Epithelium

Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial (mesothelial) tissues line the outer surfaces of many internal organs, the corresponding inner surfaces of body cavities, and the inner surfaces of blood vessels. Epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues lack blood or lymph supply, but are supplied by nerves.

There are three principal shapes of epithelial cell: squamous (scaly), columnar, and cuboidal.[1] These can be arranged in a singular layer of cells as simple epithelium, either simple squamous, simple columnar, or simple cuboidal, or in layers of two or more cells deep as stratified (layered), or compound, either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei. This sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include diffusion, filtration, secretion, selective absorption, germination, and transcellular transport. Compound epithelium has protective functions.

Epithelial layers contain no blood vessels (avascular), so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.[2][3]: 3 Cell junctions are especially abundant in epithelial tissues.

## Contents

* 1 Classification
* 1.1 Simple epithelium
* 1.2 Stratified epithelium
* 2 Structure
* 2.1 Location
* 2.2 Basement membrane
* 2.3 Cell junctions
* 2.4 Development
* 2.5 Cell turnover
* 3 Functions
* 3.1 Glandular tissue
* 3.2 Sensing the extracellular environment
* 3.3 Host immune response
* 4 Clinical significance
* 5 Etymology and pronunciation
* 6 Additional images
* 7 See also
* 8 References
* 9 Further reading
* 10 External links

## Classification

### Simple epithelium

Simple epithelium is a single layer of cells with every cell in direct contact with the basement membrane that separates it from the underlying connective tissue. In general, it is found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes.[4]

In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells.[2][4][5] The basic cell types are squamous, cuboidal, and columnar, classed by their shape.

Type  | Description

|

Squamous  | Squamous cells have the appearance of thin, flat plates that can look polygonal when viewed from above.[6] Their name comes from squāma, Latin for "scale" – as on fish or snake skin. The cells fit closely together in tissues, providing a smooth, low-friction surface over which fluids can move easily. The shape of the nucleus usually corresponds to the cell form and helps to identify the type of epithelium. Squamous cells tend to have horizontally flattened, nearly oval-shaped nuclei because of the thin, flattened form of the cell. Squamous epithelium is found lining surfaces such as skin or alveoli in the lung, enabling simple passive diffusion. Specialized squamous epithelium also forms the lining of cavities such as in blood vessels (as endothelium), in the pericardium (as mesothelium), and in other body cavities.
Cuboidal  | Cuboidal epithelial cells have a cube-like shape and appear square in cross-section. The cell nucleus is large, spherical and is in the center of the cell. Cuboidal epithelium is commonly found in secretive tissue such as the exocrine glands, or in absorptive tissue such as the pancreas, the lining of the kidney tubules as well as in the ducts of the glands. The germinal epithelium that covers the female ovary, and the germinal epithelium that lines the walls of the seminferous tubules in the testes are also of the cuboidal type. Cuboidal cells provide protection and may be active in pumping material in or out of the lumen, or passive depending on their location and specialisation. Simple cuboidal epithelium commonly differentiates to form the secretory and duct portions of glands.[7] Stratified cuboidal epithelium protects areas such as the ducts of sweat glands,[8] mammary glands, and salivary glands.
Columnar  | Columnar epithelial cells are elongated and column-shaped and have a height of at least four times their width. Their nuclei are elongated and are usually located near the base of the cells. Columnar epithelium forms the lining of the stomach and intestines. The cells here may possess microvilli for maximizing the surface area for absorption, and these microvilli may form a brush border. Other cells may be ciliated to move mucus in the function of mucociliary clearance. Other ciliated cells are found in the fallopian tubes, the uterus and central canal of the spinal cord. Some columnar cells are specialized for sensory reception such as in the nose, ears and the taste buds. Hair cells in the inner ears have stereocilia which are similar to microvilli. Goblet cells are modified columnar cells and are found between the columnar epithelial cells of the duodenum. They secrete mucus, which acts as a lubricant. Single-layered non-ciliated columnar epithelium tends to indicate an absorptive function. Stratified columnar epithelium is rare but is found in lobar ducts in the salivary glands, the eye, the pharynx, and sex organs. This consists of a layer of cells resting on at least one other layer of epithelial cells, which can be squamous, cuboidal, or columnar.
Pseudostratified  | These are simple columnar epithelial cells whose nuclei appear at different heights, giving the misleading (hence "pseudo") impression that the epithelium is stratified when the cells are viewed in cross section. Ciliated pseudostratified epithelial cells have cilia. Cilia are capable of energy-dependent pulsatile beating in a certain direction through interaction of cytoskeletal microtubules and connecting structural proteins and enzymes. In the respiratory tract, the wafting effect produced causes mucus secreted locally by the goblet cells (to lubricate and to trap pathogens and particles) to flow in that direction (typically out of the body). Ciliated epithelium is found in the airways (nose, bronchi), but is also found in the uterus and fallopian tubes, where the cilia propel the ovum to the uterus.
Summary showing different epithelial cells/tissues and their characteristics.

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, stratified cuboidal epithelium, and stratified columnar epithelium,[9]: 94, 97 and both types of layering can be made up of any of the cell shapes.[4] 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.[10]

Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium.[11]

### 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.[12]

Stratified epithelia (of columnar, cuboidal, or squamous type) can have the following specializations:[12]

Specialization  | Description

|

Keratinized | In this particular case, the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm, instead contain a tough, resistant protein called keratin. This specialization makes the epithelium somewhat water-resistant, so is found in the mammalian skin. The lining of the oesophagus is an example of a non-keratinized or "moist" stratified epithelium.[12]
Parakeratinized  | In this case, the most apical layers of cells are filled with keratin, but they still retain their nuclei. These nuclei are pyknotic, meaning that they are highly condensed. Parakeratinized epithelium is sometimes found in the oral mucosa and in the upper regions of the oesophagus.[13]
Transitional  | Transitional epithelia are found in tissues that stretch, and it can appear to be stratified cuboidal when the tissue is relaxed, or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called urothelium since it is almost exclusively found in the bladder, ureters and urethra.[12]

## Structure

Epithelial tissue cells can adopt shapes of varying complexity from polyhedral to scutoidal to punakoidal.[14] They are tightly packed and form a continuous sheet with 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.

### Location

Normal histology of the breast, with luminal epithelial cells annotated near bottom right.

See also: Medicine:Table of epithelia of human organs

Epithelium lines both the outside (skin) and the inside cavities and lumina of bodies. The outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells.[15]

In arthropods, the integument, or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle,[16] an outer covering of chitin, the rigidity of which varies as per its chemical composition.

### Basement membrane

The basal surface of epithelial tissue rests on a basement membrane and the free/apical surface faces body fluid or outside. The basement membrane acts as a scaffolding on which epithelium can grow and regenerate after injuries.[17] Epithelial tissue has a nerve supply, but no 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.[3]: 3

The basal lamina is made up of laminin (glycoproteins) secreted by epithelial cells. The reticular lamina beneath the basal lamina is made up of collagen proteins secreted by connective tissue.[citation needed]

### 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.[18]

Cell junctions are the contact points between plasma membrane and tissue cells. There are mainly 5 different types of cell junctions: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions.

* Tight junctions are a pair of trans-membrane protein fused on outer plasma membrane.
* Adherens junctions are a plaque (protein layer on the inside plasma membrane) which attaches both cells' microfilaments.
* Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein.
* Hemidesmosomes resemble desmosomes on a section. They are made up of the integrin (a transmembrane protein) instead of cadherin. They attach the epithelial cell to the basement membrane.

### Development

* 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).

### Cell turnover

Epithelia turn over at some of the fastest rates in the body. For epithelial layers to maintain constant cell numbers essential to their functions, the number of cells that divide must match those that die. They do this mechanically. If there are too few of the cells, the stretch that they experience rapidly activates cell division.[19] Alternatively, when too many cells accumulate, crowding triggers their death by activation epithelial cell extrusion.[20][21] Here, cells fated for elimination are seamlessly squeezed out by contracting a band of actin and myosin around and below the cell, preventing any gaps from forming that could disrupt their barriers. Failure to do so can result in aggressive tumors and their invasion by aberrant basal cell extrusion.[22][23]

## Functions

Forms of secretion in glandular tissue Different characteristics of glands of the body

Epithelial tissues have as their primary functions:

1. to protect the tissues that lie beneath 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 circulatory system, as well as the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts[9]: 91
4. to provide sensation[24]
5. Absorb water and digested food in the lining of digestive canal.

### Glandular tissue

Glandular tissue is the type of epithelium that forms the 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, which secrete mucus. Several glands contain myoepithelial cells that help in the contraction needed for secretion. 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 circulatory system.
* Exocrine glands secrete their products into a duct that then delivers the product to the lumen of an organ or onto the free surface of the epithelium. Their secretions include tears, saliva, oil (sebum), enzyme, digestive juices, sweat.

### Sensing the extracellular environment

"Some epithelial cells are ciliated, especially in respiratory epithelium, and they commonly exist as a sheet of polarised cells forming a tube or tubule with cilia projecting into the lumen." Primary cilia on epithelial cells provide chemosensation, thermoception, and mechanosensation of the extracellular environment by playing "a sensory role mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of the fluid flow, and mediation of fluid flow if the cilia are motile."[25]

### Host immune response

Epithelial cells express many genes that encode immune mediators and proteins involved in cell-cell communication with hematopoietic immune cells.[26] The resulting immune functions of these non-hematopoietic, structural cells contribute to the mammalian immune system ("structural immunity").[27][28] Relevant aspects of the epithelial cell response to infections are encoded in the epigenome of these cells, which enables a rapid response to immunological challenges.[citation needed]

## Clinical significance

Epithelial cell infected with Chlamydia pneumoniae

The slide shows at (1) an epithelial cell infected by Chlamydia pneumoniae; their inclusion bodies shown at (3); an uninfected cell shown at (2) and (4) showing the difference between an infected cell nucleus and an uninfected cell nucleus.

Epithelium grown in culture can be identified by examining its morphological characteristics. Epithelial cells tend to cluster together, and have a "characteristic tight pavement-like appearance". But this is not always the case, such as when the cells are derived from a tumor. In these cases, it is often necessary to use certain biochemical markers to make a positive identification. The intermediate filament proteins in the cytokeratin group are almost exclusively found in epithelial cells, so they are often used for this purpose.[3]: 9

Cancers originating from the epithelium are classified as carcinomas. In contrast, sarcomas develop in connective tissue.[29]

## 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.[30][31] The word has both mass and count senses; the plural form is epithelia.[citation needed]

## Additional images

* Squamous epithelium 100×

* Human cheek cells (nonkeratinized stratified squamous epithelium) 500×

* Histology of female urethra showing transitional epithelium

* Histology of sweat gland showing stratified cuboidal epithelium

## See also

* Dark cell
* Epithelial-mesenchymal transition
* Epithelial polarity
* Glycocalyx
* Inner and outer enamel epithelium
* Iris pigment epithelium
* Neuroepithelial cell
* Retinal pigment epithelium
* Skin cancer
* Sulcular epithelium
* List of distinct cell types in the adult human body

## References

1. ↑ Kurn, Heidi; Daly, Daniel T. (2025), "Histology, Epithelial Cell", StatPearls (Treasure Island (FL): StatPearls Publishing), PMID 32644489, https://www.ncbi.nlm.nih.gov/books/NBK559063/, retrieved 2025-03-17
2. ↑ 2.0 2.1 Dellmann's Textbook of Veterinary Histology. Wiley-Blackwell. 2006. p. 18. ISBN 978-0-7817-4148-4. https://books.google.com/books?id=FnS4uiOlRT0C.
3. ↑ 3.0 3.1 3.2 "Introduction". Culture of epithelial cells. John Wiley & Sons. 2002. ISBN 978-0-471-40121-6. https://books.google.com/books?id=KqKNxeWlU6MC&pg=PA1.
4. ↑ 4.0 4.1 4.2 Human Anatomy and Physiology (3rd ed.). Benjamin/Cummings. 1995. pp. 103–104. ISBN 0-8053-4281-8. https://archive.org/details/isbn_9780805342949/page/103.
5. ↑ Color atlas of human anatomy: Locomotor system. Thieme. 2008. p. 8. ISBN 978-3-13-533306-9. https://books.google.com/books?id=T9bb4T422j8C&pg=PA8.
6. ↑ Color atlas of cytology, histology, and microscopic anatomy. Thieme. 2003. p. 102. ISBN 978-3-13-562404-4. https://books.google.com/books?id=wUFAGmVN_aMC&pg=PA76.
7. ↑ "Simple Cuboidal Epithelium". AnatomyOne. Amirsys, Inc. http://www.anatomyone.com/a/simple-cuboidal-epithelium/.
8. ↑ "Integumentary System". DiFiore's Atlas of Histology with Functional Correlations. Lippincott Williams & Wilkins. 2008. pp. 212–234. ISBN 978-0-7817-7057-6. https://archive.org/details/difioresatlashis00eros.
9. ↑ 9.0 9.1 From cells to organs: a histology textbook and atlas. Springer. 2002. ISBN 978-1-4020-7257-4. https://books.google.com/books?id=EvYjLNKLu9sC.
10. ↑ 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. https://archive.org/details/permarsoralembry0010melf.
11. ↑ "Epithelial Cells". AnatomyOne. Amirsys, Inc.. http://www.anatomyone.com/a/epithelial-cells/.
12. ↑ 12.0 12.1 12.2 12.3 Anatomy and Physiology from Science to Life (3rd ed.). John Wiley & Sons. 2013. pp. 110–115. ISBN 978-1-118-12920-3.
13. ↑ Histology: A Text and Atlas: With Correlated Cell and Molecular Biology (7th ed.). Lippincott Williams & Wilkins. 2015. pp. 528, 604. ISBN 978-1-4511-8742-7.
14. ↑ Iber, Dagmar; Vetter, Roman (12 May 2022). "3D Organisation of Cells in Pseudostratified Epithelia". Frontiers in Physics 10. doi:10.3389/fphy.2022.898160. Bibcode: 2022FrP....10.8160I.
15. ↑ Anatomy & Physiology. Boston: Benjamin Cummings. 2011. pp. 133. ISBN 978-0-321-61640-1.
16. ↑ "Integument". Lepidoptera, Moths and Butterflies: Morphology, Physiology, and Development: Teilband. Walter de Gruyter. 1 December 2003. p. 484. ISBN 978-3-11-016210-3. https://books.google.com/books?id=5w8FgSGuH34C&pg=PA484. Retrieved 10 January 2013.
17. ↑ The nature of disease: pathology for the health professions. Lippincott Williams & Wilkins. 2006. p. 55. ISBN 978-0-7817-5317-3. https://books.google.com/books?id=chs_lilPFLwC&pg=PA55.
18. ↑ Molecular biology of the cell (4th ed.). New York [u.a.]: Garland. 2002. p. 1067. ISBN 0-8153-4072-9.
19. ↑ "Mechanical stretch triggers rapid epithelial cell division through Piezo1". Nature 543 (7643): 118–121. March 2017. doi:10.1038/nature21407. PMID 28199303. Bibcode: 2017Natur.543..118G.
20. ↑ "An epithelial cell destined for apoptosis signals its neighbors to extrude it by an actin- and myosin-dependent mechanism". Current Biology 11 (23): 1847–1857. November 2001. doi:10.1016/S0960-9822(01)00587-5. PMID 11728307. Bibcode: 2001CBio...11.1847R.
21. ↑ "Crowding induces live cell extrusion to maintain homeostatic cell numbers in epithelia". Nature 484 (7395): 546–549. April 2012. doi:10.1038/nature10999. PMID 22504183. Bibcode: 2012Natur.484..546E.
22. ↑ "KRas-transformed epithelia cells invade and partially dedifferentiate by basal cell extrusion". Nature Communications 12 (1). December 2021. doi:10.1038/s41467-021-27513-z. PMID 34893591. Bibcode: 2021NatCo..12.7180F.
23. ↑ "Defective apical extrusion signaling contributes to aggressive tumor hallmarks". eLife 4. January 2015. doi:10.7554/eLife.04069. PMID 25621765.
24. ↑ Molecular Biology of the Cell (4th ed.). New York [u.a.]: Garland. 2002. p. 1267. ISBN 0-8153-4072-9.
25. ↑ "Recent advances in the molecular pathology, cell biology and genetics of ciliopathies". Journal of Medical Genetics 45 (5): 257–267. May 2008. doi:10.1136/jmg.2007.054999. PMID 18178628.
26. ↑ "Deciphering cell-cell interactions and communication from gene expression". Nature Reviews. Genetics 22 (2): 71–88. February 2021. doi:10.1038/s41576-020-00292-x. PMID 33168968.
27. ↑ "Structural cells are key regulators of organ-specific immune responses". Nature 583 (7815): 296–302. July 2020. doi:10.1038/s41586-020-2424-4. PMID 32612232. Bibcode: 2020Natur.583..296K.
28. ↑ "A gene atlas of 'structural immunity'". Nature Reviews. Immunology 20 (9): 518–519. September 2020. doi:10.1038/s41577-020-0398-y. PMID 32661408.
29. ↑ "Types of cancer". 28 October 2014. http://www.cancerresearchuk.org/about-cancer/what-is-cancer/how-cancer-starts/types-of-cancer.
30. ↑ Foster, Michael (1874). "On the Term Endothelium". Quarterly Journal of Microscopical Science s2-14 (55): 219–223. doi:10.1242/jcs.s2-14.55.219.
31. ↑ Essential IVF: basic research and clinical applications. Boston: Kluwer Academic Publishers. 2004. p. 3. ISBN 978-1-4020-7551-3.

## Further reading

* "The birth of therapy with cultured cells". BioEssays 30 (9): 897–903. September 2008. doi:10.1002/bies.20797. PMID 18693268.
* Basement membranes: cell and molecular biology. Gulf Professional Publishing. 2005. ISBN 978-0-12-153356-4. https://books.google.com/books?id=RM-FVY47NEgC.
* "Epithelial topology". BioEssays 30 (3): 260–266. March 2008. doi:10.1002/bies.20722. PMID 18293365.
* "The regulation of skin pigmentation". The Journal of Biological Chemistry 282 (38): 27557–27561. September 2007. doi:10.1074/jbc.R700026200. PMID 17635904.

## External links

* Epithelium Photomicrographs
* Histology at KUMC epithel-epith02 Simple squamous epithelium of the glomerulus (kidney)
* Diagrams of simple squamous epithelium
* Histology at KUMC epithel-epith12 Stratified squamous epithelium of the vagina
* Histology at KUMC epithel-epith14 Stratified squamous epithelium of the skin (thin skin)
* Histology at KUMC epithel-epith15 Stratified squamous epithelium of the skin (thick skin)
* Stratified squamous epithelium of the esophagus
* Microanatomy Web Atlas

* v
* t
* e

Epithelial tissue

Cells|

* Squamous
* Cuboidal
* Columnar

Types|

* Simple squamous epithelium
* Endothelium
* Mesothelium

* Simple cuboidal epithelium

* Simple columnar epithelium

* Pseudostratified columnar epithelium
* Respiratory epithelium

* Stratified squamous epithelium

* Stratified cuboidal epithelium

* Stratified columnar epithelium

* Transitional epithelium
* Urothelium

Glands|

| Types|

| Mechanism|

* Merocrine
* Eccrine
* Apocrine
* Holocrine
* Sebaceous
* Meibomian

|

Shape|

* Tubular gland
* Alveolar gland

Secretion|

* Serous glands
* Mucous glands

Components|

* Myoepithelial cell
* Serous demilune

* Ducts
* Intralobular
* Striated
* Intercalated
* Acinus/Lobe

* 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)

* v
* t
* e

Biological tissues

Animals|

* Epithelial
* Connective
* Muscular
* Nervous

Plants|

* Dermal tissue: Epidermis
* Cuticle
* Pavement cell
* Guard cell
* Subsidiary cell
* Periderm
* Phellem
* Phelloderm

* Vascular tissue: Phloem
* Sieve tube
* Companion cell
* Phloem fiber
* Phloem parenchyma
* Xylem
* Tracheid
* Vessel element
* Xylem fiber
* Xylem parenchyma

* Ground tissue: Parenchyma
* Aerenchyma
* Chlorenchyma
* Mesophyll
* Pith
* Collenchyma
* Sclerenchyma
* Sclereid
* Fiber

* Meristematic tissue: Primary
* Protoderm
* Procambium
* Ground meristem
* Secondary
* Cork cambium
* Vascular cambium

* Mixed: Cortex
* Exodermis
* Endodermis
* Stele

* Category
* Histology

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