| Epithelium | |
|---|---|
| Code | TH H2.00.02.0.00001 |
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 "Epi" means, "on, upon," and "Theli" meaning "tissue." Epithelial layers are avascular, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.[1] Epithelia can also be organized into clusters of cells that function as exocrine and endocrine glands. Exocrine and endocrine epithelial cells are highly vascular.
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Cells in epithelium are very densely packed together like bricks in a wall, leaving very little intercellular space. The cells form continuous sheets which are attached to each other at many locations by tight junctions and desmosomes.[2] The epithelial tissues cover the interior and exterior part of our skin.
All epithelial cells rest on a basement membrane, which acts as a scaffolding on which epithelium can grow and regenerate after injuries.[3] Epithelial tissue is innervated, but avascular. This epithelial tissue 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.[4][5]
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.
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-membranar protein fused on outer plasma membrane. Adherens junctions are a plaque (protein layer on the inside plasma membrane) which attaches both protein and microfilaments. Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. Hemidesmosomes resemble desmsomes 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).
Tissues are generally 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]
Simple epithelium is one cell thick, that is, every cell is in direct contact with the underlying basement membrane. It is generally found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes.[2]
Simple epithelial tissues are generally 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.[2]
(1) simple squamous; which is found lining areas where passive diffusion of gases occur. e.g walls of capillaries, linings of the pericardial, pleural,and peritoneal cavities, as well as the linings of the alveoli of the lungs.
(2) simple cuboidal: these cells may have secretory, absorptive, or excretory functions. examples include small collecting ducts of kidney,pancreas and salivary gland.
(3) simple columnar; found in areas with extremely high secretive (as in wall of the stomach), or absorptive (as in small intestine) areas. they possess cellular extensions (e.g microvilli in the small intestine, or cilia found almost exclusively in the female reproductive tract).
(4) pseudostratified epithelia; they are also called respiratory epithelium. this is due to their almost exclusive confinement to the larger respiratory airways i.e the nasal cavity, trachea, bronchi e.t.c.
| 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 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 epithelial tissue also differs from simple epithelial tissue in that stratified epithelial tissues do not contain junctional complexes, and have their cells bound together only by desmosomes.[8]
Stratified epithelia (of columnar, cuboidal or squamous type) can have the following specializations:
| 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 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 epithelia are 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. |
The primary functions of epithelial tissues are: (1) to protect the tissues that lie beneath it from radiation, desiccation, toxins, 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 (3) the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts glandular epithelium.[10]
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.
"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."[11]
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.
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.[12]
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 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.
[editing help required: the internal wiki-link 'gastric epithelium' in the column 'Subtype' of the table below is a circular reference back to this page.]
| 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 | 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 | 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 | - |
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