Author: Adrian Rad BSc (Hons)•Reviewer: Franchesca Druggan BA, MScLast reviewed: September 30, 2021Reading time: 19 minutes


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The human being body is written of four straightforward types of tissues, epithelium being among them. The is existing on practically every part of the human being body, hence it has several vital functions. Because that example, it has roles in protection, absorption, secretion, and sensation. Its dominant presence likewise suggests the there room various varieties of epithelia in order to cater because that the varied person body structures.

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The main classifications of epithelium are basic and stratified, every one being further split into numerous subtypes follow to two main factors: cell shape and apical surface ar specialization. This write-up will define stratified (multi-layered) epithelium, focusing on its basic characteristics and also each major sub-type.

Key facts about the stratified epithelium
LocationsBody surfacesInternal cavities and also tubesParenchyma of glands
TypesSquamous - nonkeratinized (covers the mucosa)- keratinized (skin)Cuboidal (lines excretory ducts that glands)Columnar (conjunctiva that the eyelids)Transitional (urinary tract)
Squamous nonkeratinizedBasal layerSurface layer
Squamous keratinizedStratum basaleStratum spinosumStratum granulosumStratum lucidum (specific to special skin)Stratum corneum
Cuboidal2-3 layers of cuboidal cells
Columnar2-3 great of columnar cells
TransitionalBasal cellsIntermediate cellsUmbrella cells

This write-up will comment on the histology the the stratified epithelium.


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Epithelium

Functions and also location

Epithelium is one of the four straightforward types of tissues composing the human being body. It is one avascular type of tissue created of cells with tiny extracellular matrix, connected by strong intercellular adhesions. They have actually the illustration of cellular sheets. Epithelium is present virtually everywhere in the person body; that covers body surfaces, that lines interior cavities and tubes, develops the parenchyma the glands and can function as sense receptors. The prevalence and location that epithelium mirrors its principal functions:

CoverageProtectionAbsorptionSecretionSensation

In addition, any kind of substance entering or leaving any kind of tissue or organ should pass through the epithelium. This movement have the right to be either helped with or inhibited by the epithelium, making the a selective barrier.

Classification

The epithelial cells have actually several characteristics: they contain cell junctions, which enable tight intercellular adhesions. They room polar, having distinctive apical, lateral, and basal surface domains. Lastly, their basal surface is attached to a noncellular layer called the basement membrane.

Epithelium is share descriptively, follow to 3 factors: the number of cell layers developing it, the shape of surface cells, and also the expertise of the apical surface domain. The varieties of epithelium space the following:

Key facts around the types of epithelium
Simple epitheliumSquamousCuboidal (with microvili, without microvili)Columnar (with microvili, with surface ar cilia, v stereocilia, through pseudostratification)
Stratified epitheliumSquamous (nonkeratinized, keratinized)CuboidalColumnarTransitional

As the epithelium is so predominant and continuously exposed to potentially damaging factors, epithelial cell populaces are qualified of consistent self-renewal. The rate of cabinet turnover relies on the type of epithelium. Because that example, the replacement rate for straightforward columnar epithelial cells in the little intestine is 4 to 6 days, while stratified squamous epithelium of the skin is put in order every 28 days.

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Characteristics

Cell shape

Stratified epithelium is composed of two or more cell layers. There is a great amount that variability between the layers as result of various moving shapes and heights. The three species of cellular shapes are squamous, cuboidal, and columnar. Squamous cells have actually a width higher than the height and also contain an ovoid, centered nucleus. The width and also height that cuboidal cells are roughly equal and also they contain a round, centered nucleus. Because that columnar cells, the width is smaller sized than the height, when the cell nucleus is ovoid and positioned basally.To establish a typical nomenclature, only the form of the cells within the surface layer is supplied to sub-classify this tissue type (see above). For example, stratified cuboidal epithelium consists of multiple to move layers, with the surface ar layer being made up of cube-shaped cells. The layers situated at a depths layer have the right to consist of cells of different shapes, however they are typically cuboidal. Because that squamous stratified epithelium, over there is a third sub-classificational feature: the keratinization, or lack thereof, the the apical surface domain names of the cells. A common example that stratified squamous keratinized epithelium is the epidermis.

Function

The role of stratified epithelium is largely protection. In fact, this specific role is reflect in the straight influence that the form of physics stresses ~ above the degree and also nature of the stratification. To carry out their main role adequately, stratified epithelium is also quite thick, do it an especially poor for secretion or absorption. However, some stratified surface exhibit some degree of permeability for water and small molecules.

Learn everything about the stratified epithelium in the following study unit. 


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Stratified epithelium explore study unit
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It is crucial to realize that the difference in between “nonkeratinized” and “keratinized” stratified squamous epithelium (SSE) is not the lack or visibility of keratin, respectively. Instead, the difference lies in the quantity of keratinized cells current inside the epithelium due to the fact that both types actually contain this kind of fibrous protein.

Nonkeratinized SSE is composed of a variable variety of layers. The cell in the deeper, basal layer appear cuboidal through a clear cytoplasm usually, as result of their glycogen content. The cells in the surface layer room squamous, or flat. The basal great is attached to the basement membrane, a sheet of extracellular matrix proteins. It also contains stem cells that are crucial in the self-renewal process. Stem cells consistently divide within the basal layer and migrate in the direction of the apical layer. They replace old cells within this layer, which subsequently are subsequently melted as anucleated squamous cells.

Non-keratinized epithelium usually bear a mucous membrane, which serves as second protective and also lubricating layer of the epithelium. It may be checked out in the some components of the oral cavity, pharynx, esophagus, distal ureters, vagina and external woman genitalia. Nonkeratinized epithelium go contain some keratinized cells, yet the quantity of keratin deposition will vary depending on the level the dessication and abrasion it might be exposed to. For example, in instances the individuals with a history of chronic smoking cigarettes or alcohol use, the epithelia of the oropharynx and top digestive tract can become atypically keratinized. Furthermore, clenching or grinding of teeth versus the non-keratinized epithelium of the buccal mucosa may an outcome in the formation of calloused tissue.

As the lot of keratin is fairly low in this sub-class the stratified epithelium, the flat shaped cell retain their normal, characteristics nuclei and also metabolic functions.

Keratinized

Keratinzed stratified squamous epithelium is crucial in organization exposed to continuous physical abrasion, and the opportunity of desiccation (drying out) and water loss. Keratinized epithelium, is composed of plenty of layers of dead squamous cells, which space specially structured to it is in waterproof and also reduce evaporation from basic tissues. Thus they constitute an essential part of the epidermis or exterior skin. They are likewise found in certain regions of the oral cavity (e.g. Difficult palate, dorsum that the tongue) where eating, speaking and breathing could result to far-ranging loss the water. Keratinized cells space most typically identified by your anuclear appearance.

Skin
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While nonkeratinized SSE has a reasonably small quantity of keratin, the keratinized sub-class is full of it. The finest example the keratinized SSE is the epidermis the the skin. It consists of one of two people four distinct layers in thin skin or five in thick skin. They are called, starting from the deepest:

stratum basalestratum spinosumstratum granulosumstratum lucidum (specific to special skin)stratum corneumStratum basale

The stratum basale (basal layer) consists of stem cell that repetitively divide through mitosis to give rise to keratinocytes. This basal keratinocytes have a small amount of basophilic cytoplasm, carefully packed nuclei, and also a cuboidal or low columnar shapes. They space arranged in a single layer and contain very irregular and also folded basal surfaces with a high number of hemidesmosomes, which are responsible for the attachments of the stratum basale to the lamina lucida that the basement membrane.

The basal layer also contains scattered melanocytes. This cells contain particular granules referred to as melanosomes, which room responsible for the manufacturing of the precursor come pigment melanin. This pigment gives the skin its characteristic colour and protects against ultraviolet radiation. In routine H&E stains, melanocytes show up rounded through a clear cytoplasm. However, in much more detailed examinations, that is clear that melanocytes save on computer some certain cytoplasmic processes i m sorry extend between keratinocytes in ~ the stratum spinosum. These procedures are offered for delivering melanosomes to keratinocytes, which eventually situate prefer a lid over their nuclei.

Stratum spinosum

The stratum spinosum (spinous layer) is composed of keratinocytes that have actually migrated native the stratum basale, situated below. The stratum spinosum is in truth multilayered, fairly than one discrete and solitary layer. Keratinocytes also synthesize cytokeratins (intermediate filaments) that ultimately aggregated right into tonofibrils. The surface of these cells includes desmosomes, which form intercellular junctions. In this layer, the keratinocytes are shaped prefer a polyhedron, have actually round-oval nuclei, influential nucleoli and cytoplasms. They also synthesize cytokeratins (intermediate filaments) that ultimately aggregated right into tonofibrils.

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Stratum spinosum (histological slide)Stratum granulosum

As the keratinocytes continue their migration, they go into the stratum granulosum (granular layer). Together the cells mature, lock synthesize irregularly shaped keratohyalin granules (densely basophilic) that contain assorted proteins, such together involucrin, loricrin and filaggrin. This protein varieties interact with the previously created tonofibrils, to result in cross-linked intermediate filaments dubbed keratin.

The keratinocytes also produce lamellar bodies, which are tubular or ovoid shaped granules that room assembled by the Golgi complex. In fact, these bodies are heterogenous mixtures, or assemblies the probarrier lipids, lipid handling enzymes, proteins, and proteases. They deserve to be membrane bound and hence coat the cell’s membrane or secreted in ~ the extracellular space. Their contents permit the lamellar body to kind an epidermal water barrier, i beg your pardon is hydrophobic. This overall process is dubbed keratinisation (cornification). This is considered a special type of apoptosis because the common cellular fragmentation is replaced by keratin accumulation. In the epidermis, keratinisation wake up continuously. However, that rate have the right to be induced by excessive abrasion. Some examples include incorrectly fitted this within the oral cavity (nonkeratinized SSE) or high frictional levels of the skin (keratinized SSE), leading to calluses. Overall, the high amount of keratin provides the epidermis extremely sturdy to the continuous mechanical abrasion the is exposed to.

Stratum lucidum

The stratum lucidum is apparent only in thick skin, providing protection versus increased friction This layer includes visible eosinophilic cells, however as a whole, this great is very refractile and stains rather poorly. The cells contain a high lot of keratin, hence the nucleus and other organelles have actually disrupted morphologies.

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Stratum lucidum (histological slide)Stratum corneum

The stratum corneum is the topmost, non-living, cellular class of the epidermis written of terminally distinguished keratinocytes. They room filled through keratin intermediate filaments, offering them fairly an irregular and flatter form than normal. In addition, castle are fairly thin, anucleated, and also have no cytoplasmic organelles, for this reason they are metabolically inactive.

Their plasma membrane is also thickened and also their pH ranges between 4.5 to 6. In ~ this stage, lock are well-known as keratin squames. With each other they form a pattern referred to as orthokeratosis, i beg your pardon is the common presentation of squamous cells in the stratum corneum, that when together, develop a basket-weave pattern. The squames are likewise coated v an extracellular great of lipids, allowing them come repel water and also make the epidermis and efficient water barrier. In this layer, the procedure of desquamation wake up regularly. It involves the exfoliation and also loss that the squames v the destruction of their intercellular desmosomes.

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Stratified cuboidal epithelium

Stratified cuboidal epithelium is fairly thin, consist of of two or three layers the cuboidal cells. This form is fairly rare, developing specifically in the lining that excretory ducts, such together salivary and sweat glands. Its main duty is structural reinforcement, due to the fact that it is no significantly associated in absorption or secretion.

Stratified columnar epithelium

Similarly come the cuboidal sub-type, stratified columnar epithelium is rather rare. That is situated in the conjunctiva within the eyelids and areas that tissue transition. The is mainly responsible for protection and mucous secretion.

Transitional epithelium

Location and characteristics

Transitional epithelium (TE), likewise called urothelium, is a special type of stratified epithelium. That lines the urinary tract, particularly the major and minor calyces in the kidney, renal pelvis, ureters, bladder, the proximal component of the urethra, and also the prostate gland in males. Together it travels through the excretory passages, TE boosts from two cell great (minor calyces) to 4 or 5 (in the ureter) and ends up consisting the at the very least six (in the empty bladder).

It is called transitional due to the fact that it consists of mixed functions from both stratified cuboidal and stratified squamous epithelia. The attributes that do this epithelium special are:

its distention capabilityresistance to the toxicity of urineimpermeability to water and salts

Layers

When a corresponding structure of the urinary tract fills up through urine, the press inside increases, subsequently boosting the surface area. Come accommodate this phenomenon, the individual cells unfold and also flatten. This ability is the factor for the high variation in the variety of cell layers it was observed in nondistended materials of the urinary tract, stated above. In fact, this number is consistent throughout, being two or 3 layers.

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Transitional epithelium (histological slide)

Following hematoxylin and also eosin staining (H&E), surface cells from empty excretory components are termed umbrella (dome shaped) cells. They have actually a cuboidal shape and a bent apical surface that bulges into the lumen, giving them a huge and rounded appearance. Lock can additionally contain 2 nuclei. However, their appearance is influenced by the degree of distention, for this reason it have the right to vary. ~ above examination v the infection electron microscope (TEM), the cells’ apical surface contain regions of modification plasma membrane referred to as plaques. This plaques are particularly rigid and also thick. They are likewise attachment points for actin filaments, extending from your inner surfaces right into the cytoplasm of the cells. This filaments prevent extreme stretching during distension. Plaques space separated by interplaque regions, i m sorry consist of normal plasma membrane. In relaxed states, the plaques are invaginated, creating distinctive fusiform vesicles. However, they space only evident rather than fully closed vesicles because their lumens are continuous with the cells’ exterior. This vesicles unfold and disappear during distention, as the plaques become part of the cell surface. Umbrella cells additionally display tiny vesicles and mitochondria.

As urothelium is classified together stratified epithelium, it is multi-layered. Underneath the umbrella cells, i beg your pardon are located at the surface, a class of intermediate cells deserve to be found. They have a pyriform shape and also contain long, thin cytoplasmic procedures (tails) the anchor the urothelium layers to the basement membrane via unique anchoring proteins. Both of those class originate native basal cells, which kind the deepest class of TE. This single cell layer directly contacts the connective tissue and also capillary bed.


Clinical points

Psoriasis

Psoriasis is one autoimmune condition the affects the skin. This problem is dubbed parakeratosis, where nuclei are maintained in the stratum corneum, which indicates a hyperproliferative disorder. These modified squames kind an opaque, flakey and white layer, which is typically observed in psoriatic plaques. Epidermal hyperplasia is a typical manifestation, which is because of an excessively rapid maturation that keratinocytes. Normally, their maturation and also keratinisation are an extremely regulated and also slow procedures that last around 50 come 60 days. In psoriasis, this entire process takes approximately 7 days. This extremely brief timeframe results in the improper advancement of tonofibrils, keratohyalin granules, and also nuclear degeneration in the keratinocytes that the stratum granulosum. This at some point results in one abnormal basal layer and also keratin squames that room nucleated in the stratum corneum (normally they should be anucleated).

Epithelial metaplasia

Epithelial metaplasia entails a readjust from one mature form of epithelium into another. That is one adaptive process thrust by the cells’ adaptive response to stressful stimuli, such as chronic inflammation. Hence, the brand-new cell kind is an ext suited to resolve these brand-new conditions. One form is squamous to columnar epithelial metaplasia. This happens in Barrett’s syndrome, wherein stratified squamous nonkeratinized epithelium in the lower part of the esophagus transforms into basic columnar epithelium in solution to frequent gastroesophageal reflux.

Metaplasia is reversible, meaning that when the stress and anxiety is removed, the cell revert earlier to their initial type. However, if the stimuli persist because that a lengthy time, the epithelium have the right to transform right into a carcinoma. This is normally the instance in cancers affecting the lungs, cervix and the bladder.


Sources

References

Ross M. H., Pawlina W: Histology: A text and atlas: With correlated Cell and Molecular Biology, sixth edition, Lippincott Williams & WilkinsYoung B., Woodford P., O’Dowd G: Wheater’s useful Histology: A Text and also Colour Atlas, sixth edition, Churchill Livingstone ElsevierMescher Anthony L: Junqueira’s basic Histology: Text and also Atlas, 13th edition, McGraw-Hill EducationApodaca, G: The Uroepithelium: Not just a Passive Barrier. Traffic, 2004. 5(3): p. 117-128.

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