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. 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, keratinized epithelial cells.
Tissue that line the inside of the mouth, the esophagus, and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other sufaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostatified 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.
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. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.
In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous 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.
|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||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||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 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||-|
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.