Epithelium

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This article is about epithelium in animal anatomy. Epithelium_sentence_0

For the fungal structure of the same name, see Pileipellis. Epithelium_sentence_1

Epithelium_table_infobox_0

EpitheliumEpithelium_header_cell_0_0_0
PronunciationEpithelium_header_cell_0_1_0 epi- + thele + -iumEpithelium_cell_0_1_1
IdentifiersEpithelium_header_cell_0_2_0
MeSHEpithelium_header_cell_0_3_0 Epithelium_cell_0_3_1
THEpithelium_header_cell_0_4_0 Epithelium_cell_0_4_1
FMAEpithelium_header_cell_0_5_0 Epithelium_cell_0_5_1

Epithelium (/ˌɛpɪˈθiːliəm/) is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelium_sentence_2

It is a thin, continuous, protective layer of cells. Epithelium_sentence_3

Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs. Epithelium_sentence_4

An example is the epidermis, the outermost layer of the skin. Epithelium_sentence_5

There are three principal shapes of epithelial cell: squamous, columnar, and cuboidal. Epithelium_sentence_6

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. Epithelium_sentence_7

In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei. Epithelium_sentence_8

This sort of tissue is called a pseudostratified. Epithelium_sentence_9

All glands are made up of epithelial cells. Epithelium_sentence_10

Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport, and sensing. Epithelium_sentence_11

Epithelial layers contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane. Epithelium_sentence_12

Cell junctions are well employed in epithelial tissues. Epithelium_sentence_13

Classification Epithelium_section_0

In general, epithelial tissues are classified by the number of their layers and by the shape and function of the cells. Epithelium_sentence_14

The three principal shapes associated with epithelial cells are squamous, cuboidal, and columnar. Epithelium_sentence_15

Epithelium_unordered_list_0

  • 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.Epithelium_item_0_0
  • Cuboidal epithelium has cells whose height and width are approximately the same (cube shaped).Epithelium_item_0_1
  • 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.Epithelium_item_0_2

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, and both types of layering can be made up of any of the cell shapes. Epithelium_sentence_16

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. Epithelium_sentence_17

This kind of epithelium is therefore described as pseudostratified columnar epithelium. Epithelium_sentence_18

Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on the epithelium. Epithelium_sentence_19

Simple epithelium Epithelium_section_1

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. Epithelium_sentence_20

In general, it is found where absorption and filtration occur. Epithelium_sentence_21

The thinness of the epithelial barrier facilitates these processes. Epithelium_sentence_22

In general, simple epithelial tissues are classified by the shape of their cells. Epithelium_sentence_23

The four major classes of simple epithelium are (1) simple squamous, (2) simple cuboidal, (3) simple columnar, and (4) pseudostratified. Epithelium_sentence_24

Epithelium_description_list_1

  • (1) Simple squamous: 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).Epithelium_item_1_3
  • (2) Simple cuboidal: These cells may have secretory, absorptive, or excretory functions. Examples include small collecting ducts of the kidney, pancreas, and salivary gland.Epithelium_item_1_4
  • (3) Simple columnar: 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. 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.Epithelium_item_1_5
  • (4) Pseudostratified columnar epithelium: These can be ciliated or non-ciliated. The ciliated type is also called respiratory epithelium since it is almost exclusively confined to the larger respiratory airways of the nasal cavity, trachea, and bronchi.Epithelium_item_1_6

Stratified epithelium Epithelium_section_2

Stratified epithelium differs from simple epithelium in that it is multilayered. Epithelium_sentence_25

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. Epithelium_sentence_26

Cells flatten as the layers become more apical, though in their most basal layers, the cells can be squamous, cuboidal, or columnar. Epithelium_sentence_27

Stratified epithelia (of columnar, cuboidal, or squamous type) can have the following specializations: Epithelium_sentence_28

Epithelium_table_general_1

SpecializationEpithelium_header_cell_1_0_0 DescriptionEpithelium_header_cell_1_0_1
KeratinizedEpithelium_cell_1_1_0 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 esophagus is an example of a non-keratinized or "moist" stratified epithelium.Epithelium_cell_1_1_1
ParakeratinizedEpithelium_cell_1_2_0 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 esophagus.Epithelium_cell_1_2_1
TransitionalEpithelium_cell_1_3_0 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.Epithelium_cell_1_3_1

Cell types Epithelium_section_3

The basic cell types are squamous, cuboidal, and columnar, classed by their shape. Epithelium_sentence_29

Epithelium_table_general_2

TypeEpithelium_header_cell_2_0_0 DescriptionEpithelium_header_cell_2_0_1
SquamousEpithelium_cell_2_1_0 Squamous cells have the appearance of thin, flat plates that can look polygonal when viewed from above. 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 as also found in the alveolar epithelium in the lungs. 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.Epithelium_cell_2_1_1
CuboidalEpithelium_cell_2_2_0 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. Stratified cuboidal epithelium protects areas such as the ducts of sweat glands, mammary glands, and salivary glands.Epithelium_cell_2_2_1
ColumnarEpithelium_cell_2_3_0 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.Epithelium_cell_2_3_1
PseudostratifiedEpithelium_cell_2_4_0 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.Epithelium_cell_2_4_1

Structure Epithelium_section_4

Epithelial tissue is scutoid shaped, tightly packed and form a continuous sheet. Epithelium_sentence_30

It has almost no intercellular spaces. Epithelium_sentence_31

All epithelia is usually separated from underlying tissues by an extracellular fibrous basement membrane. Epithelium_sentence_32

The lining of the mouth, lung alveoli and kidney tubules are all made of epithelial tissue. Epithelium_sentence_33

The lining of the blood and lymphatic vessels are of a specialised form of epithelium called endothelium. Epithelium_sentence_34

Location Epithelium_section_5

See also: Table of epithelia of human organs Epithelium_sentence_35

Epithelium lines both the outside (skin) and the inside cavities and lumina of bodies. Epithelium_sentence_36

The outermost layer of human skin is composed of dead stratified squamous, keratinized epithelial cells. Epithelium_sentence_37

Tissues that line the inside of the mouth, the esophagus, the vagina, and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Epithelium_sentence_38

Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Epithelium_sentence_39

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. Epithelium_sentence_40

The outer surface of the cornea is covered with fast-growing, easily regenerated epithelial cells. Epithelium_sentence_41

A specialised form of epithelium, endothelium, forms the inner lining of blood vessels and the heart, and is known as vascular endothelium, and lining lymphatic vessels as lymphatic endothelium. Epithelium_sentence_42

Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum. Epithelium_sentence_43

In arthropods, the integument, or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle, an outer covering of chitin, the rigidity of which varies as per its chemical composition. Epithelium_sentence_44

Basement membrane Epithelium_section_6

Epithelial tissue rests on a basement membrane, which acts as a scaffolding on which epithelium can grow and regenerate after injuries. Epithelium_sentence_45

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. Epithelium_sentence_46

The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium. Epithelium_sentence_47

Cell junctions Epithelium_section_7

Cell junctions are especially abundant in epithelial tissues. Epithelium_sentence_48

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. Epithelium_sentence_49

Cell junctions are the contact points between plasma membrane and tissue cells. Epithelium_sentence_50

There are mainly 5 different types of cell junctions: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions. Epithelium_sentence_51

Tight junctions are a pair of trans-membrane protein fused on outer plasma membrane. Epithelium_sentence_52

Adherens junctions are a plaque (protein layer on the inside plasma membrane) which attaches both cells' microfilaments. Epithelium_sentence_53

Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. Epithelium_sentence_54

Hemidesmosomes resemble desmosomes on a section. Epithelium_sentence_55

They are made up of the integrin (a transmembrane protein) instead of cadherin. Epithelium_sentence_56

They attach the epithelial cell to the basement membrane. Epithelium_sentence_57

Gap junctions connect the cytoplasm of two cells and are made up of proteins called connexins (six of which come together to make a connexion). Epithelium_sentence_58

Development Epithelium_section_8

Epithelial tissues are derived from all of the embryological germ layers: Epithelium_sentence_59

Epithelium_unordered_list_2

However, it is important to note that pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. Epithelium_sentence_60

This is because such tissues present very different pathology. Epithelium_sentence_61

For that reason, pathologists label cancers in endothelium and mesothelium sarcomas, whereas true epithelial cancers are called carcinomas. Epithelium_sentence_62

Additionally, the filaments that support these mesoderm-derived tissues are very distinct. Epithelium_sentence_63

Outside of the field of pathology, it is generally accepted that the epithelium arises from all three germ layers. Epithelium_sentence_64

Functions Epithelium_section_9

Epithelial tissues have as their primary functions: Epithelium_sentence_65

Epithelium_ordered_list_3

  1. to protect the tissues that lie beneath from radiation, desiccation, toxins, invasion by pathogens, and physical traumaEpithelium_item_3_10
  2. the regulation and exchange of chemicals between the underlying tissues and a body cavityEpithelium_item_3_11
  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 ductsEpithelium_item_3_12
  4. to provide sensationEpithelium_item_3_13
  5. Absorb water and digested food in the lining of digestive canal.Epithelium_item_3_14

Glandular tissue Epithelium_section_10

Glandular tissue is the type of epithelium that forms the glands from the infolding of epithelium and subsequent growth in the underlying connective tissue. Epithelium_sentence_66

There are two major classifications of glands: endocrine glands and exocrine glands: Epithelium_sentence_67

Epithelium_unordered_list_4

  • Endocrine glands secrete their product into the extracellular space where it is rapidly taken up by the circulatory system.Epithelium_item_4_15
  • 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.Epithelium_item_4_16

Sensing the extracellular environment Epithelium_section_11

Clinical significance Epithelium_section_12

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_sentence_68

Epithelium grown in culture can be identified by examining its morphological characteristics. Epithelium_sentence_69

Epithelial cells tend to cluster together, and have a "characteristic tight pavement-like appearance". Epithelium_sentence_70

But this is not always the case, such as when the cells are derived from a tumor. Epithelium_sentence_71

In these cases, it is often necessary to use certain biochemical markers to make a positive identification. Epithelium_sentence_72

The intermediate filament proteins in the cytokeratin group are almost exclusively found in epithelial cells, so they are often used for this purpose. Epithelium_sentence_73

Cancers originating from the epithelium are classified as carcinomas. Epithelium_sentence_74

In contrast, sarcomas develop in connective tissue. Epithelium_sentence_75

When epithelial cells or tissues are damaged from cystic fibrosis, sweat glands are also damaged, causing a frosty coating of the skin. Epithelium_sentence_76

Etymology and pronunciation Epithelium_section_13

The word epithelium uses the Greek roots ἐπί (epi), "on" or "upon", and θηλή (thēlē), "nipple". Epithelium_sentence_77

Epithelium is so called because the name was originally used to describe the translucent covering of small "nipples" of tissue on the lip. Epithelium_sentence_78

The word has both mass and count senses; the plural form is epithelia. Epithelium_sentence_79

Additional images Epithelium_section_14

Epithelium_unordered_list_5

  • Epithelium_item_5_17
  • Epithelium_item_5_18
  • Epithelium_item_5_19
  • Epithelium_item_5_20

See also Epithelium_section_15

Epithelium_unordered_list_6


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Epithelium.