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Epithelium

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This article is about the epithelium as it relates to animal anatomy. For the fungal structure of the same name, see Pileipellis.
Types of epithelium

Epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body.[1] Many glands are also formed from epithelial tissue.[2] It lies on top of connective tissue, and the two layers are separated by a basement membrane.[3] In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue.

Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport and detection of sensation. As a result, they commonly present extensive apical-basolateral polarity (e.g. different membrane proteins expressed) and specialization.

General characters of epithelial tissue

  • It may develop from mesoderm, ectoderm or endoderm.
  • The epithelial cells rest on a basement membrane which may be clear or not clear.
  • No blood vessels can enter in between epithelial cells but nerves can, so epithelial tissue is avascular tissue.
  • Epithelial tissue receives nutrition by diffusion from the underlying connective tissue.

Classification (structural)

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

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.

Regardless of the type, any epithelium is separated from the underlying tissue by a thin layer of connective tissue known as the basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.

Simple epithelium

Simple epithelium is one cell thick; that is, every cell is in contact with the underlying basement membrane. Simple epithelium can be subdivided further according to the shape and function of its cells.

Type Description
Squamous Squamous cells have the appearance of thin, flat plates. They 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, elliptical (oval or shaped like an egg) nuclei because of the thin flattened form of the cell. Classically, squamous epithelia are found lining surfaces utilizing simple passive diffusion such as the alveolar epithelium in the lungs. Specialized squamous epithelia also form the lining of cavities such as the blood vessels (endothelium) and pericardium (mesothelium) and the major cavities found within the body.
Cuboidal As their name implies, cuboidal cells are roughly cuboidal in shape, appearing square in cross section. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is commonly found in secretive or absorptive tissue: for example the (secretive) exocrine gland the pancreas and the (absorptive) lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium that covers the female ovary
Columnar Columnar epithelial cells are elongated and column-shaped. Their 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 specialized 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, which acts as a lubricant.
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. Pseudostratified epithelium can also possess fine hair-like extensions of their apical (luminal) membrane called cilia. In this case, the epithelium is described as "ciliated" pseudostratified epithelium. Cilia are capable of energy dependent pulsatile beating in a certain direction through interaction of cytoskeletal microtubules and connecting structural proteins and enzymes. 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 of females, where the cilia propel the ovum to the uterus.

Stratified epithelium

Stratified epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers the cells can be squamous, cuboidal or columnar.

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

Specialization Description
Keratinized In this 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 waterproof, so is found in the mammalian skin. The lining of the esophagus is an example of a non-keratinized or "moist" stratified epithelium.
Transitional Transitional epithelium, like pseudostratified epithelium, is almost a class of its own; it is found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters and urethra.[citation needed]

Functions

  • Protection: covering and lining surfaces. protect underlying tissues.
  • Sensation: Sensory stimuli are detected by specialized epithelial cells. Specialized epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears and nose and on the tongue.
  • Secretion: In glands, epithelial tissue is specialized 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 (e.g. walls of capillaries and lungs).

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.

Tissues that line the inside of the mouth, the oesophagus 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.

Click the table headings to sort

System Tissue Epithelium Subtype
circulatory blood vessels Simple squamous endothelium
digestive ducts of submandibular glands Stratified 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 -
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, keratinised 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-keratinised -
reproductive - female vagina 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 ductuli efferentes 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 respiratory bronchioles Simple cuboidal -
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 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 external urethral orifice Stratified squamous -

Additional information

Cell junctions

Main article: Cell junction

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 build up the paracellular barrier of epithelia and control the paracellular transport.

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 are glands that secrete their product directly onto a surface rather than through a duct. This group contains the glands of the Endocrine system.

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."[4]

Embryology

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

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.

Additional images

References

  1. ^ "epithelium" at Dorland's Medical Dictionary
  2. ^ "Epithelia". Retrieved 2008-12-12.
  3. ^ "Blue Histology". Retrieved 2008-12-12.
  4. ^ Adams, M (2008). "Recent advances in the molecular pathology, cell biology and genetics of [[ciliopathy|ciliopathies]]". Journal of Medical Genetics. 45 (5): 257–267. doi:10.1136/jmg. PMID 18178628. {{cite journal}}: URL–wikilink conflict (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

Further reading

Look up epithelium in Wiktionary, the free dictionary.
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