Epithelium is a tissue (collection of interconnected cells that perform a similar function within an organism) that covers organs and surfaces of the bodies of animals, including both outside surfaces (the skin) and inside cavities and lumen (interior of a vessel, such as the small central space in an artery or vein through which blood flows). Epithelial cells are close together, with very little intercellular material. The epithelium is generally free of blood vessels, with very few exceptions. Functions of epithelial cells include protection, secretion, absorption, transcellular transport, sensation detection, and selective permeability.
Epithelium is one of four primary body tissues of animals, including the human body and lower multicellular organisms, such as insects. The other three basic tissues are muscle tissue (contain contractile filaments that move past each other and change the size of the cell), nervous tissue (forming the brain, spinal cord, and peripheral nervous system), and connective tissue (holds everything together).
Despite the same DNA, epithelial cells offer an array of shapes, from flattened squamous cells, to cube-shaped cuboidal cells, to tall columnar cells, to transitional cells whose appearance depends on the amount they are stretched. Simple epithelial tissue, with a single layer of cells, is useful in locations requiring diffusion, while the outermost layer of our skin, the epidermis, is composed of stratified (more than one layer) squamous epithelial cells (with an underlying basal lamina), which can withstand stress. Other stratified squamous epithelial cells comprise the mucous membranes lining the inside of mouths and body cavities.
Endothelium (the inner lining of blood vessels) is a specialized form of epithelium.
Epithelial cells and the epithelium (collection of cells, or tissue) are classified by the following three factors:
- Squamous: Squamous cells are flat cells with an irregular flattened shape. The one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane in the lungs, and the endothelium of capillaries, and is a minimal barrier to diffusion. Places where squamous cells can be found also include 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 about 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. The nucleus is also closer to the base of the cell. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. 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, which 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.
- Simple: There is a single layer of cells. As noted above, simple squamous epithelium forms the alveoli of the respiratory membrane, as they offer minimal barrier to diffusion.
- Stratified: There is more than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina (layer on which epithelium sits and which is secreted by the epithelial cells). Stratified cells can usually withstand large amounts of stress. For example, stratified squamous epithelial cells make up the outer layer of skin.
- 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.
- 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.
|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||esophagus||Stratified squamous, non-keratinised||-|
|digestive||stomach||Simple columnar, non-ciliated||-|
|digestive||small intestine||Simple columnar, non-ciliated||-|
|digestive||large intestine||Simple columnar, non-ciliated||-|
|digestive||rectum||Stratified squamous, non-keratinised||-|
|digestive||anus||Stratified squamous, keratinised||-|
|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||-|
|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||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 columnar, ciliated||-|
|urinary||kidney - ascending thin limb||Simple squamous||-|
|urinary||kidney - distal convoluted tubule||Simple columnar, 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||-|
Embryology, secretory epithelia, and cell junctions
There are epithelial tissues deriving from all three 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).
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. Exocrine glands are glands that secrete their products into ducts (duct glands). Typical exocrine glands include sweat glands, salivary glands, mammary glands, and many glands of the digestive system.
Cell junctions are especially abundant in epithelial tissues. A cell junction is a structure within a tissue of a multicellular organism that consists of protein complexes and that provides contact between neighboring cells or between a cell and the extracellular matrix, or they build up the paracellular barrier of epithelia and control the paracellular transport.
- Alberts, B. 2002. Molecular Biology of the Cell, 4th edition. Garland Science. ISBN 0815332181.
- Lodish, H., A. Beck, L. S. Zipursky, P. Matsudaira, D. Baltimore, and J. Darnell. 2000. Molecular Cell Biology, 4th edition. New York: W. H. Freeman. ISBN 0716731363.
- Towle, A. 1989. Modern Biology. Austin, TX: Holt, Rinehart and Winston. ISBN 0030139198.
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