In zootomy, epithelium is a tissue composed of a layer of cells. In humans, it is one of four primary body tissues. 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 epithelial cells, as are the mucous membranes lining the inside of mouths and body cavities. 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. Zootomy is a contraction of zoological and anatomy and refers to the dissection of animals as opposed to that of plants (phytotomy) See also: Androtomy, zootomical terms for location, Cat anatomy La Anatomía comparada estudia diversas especies. ... Biological tissue is any substance made up of cells that perform a similar function within an organism. ... Cells in culture, stained for keratin (red) and DNA (green). ... A close-up of human skin. ... The lumen is the interior of a vessel within the body, such as the small central space in an artery or vein, or any of their relating vessels, down which blood flows. ... A close-up of human skin. ... Squamous means covered with or formed of scales; scaly. ... The mucous membranes (or mucosa) are linings of ectodermic origin, covered in epithelium, that line various body cavities and internal organs. ... The lungs flank the heart and great vessels in the chest cavity. ... The gastrointestinal tract or digestive tract, also referred to as the GI tract or the alimentary canal, (nourishment canal) or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and expels the remaining waste. ... Exocrine gland refers to glands that secrete their products via a duct. ... The endocrine system is a control system of ductless endocrine glands that secrete chemical messengers called hormones that circulate within the body via the bloodstream to affect distant organs. ...

Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability. Endothelium (the inner lining of blood vessels) is not related to epithelium except by name. The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. ... The arterial system The blood vessels are part of the circulatory system and function to transport blood throughout the body. ...

Characteristics

Epithelial cells:

• Sit on a basal lamina (formerly called a basement membrane).
• Have a free surface (if a cell has all of the other properties, but no free surface, it is called an epithelioid).
• Are avascular (contain no blood vessels).
• Have almost no extracellular space.
• Renew basally.
• Are derived from all three germ layers.
• Are named for the most superficial live (nucleus-containing) layer.

The basal lamina (often erroneously called basement membrane) is a layer on which epithelium sits. ... It has been suggested that organogenesis be merged into this article or section. ...

Classification

Epithelial cells are classified by the following three factors:

• Shape
• Stratification (number of layers)
• Specialization

Shape

• Squamous: Squamous cells are flat cells with an irregular flattened shape. The one-cell layer of simple squamous epithelium that forms the alveoli of the respiratory membrane, and the endothelium of capillaries, and is a minimal barrier to diffusion. Places where squamous cells can be found include the alveoli of the lungs, the filtration tubules of the kidneys, and the major cavities of the body. These cells are relatively inactive metabolically, and are associated with the diffusion of water, electrolytes, and other substances.
• Cuboidal: As the name suggests, these cells have a shape similar to a cube, meaning its width is the same size as its height. The nuclei of these cells are usually located in the center.
• Columnar: These cells are taller than they are wide. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. The nucleus is also closer to the base of the cell. The small intestine is a tubular organ lined with this type of tissue. Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete mucus. The free surface of the columnar cell has tiny hairlike projections called microvilli. They increase the surface area for absorption.
• Transitional: This is a specialized type of epithelium found lining organs that can stretch, such as the urothelium that lines the bladder and ureter of mammals. Since the cells can slide over each other, the appearance of this epithelium depends on whether the organ is distended or contracted: if distended, it appears as if there are only a few layers; when contracted, it appears as if there are several layers.

The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. ... This article or section does not cite its references or sources. ... The alveoli (singular:alveolus), tiny hollow sacs which are continuous with the airways, are the sites of gas exchange with the blood. ... The heart and lungs (from an older edition of Grays Anatomy) The lung is an organ belonging to the respiratory system and interfacing to the circulatory system of air-breathing vertebrates. ... Kidneys viewed from behind with spine removed The kidneys are bean-shaped excretory organs in vertebrates. ... An electrolyte is a substance which dissociates free ions when dissolved (or molten), to produce an electrically conductive medium. ... In cell biology, the nucleus (from Latin nucleus or nuculeus, kernel) is found in all eukaryotic cells and contains the nuclear genes which form most of the cells genetic material. ... Diagram showing the small intestine In biology the small intestine is the part of the gastrointestinal tract between the stomach and the large intestine (colon). ... In biology, an organ (Latin: organum, instrument, tool) is a group of tissues that perform a specific function or group of functions. ... Mucus is a slippery secretion of the lining of various membranes in the body (mucous membranes). ... Categories: Stub ... The urothelium is the lining of the ureters, urethra, and urinary bladder. ... Transverse section of ureter. ...

Stratification

• Simple: There is a single layer of cells.
• Stratified: More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina. Stratified cells can usually withstand large amounts of stress.
• Pseudostratified with cilia: This is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, but the position of the nuclei gives the impression that it is stratified. If a specimen looks stratified, but you can identify cilia, the specimen is pseudostratified ciliated epithelium since stratified epithelium cannot have cilia.

The basal lamina (often erroneously called basement membrane) is a layer on which epithelium sits. ...

Specializations

• Keratinized cells contain keratin (a cytoskeletal protein). While keratinized epithelium occurs mainly in the skin, it is also found in the mouth and nose, providing a tough, impermeable barrier.
• Ciliated cells have apical plasma membrane extensions composed of microtubules capable of beating rhythmically to move mucus or other substances through a duct. Cilia are common in the respiratory system and the lining of the oviduct.

Microscopy of keratin filaments inside cells. ... Microscopy of keratin filaments inside cells. ... The eukaryotic cytoskeleton. ... Sagittal section of nose mouth, pharynx, and larynx. ... Human nose in profile A typical bulbous human nose, here used to support a pair of glasses. ... cross-section of two cilia, showing 9+2 structure A cilium (plural cilia) is a fine projection from a eukaryotic cell that constantly beats in one direction. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... Microtubules are protein structures found within cells, one of the components of the cytoskeleton. ... Mucus is a slippery secretion of the lining of various membranes in the body (mucous membranes). ... To meet Wikipedias quality standards, this article or section may require cleanup. ... In oviparous animals (those that lay eggs), the passage from the ovaries to the outside of the body is known as the oviduct. ...

Examples

• Simple squamous: Found in blood vessels & lymph channels (called endothelium) and body cavities (called mesothelium)
• Keratinized stratified squamous: Found in human skin (specifically, the dead superficial layer); also found in masticatory oral mucosa (attached gingiva, dorsum of tongue, hard palate, etc.)
• Non-Keratinised stratified squamous: Found in human oesophagus (Oral Mucosa) specfically non-masticatory "movable" mucosa, and vagina
• Simple cuboidal: Found in thyroid follicles
• Stratified cuboidal: Exclusively found in sweat gland ducts
• Ciliated simple columnar: Found in intestine and kidney (specifically, proximal convoluted tubule)
• Stratified columnar: Ducts of submandibular glands
• Transitional: Specialized to distend (stretch) as the urinary bladder fills

The lymphatic system is a complex network of lymphoid organs, lymph nodes, lymph ducts, and lymph vessels that produce and transport lymph fluid from tissues to the circulatory system. ... The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. ... The mesothelium is a membrane that forms the lining of several body cavities: the pleura (thoracal cavity), peritoneum (abdominal cavity) and pericardium (heart sac). ... The esophagus, oe/œsophagus*, or gullet is the muscular tube in vertebrates through which ingested food passes from the mouth area to the stomach. ... The vagina, (from Latin, literally sheath or scabbard ) is the tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. ... In the biology of the kidney, the proximal convoluted tubule is the segment of the renal tubule that drains Bowmans capsule. ... The submandibular gland is one of the salivary glands, responsible for producing saliva. ... In the anatomy of mammals, the urinary bladder is the organ that collects urine excreted by the kidneys prior to disposal by urination. ...

Junctional complexes

Main article: Junctional complex

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 built up the paracellular barrier of epithelia and control the paracellular transport. Junctional complexes (JC) are the connections between two epithelial cells. ...

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 classification of glands: endocrine glands and exocrine glands. A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... An endocrine gland is one of a set of internal organs involved in the secretion of hormones into the blood. ... Exocrine gland refers to glands that secrete their products via a duct. ...

Embryology

There are epithelial tissues deriving from all of the embrological germ layers: Organs derived from each germ layer. ...

• from ectoderm (e.g., the epidermis);
• from endoderm (e.g., the lining of the gastrointestinal tract);
• from mesoderm (e.g., the inner linings of body cavities).

The ectoderm is outermost of the three germ layers of the developing embryo, the other two being the mesoderm and the endoderm. ... Epidermis could refer to: In plants, the outermost layer of cells covering the leaves and young parts of a plant is the epidermis. ... The endoderm, sometimes refered to as entoderm, is one of the three germ layers of the developing embryo, the other two being the ectoderm and the mesoderm. ... The gastrointestinal tract or digestive tract, also referred to as the GI tract or the alimentary canal, (nourishment canal) or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and expels the remaining waste. ... The mesoderm is one of the three germ layers in the early developing embryo, the other two layers being the ectoderm and the endoderm. ... A body cavity is an aspect of a number of basic animal body plans (phyla) that incorporate a central body cavity, known as a coelom. ...

See also

• Squamous cell
• Columnar epithelia
• Cuboidal epithelia
• Transitional epithelia
• Corneal epithelium

Squamous cells are basically types of cells which can be identified histologically by the fact that they look flattened and thin under a microscope. ... Columnar epithelia are cells whose heights are at least twice their width. ... Cuboidal epithelia are cube-shaped epithelial cells present in single layers or multiple layers, depending on their location in the body. ... Transitional epithelia are multi-layered cells that can contract and expand. ... The corneal epithelium (epithelium corneæ anterior layer) covers the front of the cornea and consists of several layers of cells. ...

References

• Molecular Biology of the CELL, 4th edition, Alberts et al., 2002