Contain cheek cells microvilli

Epithelium - epithelium

Tissue that lines the surfaces of organs in animals

epithelium (/ ˌƐpɪˈθiːliəm /) is next to connective tissue, one of the four basic types of animal tissue, muscle tissue and nerve tissue. It's a thin, continuous protective layer of cells. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of the cavities in many internal organs. One example is the epidermis, the outermost layer of the skin.

. There are three main types of epithelial cells: squamous, columnar, and cuboid. These can be arranged in a single layer of cells as a simple epithelium, either as a squamous epithelium, columnar or cuboid, or in layers of two or more cells that are deeply layered (stratified) or composite, either as squamous, columnar or cuboid. In some tissues, a layer of columnar cells appears to be layered due to the placement of the nuclei. This type of tissue is known as pseudostratified. All glands are made up of epithelial cells. The functions of epithelial cells include secretion, selective absorption, protection, transcellular transport and detection.

. Epithelial layers do not contain blood vessels and must be nourished by diffusion of substances from the underlying connective tissue through the basement membrane. Cell contacts are well used in epithelial tissues.

Classification

Summary with different epithelial cells / tissues and their properties.

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

The three main shapes associated with epithelial cells are squamous, cuboid, and columnar.

  • Squamous epithelium has cells wider than their height (flat and scale-like). This is found as the lining of the mouth, esophagus and including blood vessels and in the alveoli of the lungs.
  • The cuboid epithelium has cells that are roughly the same height and width (cube-shaped).
  • The columnar epithelium has cells that are larger than they are wide (columnar). The columnar epithelium can be further divided into ciliated epithelium and glandular columnar epithelium.

By stratification, the epithelium is classified as either simple epithelium, only one cell thick (not stratified), or stratified epithelium with two or more cells thickness, or as multiple epithelium - stratified - stratified squamous epithelium, stratified cuboid epithelium, and stratified columnar epithelium, and both types of Layering can consist of any of the cell shapes. However, when larger simple columnar epithelial cells are viewed in cross section and multiple nuclei appear at different heights, they may be mistaken for stratified epithelia. This type of epithelium is therefore called pseudostratified columnar epithelium.

described. The transitional epithelium has cells that, depending on the tension of the epithelium, can change from squamous epithelium to cuboid.

Simple epithelium

Simple epithelium is a single layer of cells, with each cell in direct contact with the basement membrane, which separates it from the underlying connective tissue. Generally found where absorption and filtration take place. The thinness of the epithelial barrier facilitates these processes.

Generally, simple epithelial tissues are classified based on the shape of their cells. The four main classes of simple epithelium are (1) simple squamous epithelium, (2) simple cuboid, (3) simple columnar, and (4) pseudostratified.

(1) simple squamous epithelium: squamous cells appear like scales, flattened or rounded (e.g. walls of capillaries, linings of the pericardial, pleural and peritoneal cavities, linings of the alveoli).
(2) Simply cuboid: These cells can have secretory, absorbent or excretory functions. Examples of this are small collecting ducts in the kidney, pancreas and salivary gland.
(3) Simple column: cells can be secretory, absorbent, or excretory. Simple columnar epithelium may or may not be ciliated; ciliated column is found in the female reproductive tract and in the uterus. Non-ciliated epithelium can also have microvilli. Some tissues contain goblet cells and are called simple glandular columnar epithelium. These secrete mucus and are found in the stomach, colon, and rectum.
(4) Pseudostratified columnar epithelium: These may or may not be ciliated. The fibrillation type is also known as the airway epithelium because it is almost exclusively restricted to the larger airways of the nasal cavity, the trachea and the bronchi.

Layered epithelium

Layered epithelium differs from simple epithelium in that it is multilayered. It is therefore stated that body linings must withstand mechanical or chemical influences so that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, although the cells in their most basal layers may be squamous, cuboid, or columnar.

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

specialization description
Keratinized In this particular case, the most apical layers (outside) of the cells are dead and lose their nucleus and cytoplasm. Instead, they contain a tough, resistant protein called keratin. This specialization makes the epithelium somewhat water resistant, as is found in the skin of mammals. The lining of the esophagus is an example of a non-keratinized or "moist" stratified epithelium.
Parakeratinized In this case, the most apical layers of cells are filled with keratin, but retain their nuclei. These nuclei are pyknotic, which means that they are highly condensed. Parakeratinized epithelium is sometimes found in the lining of the mouth and in the upper regions of the esophagus.
Transitional epithelia Transitional epithelia are found in tissues that stretch, and they can appear layered in a cuboid shape when the tissue is relaxed or layered as the organ expands and the tissue expands. It is sometimes referred to as the urothelium because it is almost entirely in the bladders, ureters, and urethra.

Cell types

Occurs The basic cell types are squamous epithelium, cuboid and columnar, classified according to their shape.

Type description
Squamous epithelium Squamous cells have the appearance of thin, flat plates that may appear polygonal when viewed from above. Its name comes from squāma, Latin for "scale" - as on fish or snake skin. The cells fit tightly together in tissues and provide a smooth, low-friction surface that fluids can move easily over. The shape of the nucleus usually matches the shape of the cell and helps identify the type of epithelium. Squamous cell carcinomas, due to the thin, flattened shape of the cell, tend to have horizontally flattened, almost oval-shaped nuclei. Squamous epithelium is located in the lining of surfaces such as skin or alveoli in the lungs and thus enables simple passive diffusion, as can also be 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.
Cuboid Cuboid epithelial cells have a cube-shaped shape and appear square in cross-section. The nucleus is large, spherical and is located in the center of the cell. Cuboid epithelium is usually found in secretory tissue such as the exocrine glands or in absorbent tissue such as the pancreas, the lining of the renal tubules and in the gland ducts. The germinal epithelium, which covers the female ovary, and the germinal epithelium, which lines the walls of the seminiferous tubules in the testes, are also cuboid. Cuboid cells offer protection and, depending on location and specialization, can actively pump material into or out of the lumen or be passive. Simple cuboid epithelium usually differentiates to form the secretory and ductal sections of the glands. Layered cuboid epithelium protects areas such as the ducts of the sweat glands, mammary glands and salivary glands.
Columnar epithelial cells Columnar epithelial cells are elongated and columnar and have a height of at least four times their width. Their nuclei are elongated and are usually near the base of the cells. The columnar epithelium forms the lining of the stomach and intestines. The cells here can have microvilli to maximize surface area for absorption, and these microvilli can form a brush rim. Other cells may be ciliated to move mucus in the function of mucociliary clearance. Other ciliated cells are found in the fallopian tubes, in the uterus, and in the central canal of the spinal cord. Some column cells specialize in sensory reception, e.g. B. in the nose, ears and the taste buds. Hair cells in the inner ears have stereocilia that are similar to microvilli. Goblet cells are modified columnar cells and are located between the columnar epithelial cells of the duodenum. They secrete mucus, which acts as a lubricant. Unilamellar columnar epithelium that is not ciliated tends to indicate an absorption function. Stratified columnar epithelium is rare, but is found in lobe ducts in the salivary glands, eye, pharynx, and genital organs. This consists of a layer of cells that rest on at least one other layer of epithelial cells, which can be squamous, cuboid or columnar.
Pseudostratified These are simple columnar epithelial cells, the nuclei of which appear at different heights, giving the misleading (hence "pseudo") impression that the epithelium is stratified when viewed in cross section. Ciliated pseudostratified epithelial cells have cilia. Due to the interaction of microtubules of the cytoskeleton and the connection of structural proteins and enzymes, cilia are able to pulsate in a certain direction depending on the energy. In the airways, the wave effect generated causes mucus secreted locally by the goblet cells (to lubricate and trap pathogens and particles) to flow in this direction (typically out of the body). The ciliated epithelium is located in the airways (nose, bronchi), but also in the uterus and in the fallopian tubes

structure

Epithelial tissue is scutoid-shaped, densely packed and forms a continuous layer. It has almost no intercellular spaces. All epithelia are usually separated from the underlying tissue by an extracellular fibrous basement membrane. The lining of the mouth, pulmonary alveoli and renal tubules consists of epithelial tissue. The lining of the blood and the lymphatic vessels consists of a special form of epithelium called the endothelium.

referred to as. location

The epithelium draws both the outer (skin) and the inner cavities and lumina of bodies. The outermost layer of human skin consists of dead stratified squamous cells, keratinized epithelial cells.

Tissues that line the inside of the mouth, the esophagus, vagina, and part of the rectum are made up of non-keratinized stratified squamous epithelium. Other surfaces that separate body cavities from the external environment are lined with simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs, gastrointestinal, reproductive, and urinary tracts and make up the exocrine and endocrine glands. The outer surface of the cornea is covered with rapidly growing, easily regenerable epithelial cells. A special form of epithelium, endothelium, forms the inner lining of blood vessels and the heart and is known as the vascular endothelium and the lining of the lymphatic vessels as the lymphatic endothelium. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.

. In arthropods, the integument or outer "skin" consists of a single layer of epithelial ectoderm from which the cuticle emerges, an outer shell made of chitin, the stiffness of which varies depending on its chemical composition.

Basement membrane

Epithelial tissue rests on a basement membrane, which serves as a scaffold on which epithelium can grow and regenerate after injury. Epithelial tissue has a nerve supply but no blood supply and must be nourished by substances that diffuse from the blood vessels in the underlying tissue. The basement membrane acts as a selectively permeable membrane that determines which substances can enter the epithelium.

Cell contacts

Cell contacts are particularly common in epithelial tissues. They consist of protein complexes and establish contact between neighboring cells, between a cell and the extracellular matrix, or they build up the paracellular barrier of epithelia and control paracellular transport.

Contact points between the plasma membrane and tissue cells. There are mainly 5 different types of cell junctions: tight junctions, adherent junctions, desmosomes, hemidesmosomes and gap junctions. Tight junctions are a pair of transmembrane proteins that are fused on the outer plasma membrane. Adherens junctions are plaques (protein layers on the inner plasma membrane) that attach the microfilaments of both cells. Desmosomes attach themselves to the microfilaments of the cytoskeleton, which are made up of keratin protein. Hemidesmosomes resemble desmosomes in one section. They consist of the integrin (a transmembrane 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 create a connection).

development

Epithelial tissues are derived from all embryological germ layers:

However, it is important to note that pathologists do not consider the endothelium and mesothelium (both derived from the mesoderm) as true epithelium. This is due to the fact that such tissues have very different pathology. For this reason, pathologists label cancers as endothelial and mesothelial sarcomas, while true epithelial carcinomas are referred to as carcinomas. In addition, the filaments that these mesoderm-derived tissues carry are very different. Outside the realm of pathology, it is generally the case that the epithelium originates from all three germinal layers.

Functions

Forms of secretion in the glandular tissue
Different properties of the body's glands

The main functions of epithelial tissues are:

  1. to protect underlying tissues from radiation, dehydration, toxins, pathogen invasion, and physical trauma
  2. the regulation and exchange of chemicals between the underlying tissues and protect a body cavity
  3. the secretion of hormones into the circulatory system, as well as the secretion of sweat, mucus, enzymes and other products from ducts

Glandular tissue

Glandular tissue is the type of epithelium that the glands form from the folds of the epithelium and subsequent growth in the underlying connective tissue. There are two main classifications of glands: endocrine glands and exocrine glands:

  • Endocrine glands secrete their product into the extracellular space, where it is quickly absorbed by the circulatory system.
  • Exocrine glands secrete their products into a duct, which then delivers the product to the lumen of an organ or onto the free surface of the epithelium.

Perception of the extracellular environment

"Some epithelial cells are ciliated, particularly in respiratory epithelium, and they usually exist as a layer of polarized cells that form a tube or tubule, with cilia in the lumen

Clinical Significance

The slide shows at (1) an epithelial cell infected with Chlamydia pneumoniae, the inclusion bodies of which are shown at (3), an uninfected cell at (2) and (4) The difference between an infected cell nucleus and an uninfected cell nucleus.

Epithelium grown in culture can be identified by examining its morphological properties. Epithelial cells tend to group together and have a "property of" dense, patch-like appearance. "But this is not always the case, for example 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 occur almost exclusively in epithelial cells, so they are often used for this purpose.

Cancers derived from the epithelium are called carcinomas

. When epithelial cells or tissues are damaged by cystic fibrosis, sweat glands are also damaged, resulting in a frosty coating on the skin

Etymology and Pronunciation

The word epithelium uses the Greek roots ἐπί (epi), "on" or "upon" and θηλή (thēlē), "nipple". Epithelium is so named because the name was originally used to describe the translucent covering of tiny "nipples" of tissue on the lip. The word has both mass and number of senses; The plural form is epithelium .

Additional pictures

  • Squamous epithelium 100x

  • Human cheek cells (non-keratinized stratified squamous epithelium) 500x

  • Histology of the female urethra with transitional epithelium

  • Histology of the sweat gland: stratified cuboid epithelium

See also

literature

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further reading

External links

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