Types of epithelium

In biology and medicine, epithelium is a tissue composed of layers of cells that line the cavities and surfaces of structures throughout the body. Tissue is a cellular organizational level intermediate between cells and a complete organism The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called It is also the type of tissue of which many glands are formed. A gland is an organ in an animal's body that synthesizes a substance for release such as Hormones or Breast milk, often into the Bloodstream Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The skin is the outer covering of living tissue of an animal (or plant A lumen (Lat lūmen, an opening or light (pl lumina is the inside space or lining of a tubular structure such as an artery or intestine The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells. The skin is the outer covering of living tissue of an animal (or plant In Anatomy, squamous epithelium (from Latin squama, "scale" is an Epithelium characterised by its most superficial layer consisting Epidermis is the outermost layer of the Skin. It forms the waterproof protective wrap over the body's surface and is made up of stratified squamous Epithelium with

Mucous membranes lining the inside of the mouth, the oesophagus, and part of the rectum are lined by nonkeratinized stratified squamous epithelium. The mucous membranes (or mucosae; singular mucosa) are linings of mostly endodermal origin covered in Epithelium, which are involved in Epidermis is the outermost layer of the Skin. It forms the waterproof protective wrap over the body's surface and is made up of stratified squamous Epithelium with Other, open to outside body cavities are lined by simple squamous or columnar 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. lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive Exocrine glands are Glands that secrete their products ( Enzymes into ducts (duct glands The endocrine system is an integrated system of small organs that involve the release of extracellular signaling molecules known as Hormones The endocrine system is instrumental The outer surface of the cornea is covered with fast-growing, easily-regenerated epithelial cells. The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber.

Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability.

Endothelium (the inner lining of blood vessels, the heart, and lymphatic vessels) is a specialized form of epithelium. The endothelium is the thin layer of cells that line the interior surface of Blood vessels forming an interface between circulating Blood in the The blood vessels are part of the Circulatory system and function to transport Blood throughout the body The heart is a muscular organ in all Vertebrates responsible for pumping Blood through the Blood vessels by repeated rhythmic The lymphatic system in Vertebrates is a network of conduits that carry a clear fluid called Lymph. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum. The mesothelium is a membrane that forms the lining of several body cavities the Pleura (thoracal cavity Peritoneum (abdominal cavity including the mesentery and The pericardium is a double-walled sac that contains the Heart and the roots of the Great vessels. In higher Vertebrates the peritoneum is the Serous membrane that forms the lining of the abdominal cavity &mdash it covers most of the intra-abdominal

In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue. Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, Muscle, and Nervous tissue) Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the Nervous tissue is the fourth major class of Vertebrate tissue. Epithelium is often defined by the expression of the adhesion molecule e-cadherin, as opposed to n-cadherin, which is used by cells of the connective tissue. Cell Adhesion Molecules ( CAM s are Proteins located on the cell surface involved with the Binding with other cells or with the Extracellular Cadherin 1 type 1 E-cadherin (epithelial, also known as CDH1, is a human Gene. Cadherin 2 type 1 N-cadherin (neuronal, also known as CDH2, is a human Gene. Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, Muscle, and Nervous tissue)

Classification

Epithelial cells are classified by the following three factors:

Shape (of most superficial cells)

• Squamous: All Squamous cells are flat cells with an irregular flattened shape. A one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane, and the endothelium of capillaries, and is a minimal barrier to diffusion. The endothelium is the thin layer of cells that line the interior surface of Blood vessels forming an interface between circulating Blood in the Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement Squamous cells can be found included in the filtration tubules of the kidneys, and the major cavities of the body. The kidneys are complicated organs that have numerous biological roles These cells are relatively inactive metabolically, and are associated with the diffusion of water, electrolytes, and other substances. An electrolyte is any substance containing free Ions that behaves as an electrically conductive medium
• 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. In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed The cuboidal epithelium forms the smallest duct glands and many kidney tubules.
• 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. In Biology the small Intestine is the part of the Gastrointestinal tract (gut between the Stomach and the Large intestine, and comprises In Biology, an organ ( Latin: organum, "instrument tool" from Greek όργανον - organon "organ instrument Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete mucus. In vertebrates mucus is a slippery secretion produced by and covering Mucous membranes It is a viscous Colloid containing Antiseptic enzymes (such as The free surface of the columnar cell has tiny hairlike projections called microvilli. Microvilli (singular microvillus) are microscopic cellular membrane protrusions that increase the surface area of cells and are involved in a wide variety of functions including 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.

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. The basal lamina is a layer of extracellular matrix on which Epithelium sits and which is secreted by the epithelial cells 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. Stratified epithelium cannot have cilia but may be very rarely found in the fetal esophagus. A cell that contains hairs will be around ten times stronger than a regular cell.

Specializations

• Keratinized cells contain keratin (a cytoskeletal protein). Keratins are a family of fibrous structural proteins; tough and insoluble they form the hard but nonmineralized structures found in Reptiles Birds Keratins are a family of fibrous structural proteins; tough and insoluble they form the hard but nonmineralized structures found in Reptiles Birds cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. While keratinized epithelium occurs mainly in the skin, it is also found in the mouth and nose, providing a tough, impermeable barrier. The mouth, buccal cavity, or oral cavity is the first portion of the Alimentary canal that receives food and begins digestion by mechanically breaking up Anatomically a nose is a protuberance in Vertebrates that houses the Nostrils or nares which admit and expel air for respiration in conjunction with the

• Ciliated cells have apical plasma membrane extensions composed of microtubules capable of beating rhythmically to move mucus or other substances through a duct. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Microtubules are one of the components of the Cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers In vertebrates mucus is a slippery secretion produced by and covering Mucous membranes It is a viscous Colloid containing Antiseptic enzymes (such as Cilia are common in the respiratory system and the lining of the oviduct. In living organisms a respiratory system functions to allow Gas exchange. Oviduct is also another name for Fallopian tube In Oviparous Animals (those that lay eggs, the passage from the ovaries

Examples

Cell junctions

Main article: Cell junction

A cell junction is a structure within a tissue of a multicellular organism. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body The endothelium is the thin layer of cells that line the interior surface of Blood vessels forming an interface between circulating Blood in the In Anatomy and Physiology, a duct is a circumscribed channel leading from an Exocrine gland or organ. The paired submandibular glands (or submaxillary glands) are Salivary glands located beneath the floor of the mouth The gingiva (sing and plur: gingiva) or gums, consists of the Mucosal tissue that lies over the Alveolar bone. The tongue is the large bundle of Skeletal muscles on the floor of the Mouth that manipulates Food for chewing and swallowing (deglutition The hard palate is a thin horizontal bony plate of the Skull, located in the roof of the mouth The esophagus or oesophagus (see American and British English spelling differences) sometimes known as the gullet, is an organ in In Human anatomy, the stomach is a J-shaped hollow muscular organ of the Gastrointestinal tract involved in the second phase of Digestion, following In Biology the small Intestine is the part of the Gastrointestinal tract (gut between the Stomach and the Large intestine, and comprises The intestinal epithelium is the Epithelium that covers the small and Large intestine. The large intestine is the last part of the Digestive system: the final stage of the Alimentary canal in Vertebrate Animals Its function is to The intestinal epithelium is the Epithelium that covers the small and Large intestine. The rectum (from the Latin rectum intestinum, meaning straight intestine) is the final straight portion of the Large intestine in some Mammals The anus is an opening at the opposite end of an Animal 's Digestive tract from the Mouth. The gallbladder (or cholecyst sometimes gall bladder is a small organ whose function in the body is to store Bile and aid in the digestive process The thyroid is one of the largest Endocrine glands in the body Ependyma is the thin epithelial membrane lining the Ventricular system of the Brain and the Spinal cord. In Anatomy, lymph vessels are thin walled Valved structures that carry Lymph. The endothelium is the thin layer of cells that line the interior surface of Blood vessels forming an interface between circulating Blood in the The skin is the outer covering of living tissue of an animal (or plant The skin contains two different groups of sweat glands: Apocrine sweat glands and Merocrine sweat glands. The mesothelium is a membrane that forms the lining of several body cavities the Pleura (thoracal cavity Peritoneum (abdominal cavity including the mesentery and By the broadest definition a body cavity is any fluid filled space in a Multicellular organism. The mesothelium is a membrane that forms the lining of several body cavities the Pleura (thoracal cavity Peritoneum (abdominal cavity including the mesentery and "Ovaria" redirects here This is also a proposed section and a Synonym of Solanum. The surface of the Ovary is covered by a layer of Simple cuboidal cells which constitutes the germinal epithelium of Waldeyer The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges ( singular salpinx) are two very fine tubes lined with ciliated The uterus (from the Latin word for womb) is the major Female reproductive organ of most Mammals including Humans One end the The endometrium is the inner membrane of the Mammalian Uterus. The cervix (from Latin "neck" is the lower narrow portion of the Uterus where it joins with the top end of the Vagina. In the anatomy of the Female reproductive system, the canal of the cervix (also called the endocervical canal, cervical canal, cervical canal The cervix (from Latin "neck" is the lower narrow portion of the Uterus where it joins with the top end of the Vagina. The cervix (from Latin "neck" is the lower narrow portion of the Uterus where it joins with the top end of the Vagina. The vagina (from Latin, literally " Sheath " or " Scabbard " is a fibromuscular tubular tract leading from the Uterus The labia majora (singular labium majus) are two prominent Longitudinal Cutaneous folds which extend downward and backward from the Mons pubis The tubuli seminiferi recti (also known as the tubuli recti, tubulus rectus, or straight seminiferous tubules) is a structure in the Testicle The germinal epithelium is the outermost layer of the Testicle. Rete testis is an anastomosing network of delicate Tubules located in the hilum of the Testicle ( Mediastinum testis) that carries sperm The efferent ducts (or efferent ductules or ductuli efferentes) connect the Rete testis with the initial section of the Epididymis. The epididymis is part of the Human Male Reproductive system and is present in all male Mammals It is a narrow tightly-coiled Tube Stereocilia are apical modifications of the cell which are distinct from Microvilli and Cilia. The vas deferens (plural vasa deferentia also called ductus deferens, ( Latin: "carrying-away vessel" is part of the Male Anatomy The Ejaculatory ducts ( ductus ejaculatorii) are paired structures in male Anatomy, about 2 cm in length A bulbourethral Gland (or Cowper 's gland) is one of two small Exocrine glands present in the Reproductive system of human males The seminal vesicles ( glandulae vesiculosae) are a pair of simple tubular glands posteroinferior to the Urinary bladder of males The Oropharynx ( oral part of the Pharynx) reaches from the Soft palate to the level of the Hyoid bone. The larynx (plural larynges) colloquially known as the voicebox, is an organ in the Neck of Mammals involved in protection of the Respiratory Epithelium is a type of Epithelium found lining the Respiratory tract, where it serves to moisten and protect the airways The traceartes, or windpipe, is a tube that has an inner diameter of about 20-25 mm and a length of about 10-16 cm in humans Respiratory Epithelium is a type of Epithelium found lining the Respiratory tract, where it serves to moisten and protect the airways The respiratory bronchioles are the beginning of the respiratory segment of the airway and are just distal to the terminal bronchioles (which are the last segment of the conducting airway The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. The corneal epithelium ( epithelium corneæ anterior layer) is made up of epithelial tissue and covers the front of the Cornea. Anatomically a nose is a protuberance in Vertebrates that houses the Nostrils or nares which admit and expel air for respiration in conjunction with the The olfactory epithelium is a specialized epithelial tissue inside the Nasal cavity that is involved in smell. The kidneys are complicated organs that have numerous biological roles The proximal tubule is the portion of the duct system of the Nephron leading from Bowman's capsule to the Loop of Henle. The kidneys are complicated organs that have numerous biological roles As the filtrate passes back up the thin ascending limb of loop of Henle, the concentration of the surrounding medulla decreases The kidneys are complicated organs that have numerous biological roles The distal convoluted tubule ( DCT) is a portion of Kidney Nephron between the Loop of Henle and the Collecting duct system. The kidneys are complicated organs that have numerous biological roles The collecting duct system of the Kidney consists of a series of tubules and ducts that connect the Nephrons to the Ureter. The renal pelvis is the funnel-like dilated proximal part of the Ureter in the Kidney. The urothelium is the tissue layer that lines much of the Urinary tract, including the Renal pelvis, the Ureters the bladder, and In human Anatomy, the ureters are muscular ducts that propel Urine from the Kidneys to the Urinary bladder. The urothelium is the tissue layer that lines much of the Urinary tract, including the Renal pelvis, the Ureters the bladder, and In Anatomy, the urinary bladder is a hollow muscular, and distensible (or elastic organ that sits on the Pelvic floor in Mammals It is the The urothelium is the tissue layer that lines much of the Urinary tract, including the Renal pelvis, the Ureters the bladder, and The prostatic urethra, the widest and most dilatable part of the Urethra canal is about 3 cm The urothelium is the tissue layer that lines much of the Urinary tract, including the Renal pelvis, the Ureters the bladder, and The intermediate part of male urethra ( membranous portion) is the shortest least dilatable and with the exception of the external orifice the narrowest part of the canal The spongy urethra ( cavernous portion of urethra, penile urethra) is the longest part of the male Urethra, and is contained in the Corpus spongiosum 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. Paracellular transport refers to the transfer of substances between cells of an Epithelium.

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. A gland is an organ in an animal's body that synthesizes a substance for release such as Hormones or Breast milk, often into the Bloodstream There are two major classification of glands: endocrine glands and exocrine glands. Endocrine glands are Glands that secrete their product ( Hormones, directly into the blood rather than through a duct Exocrine glands are Glands that secrete their products ( Enzymes into ducts (duct 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

Embryology

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

• from ectoderm (e. A germ layer is a collection of cells formed during animal Embryogenesis. The ectoderm is the start of a tissue that covers the body surfaces g. , the epidermis);
• from endoderm (e. Endoderm, (sometimes called Entoderm) is one of the Germ layers formed during animal Embryogenesis. g. , the lining of the gastrointestinal tract);
• from mesoderm (e. One of the three Germ layers found in the Embryos of Animals more complex than Cnidarians making them Triploblastic. g. , the inner linings of body cavities). By the broadest definition a body cavity is any fluid filled space in a Multicellular organism.

However, it is important to note that pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. Max Bielschowsky Francis Camps Paul Ehrlich - (1854 - 1915 Gustav Giemsa - (1867 - 1948 (see Giemsa The endothelium is the thin layer of cells that line the interior surface of Blood vessels forming an interface between circulating Blood in the The mesothelium is a membrane that forms the lining of several body cavities the Pleura (thoracal cavity Peritoneum (abdominal cavity including the mesentery and 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. A sarcoma (from the Greek 'sarx' meaning "flesh" is a Cancer of the connective or supportive tissue ( Bone, Cartilage, Fat A carcinoma is any Malignant Cancer that arises from epithelial cells. 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

Links

• Parietal cell antibody
• Antibody to GPC
• An Example (cholesteatoma)

References

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