithelium
respiratory	bronchi	Pseudostratified columnar, ciliated
respiratory	terminal bronchioles	Simple cuboidal, ciliated
respiratory	respiratory bronchioles	Simple cuboidal, ciliated	-
respiratory	alveoli	Simple squamous
sensory	cornea	Stratified squamous, non-keratinized	corneal epithelium
sensory	nose	Pseudostratified columnar	olfactory epithelium
urinary	kidney - proximal convoluted tubule	Simple cuboidal, with microvilli	-
urinary	kidney - ascending thin limb	Simple squamous	-
urinary	kidney - distal convoluted tubule	Simple cuboidal, without microvilli	-
urinary	kidney - collecting duct	Simple cuboidal	-
urinary	kidney - Bowman's capsule	Simple squamous	-
urinary	kidney - Loop of Henle	Simple squamous	-
urinary	kidney - descending thin limb	Simple squamous	-
urinary	kidney - descending thick limb	simple cuboidal	-
urinary	renal pelvis	Transitional	urothelium
urinary	ureter	Transitional	urothelium
urinary	urinary bladder	Transitional	urothelium
urinary	prostatic urethra	Transitional	urothelium
urinary	membranous urethra	Pseudostratified columnar, non-ciliated	-
urinary	penile urethra	Pseudostratified columnar, non-ciliated	-
urinary	urinary meatus	Stratified squamous	-

Additional images

Squamous epithelium is one of several types of epithelia.
Squamous Epithelium 100x
Human cheek cells (Nonkeratinized stratified squamous epithelium) 500x

References

Notes

^ Epithelium at Wiktionary
^ ^a ^b ^c Dellmann's textbook of veterinary histology. Wiley-Blackwell. 2006. p. 18. ISBN 978-0-7817-4148-4. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)
^ ^a ^b Freshney, 2002: p. 3
^ ^a ^b ^c ^d Marieb, Elaine M. (1995). Human Anatomy and Physiology (3rd ed.). Benjamin/Cummings. pp. 103–104. ISBN 0-8053-4281-8.
^ McConnell, Thomas H. (2006). The nature of disease: pathology for the health professions. Lippincott Williams & Wilkins. p. 55. ISBN 978-0-7817-5317-3.
^ Platzer, Werner (2008). Color atlas of human anatomy: Locomotor system. Thieme. p. 8. ISBN 978-3-13-533306-9.
^ van Lommel, 2002: p. 94
^ van Lommel, 2002: p. 97
^ Permar's oral embryology and microscopic anatomy: a textbook for students in dental hygiene. Lippincott Williams & Wilkins. 2000. p. 9. ISBN 978-0-683-30644-6. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)
^ Pratt, Rebecca. "Epithelial Cells". AnatomyOne. Amirsys, Inc. Retrieved 2012-09-28.
^ van Lommel, 2002: p. 91
^ Kristensen, Niels P.; Georges, Chauvin (1 December 2003). "Integument". Lepidoptera, Moths and Butterflies: Morphology, Physiology, and Development : Teilband. Walter de Gruyter. p. 484. ISBN 978-3-11-016210-3. Retrieved 10 January 2013.
^ Adams, M.; Smith, U.M.; Logan, C.V.; Johnson, C.A. (2008). "Recent advances in the molecular pathology, cell biology and genetics of ciliopathies". Journal of Medical Genetics. 45 (5): 257–267. doi:10.1136/jmg.2007.054999. PMID 18178628.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ Freshney, 2002: p. 9

Bibliography

Freshney, R.I. (2002). "Introduction". Culture of epithelial cells. John Wiley & Sons. ISBN 978-0-471-40121-6. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)
van Lommel, Alfons T.L. (2002). From cells to organs: a histology textbook and atlas. Springer. ISBN 978-1-4020-7257-4.

@@ Line 9: / Line 9: @@
 ===Basement membrane===
-All epithelial cells rest on a [[basement membrane]], which acts as a scaffolding on which epithelium can grow and regenerate after injuries.<ref>{{Cite book|author=McConnell, Thomas H.|title=The nature of disease: pathology for the health professions|publisher=Lippincott Williams & Wilkins|year=2006|isbn=978-0-7817-5317-3|page=55|url=http://books.google.com/books?id=chs_lilPFLwC&pg=PA55}}</ref> Epithelial tissue is [[innervated]], but [[avascular]]. This epithelial tissue must be nourished by substances diffusing from the blood vessels in the underlying tissue, but they don't have their own blood supply. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium.<ref name="Eurell-2006-p18"/><ref name="p. 3"/>
+All epithelial hey hay  cells rest on a [[basement membrane]], which acts as a scaffolding on which epithelium can grow and regenerate after injuries.<ref>{{Cite book|author=McConnell, Thomas H.|title=The nature of disease: pathology for the health professions|publisher=Lippincott Williams & Wilkins|year=2006|isbn=978-0-7817-5317-3|page=55|url=http://books.google.com/books?id=chs_lilPFLwC&pg=PA55}}</ref> Epithelial tissue is [[innervated]], but [[avascular]]. This epithelial tissue must be nourished by substances diffusing from the blood vessels in the underlying tissue, but they don't have their own blood supply. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium.<ref name="Eurell-2006-p18"/><ref name="p. 3"/>
 ===Cell junctions===

v t e Biological tissues
Animals	Connective Epithelial Muscular Nervous
Plants	Dermal tissue: Epidermis Bulliform cell Cuticle Guard cell Pavement cell Subsidiary cell Periderm Phellem Phelloderm Vascular tissue: Phloem Companion cell Phloem fiber Phloem parenchyma Sieve tube Xylem Tracheid Vessel element Xylem fiber Xylem parenchyma Ground tissue: Parenchyma Aerenchyma Chlorenchyma Mesophyll Pith Collenchyma Sclerenchyma Fiber Sclereid Meristematic tissue: Primary Ground meristem Procambium Protoderm Secondary Cork cambium Vascular cambium Mixed: Cortex Endodermis Exodermis Stele
Category Histology

v t e Proteins of epithelium
Lateral/cell–cell	Cell adhesion molecules: Adherens junction Cadherin Desmosome Desmoglein Ion channels: Gap junction/Connexon Connexin Cytoskeleton: Desmosome Desmoplakin Plakoglobin Tonofibril other membrane proteins: Tight junction Claudin Occludin MARVELD2
Basal/cell–matrix	Basal lamina Hemidesmosome/Tonofibril Focal adhesion Costamere
Apical	Cilia/Kinocilium Microvilli/Stereocilia (STRC)