Epithelium

The outside layer of cells that covers all the free, open surfaces of the body including the skin, and mucous membranes that communicate with the outside of the body. By contrast the endothelium is the layer of cells lining the closed internal spaces of the body such as the blood vessels and lymphatic vessels (that convey the lymph, a milky fluid).

* * *

The purely cellular avascular layer covering all free surfaces, cutaneous, mucous, and serous, including the glands and other structures derived therefrom. [G. epi, upon, + thele, nipple, a term applied originally to the thin skin covering the nipples and the papillary layer of the border of the lips]

- anterior e. of cornea the stratified squamous e. covering the outer surface of the cornea; it is smooth, consists usually of five layers of cells, and contains numerous free nerve endings. SYN: e. anterius corneae.

- e. anterius corneae SYN: anterior e. of cornea.

- Barrett e. columnar esophageal e. seen in Barrett syndrome.

- ciliated e. any e. having motile cilia on the free surface.

- columnar e. e. formed of a single layer of prismatic cells taller than they are wide. SYN: cylindrical e..

- crevicular e. the stratified squamous e. lining the inner aspect of the soft tissue wall of the gingival sulcus. SYN: sulcular e..

- cuboidal e. simple e. with cells appearing as cubes in a vertical section but as polyhedra in surface view.

- cylindrical e. SYN: columnar e..

- e. ductus semicircularis SYN: e. of semicircular duct.

- enamel e. the several layers of the enamel organ remaining on the enamel surface after formation of enamel is completed. SYN: reduced enamel e..

- external dental e., external enamel e. the cuboidal cells of the outer layer of the odontogenic organ of a developing tooth.

- germinal e. a cuboidal layer of peritoneal e. covering the gonads, once thought to be the source of germ cells.

- gingival e. a stratified squamous e. that undergoes some degree of keratinization and covers the free and attached gingiva.

- glandular e. e. composed of secretory cells.

- inner dental e., inner enamel e. the columnar epithelial layer of enamel matrix of the odontogenic organ of a developing tooth that develops into the enamel-producing ameloblasts.

- junctional e. a collar of epithelial cells attached to the tooth surface and subepithelial connective tissue found at the base of the gingival crevice. SYN: epithelial attachment of Gottlieb, epithelial attachment.

- laminated e. SYN: stratified e..

- e. of lens the layer of cuboidal cells lying on the anterior surface of the crystalline lens inside the lens capsule. At the equator the cells elongate and give rise to the lens fibers. SYN: e. lentis.

- e. lentis SYN: e. of lens.

- mesenchymal e. the flat e. derived from mesenchymal cells found lining certain connective tissue spaces such as the anterior chamber of eye, perilymph spaces in the ear, and subdural and subarachnoid spaces.

- muscle e. SYN: myoepithelium.

- olfactory e. an e. of the pseudostratified type that contains olfactory, receptor, nerve cells whose axons extend to the olfactory bulb of the brain.

- pavement e. SYN: simple squamous e..

- pigment e. e. composed of cells containing granules of pigment or melanin, as in the retinal or iris pigment layer.

- pigment e. of optic retina See retina.

- pseudostratified e. an e. that gives a superficial appearance of being stratified because the cell nuclei are at different levels, but in which all cells reach the basement membrane, hence it is classed as a simple e..

- reduced enamel e. SYN: enamel e..

- respiratory e. the pseudostratified ciliated e. that lines the conducting portion of the airway, including part of the nasal cavity and larynx, the trachea, and bronchi.

- e. of semicircular duct the simple squamous e. of the semicircular ducts. SYN: e. ductus semicircularis.

- seminiferous e. the e. lining the convoluted tubules of the testis where spermatogenesis and spermiogenesis occur.

- simple e. an e. having one layer of cells.

- simple squamous e. e. composed of a single layer of flattened scalelike cells, such as mesothelium, endothelium, and that in the pulmonary alveoli. SYN: pavement e..

- stratified e. a type of e. composed of a series of layers, the cells of each varying in size and shape. It is named more specifically according to the type of cells at the surface, e.g., stratified squamous e., stratified columnar e., stratified ciliated columnar e.. SYN: laminated e..

- stratified ciliated columnar e. an e. consisting of several layers of cells with the deeper cells being polyhedral in form and the surface ones columnar with motile cilia, such as that which lines the fetal esophagus.

- stratified squamous e. an e. consisting of several layers of keratin containing cells in which the surface cells are flattened and scale-like and the deeper cells are polyhedral in form. Keratin filaments become progressively more abundant toward the surface, which on the dry surfaces of the body may consist of a layer of dead corneocytes.

- sulcular e. SYN: crevicular e..

- surface e. 1. a layer of celomic epithelial cells covering the gonadal ridges; 2. the mesothelial covering of the definitive ovary.

- transitional e. a highly distensible pseudostratified e. with large polyploid superficial cells that are cuboidal in the relaxed state but broad and squamous in the distended state; occurs in the kidney, ureter, and bladder.

* * *

ep·i·the·li·um .ep-ə-'thē-lē-əm n, pl -lia -lē-ə a membranous cellular tissue that covers a free surface or lines a tube or cavity of an animal body and serves esp. to enclose and protect the other parts of the body, to produce secretions and excretions, and to function in assimilation

* * *

n.

the tissue that covers the external surface of the body and lines hollow structures (except blood and lymphatic vessels). It is derived from embryonic ectoderm and endoderm. Epithelial cells may be flat and scalelike (squamous), cuboidal, or columnar. The latter may bear cilia or brush borders or secrete mucus or other substances (see goblet cell). The cells rest on a common basement membrane, which separates epithelium from underlying connective tissue. Epithelium may be either simple, consisting of a single layer of cells; stratified, consisting of several layers; or pseudostratified, in which the cells appear to be arranged in layers but in fact share a common basement membrane. See also endothelium, <

• epithelial adj.

* * *

ep·i·the·li·um (ep″ĭ-theґle-əm) pl. epitheґlia [epi- + Gr. thēlē nipple] the covering of internal and external surfaces of the body, including skin and the lining of vessels and other small cavities. It consists of cells joined by small amounts of cementing substances. Epithelium is classified into types on the basis of the number of layers deep and the shape of the superficial cells; see illustration. epithelial adj

Epithelium of different types. (A), Simple squamous epithelium (arrows) and simple cuboidal epithelium (arrowheads); (B), simple columnar epithelium; (C), pseudostratified columnar epithelium; (D), stratified squamous (nonkeratinized) epithelium; (E), stratified cuboidal epithelium; (F), transitional epithelium.