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In biology and medicine, epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue. It lies on top of connective tissue, and the two layers are separated by a basement membrane.
Tissue 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.
Epithelial tissue covers the whole surface of the body. It is made up of cells closely packed and ranged in one or more layers. This tissue is specialised to form the covering or lining of all internal and external body surfaces. Epithelial tissue that occurs on surfaces on the interior of the body is known as endothelium. Epithelial cells are packed tightly together, with almost no intercellular spaces and only a small amount of intercellular substance. Epithelial tissue, regardless of the type, is usually separated from the underlying tissue by a thin sheet of connective tissue; basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.
Types of Epithelial Tissue Epithelial tissue can be divided into two groups depending on the number of layers of which it is composes. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick such as the skin, it is known as stratified epithelium.
Simple epithelium can be subdivided according to the shape and function of its cells.
Simple squamous epithelium
Squamous (pavement) epithelium. Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually corresponds to the cell form and help to identify the type of epithelium. Squamous cells, for example, tend to have horizontall flattened, elliptical nuclei because of the thin flattened form of the cell. They form the lining of cavities such as the mouth, blood vessels, heart and lungs and make up the outer layers of the skin.
Simple cuboidal epithelium
As their name implies, cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus in the centre. Cuboidal epithelium is found in glands and in the lining of the kidney tubules as well as in the ducts of the glands. They also constitute the germinal epithelium which produces the egg cells in the female ovary and the sperm cells in the male testes.
Simple columnar epithelium
Columnar epithelial cells occur in one or more layers. The cells are elongated and column-shaped. The 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 specialised 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 or slime, a lubricating substance which keeps the surface smooth.
Ciliated columnar epithelium
These are simple columnar epithelial cells, but in addition, they posses fine hair-like outgrowths, cilia on their free surfaces. These cilia are capable of rapid, rhythmic, wavelike beatings in a certain direction. This movement of the cilia in a certain direction causes the mucus,
Columnar epithelium with goblet cells is called glandular epithelium. Some parts of the glandular epithelium consist of such a large number of goblet cells that there are only a few normal epithelial cells left. Columnar and cuboidal epithelial cells often become specialised as gland cells which are capable of synthesising and secreting certain substances such as enzymes, hormones, milk, mucus, sweat, wax and saliva. Unicellular glands consist of single, isolated glandular cells such as the goblet cells. Sometimes a portion of the epithelial tissue becomes invaginated and a multicellular gland is formed. Multicellular glands are composed of clusters of cells. Most glands are multicellular including the the salivary glands.
Where body linings have to withstand wear and tear, the epithelia are composed of several layers of cells and are then called compound or stratified epithelium. The top cells are flat and scaly and it may or may not be keratinised (i.e. containing a tough, resistant protein called keratin). The mammalian skin is an example of dry, keratinised, stratified epithelium. The lining of the mouth cavity is an example of an unkeratinisied, stratified epithelium.
Functions of Epithelial Tissue
Protection Epithelial cells from the skin protect underlying tissue from mechanical injury, harmful chemicals, invading bacteria and from excessive loss of water.
Sensation Sensory stimuli penetrate specialised epithelial cells. Specialised epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears, nose and on the tongue.
Secretion In glands, epithelial tissue is specialised to secrete specific chemical substances such as enzymes, hormones and lubricating fluids.
Absorption Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food.
Excretion Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands.
Diffusion Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (eg. walls of capillaries and lungs).
Cleaning Ciliated epithelium assists in removing dust particles and foreign bodies which have entered the air passages.
Reduces Friction The smooth, tightly-interlocking, epithelial cells that line the entire circulatory system reduce friction between the blood and the walls of the blood vessels.
Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability.
Epithelial cells are classified by the following three factors:
Shape (of most superficial cells)
|Squamous||Squamous epithelium covers many surfaces in the human body, including the cheeks, inner surfaces of blood vessels, filtration tubules and the major cavities found within the body. It also form the walls of the alveoli in the lungs. The individual cells are smooth, flat and very thin, as well as typically soft. They fit closely together in tissues; providing a smooth, low-friction surface over which fluids can move easily. In the alveoli, the thiness of the cells allows for rapid diffusion of gases between the alveoli and the blood. They are not regarded as being relatively active in terms of metabolism, but are highly key in the diffusion of water and specific substances through certain tissues.|
|Cuboidal||As the name suggests, these cells have a shape similar to a cube, meaning their width is the same size as their length and height. The Cell nucleus nuclei of these cells are usually located in the center. The cuboidal epithelium forms are the smallest.|
|Columnar||These cells are taller than they are wide like a cylindrical structure. 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 projections called microvilli.|
|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||This term is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, with each cell touching the basal lamina directly. However, without careful observation, the location of the nuclei within the cells may give the impression that the epithelium is stratified. For example, the epithelium lining the bronchi in mammals is pseudostratified epithelium composed of at least three cells types (basal cells with nuclei located close to the basal lamina; ciliated cells with nuclei halfway between the basal lamina and the cellular layer; and mucous cells so full of mucous granules that the nuclei are often difficult to clearly discern but are almost always near the basal lamina).|
|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. Additionally, the structure of the cilia found on ciliated epithelial cells is identical to that covering the single-cell organisms collectively known as ciliated prozoans. this cell secretes out of pores and hairs . The have ordor fission. Absorption is also a main key in this.|
|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-keratinised||-|
|digestive||stomach||Simple columnar, non-ciliated||-|
|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||-|
|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||uterus||Simple columnar, ciliated||-|
|reproductive - female||endometrium||Simple columnar||-|
|reproductive - female||cervix (endocervix)||Simple columnar||-|
|reproductive - female||cervix (ectocervix)||Stratified squamous, non-keratinised||-|
|reproductive - female||vagina||Stratified squamous, non-keratinised||-|
|reproductive - female||labia majora||Stratified squamous, keratinised||-|
|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-keratinised||-|
|respiratory||larynx||Pseudostratified columnar, ciliated||respiratory epithelium|
|respiratory||larynx - True vocal cords||Stratified squamous, non-keratinised||-|
|respiratory||trachea||Pseudostratified columnar, ciliated||respiratory epithelium|
|respiratory||respiratory bronchioles||Simple cuboidal||-|
|sensory||cornea||Stratified squamous, non-keratinised||corneal epithelium|
|sensory||nose||Pseudostratified columnar||olfactory epithelium|
|urinary||kidney - proximal convoluted tubule||Simple cuboidal, ciliated||-|
|urinary||kidney - ascending thin limb||Simple squamous||-|
|urinary||kidney - distal convoluted tubule||Simple cuboidal, non-ciliated||-|
|urinary||kidney - collecting duct||Simple cuboidal||-|
|urinary||membranous urethra||Pseudostratified columnar, non-ciliated||-|
|urinary||penile urethra||Pseudostratified columnar, non-ciliated||-|
|urinary||external urethral orifice||Stratified squamous||-|
- Main article: Cell junction
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.
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. 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
In general, there are epithelial tissues deriving from all of the embryological germ layers:
- 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).
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.
- ↑ Template:DorlandsDict
- ↑ Epithelia. URL accessed on 2008-12-12.
- ↑ Blue Histology. URL accessed on 2008-12-12.
- Molecular Biology of the Cell, 4th edition, Alberts et al., 2002
|Animals : Epithelium - Connective - Muscular - Nervous|
|Plants : Epidermis - Vascular tissue - Ground tissue|
Histology: epithelial tissue
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