epithelium

(ĕp'əthē`lēəm), sheet of tissue that covers or lines the external and internal body surfaces. The epithelium is closely packed, has little intercellular material, and is lacking in blood vessels. There are three characteristic types of epithelial cells: squamous, cuboidal, and columnar. Squamous epithelial cells are flat and often overlapping; they compose the outer layer of skin (epidermis) and line certain internal cavities, e.g., the mouth. Cuboidal epithelial cells are rounded and elastic and line such structures as the urinary bladder, where, by stretching and becoming flatter, they increase the organ's capacity to hold fluid. The cells of the columnar epithelium are long and thin; they are found as a single layer of secretory and absorptive cells in the gastrointestinal tract, and they form the ciliated lining of the respiratory tract. Embryologically, epithelium may be derived from any of the three germ layers, i.e., ectoderm, mesoderm, and endoderm, and may be classified accordingly as epithelium proper, mesothelium, and endothelium.

Epithelium

One of the four primary tissues of the body, which constitutes the epidermis and the lining of respiratory, digestive, and genitourinary passages. The major characteristic of epithelium is that the cells are close together, separated by a very small amount of intercellular substance. Epithelium may be derived from any of the three primary germ layers of the very early embryo—ectoderm, entoderm, or mesoderm. With very few exceptions, epithelium is free of blood vessels.

The functions of epithelium are varied and include (1) protective function, by completely covering the external surface (including the gastrointestinal surface—and the surface of the whole pulmonary tree including the alveoli); (2) secretory function, by secreting fluids and chemical substances necessary for digestion, lubrication, protection, excretion of waste products, reproduction, and the regulation of metabolic processes of the body; (3) absorptive function, by absorbing nutritive substances and preserving water and salts of the body; (4) sensory function, by constituting important parts of sense organs, especially of smell and taste; and (5) lubricating function, by lining all the internal cavities of the body, including the peritoneum, pleura, pericardium, and the tunica vaginalis of the testis.

The forces which hold the epithelial cells together are not satisfactorily understood. The intercellular substance between the cells, also called cement substance, is undoubtedly important. The interdigitation of adjacent cell surfaces and the occurrence of intercellular bridges in certain cells may also be important in holding the cells together. Finally, in certain cells local modifications of contiguous surfaces and the intervening intercellular substances, which together form the terminal bars, may be effective in the same way.

The outstanding property of the arrangement of most of the epithelium of the body is the economy of space achieved in the face of a broad exposure of the cell surfaces. The efficiency is achieved by the presence of numerous folds, which may be gross or microscopic and temporary or permanent. A part must also be attributed to the surface specialization of the epithelial cells themselves, such as their minute, fingerlike processes. Another specialization of the surface or epithelial cell is the occurrence of motile cilia. See Cilia and flagella

Cellular arrangements in epithelial tissues

Classification of epithelia is based on morphology, that is, on the shape of the cells and their arrangement (see illustration):

I. Single-layered.
- A. Squamous (mesothelium, descending loop of Henle in the kidney)—thin, flat.
- B. Cuboidal (duct, thyroid, choroid plexus)—cubelike.
- C. Columnar (intestine), sometimes ciliated (Fallopian tube, or oviduct)—tall.
II. Multiple-layered or stratified.
- A. Squamous (skin, esophagus, vagina)—superficial cells thin and flat, deeper cells cuboidal and columnar.
- B. Columnar (pharynx, large ducts of salivary glands), sometimes ciliated (larynx)—two or more layers of tall cells.
III. Pseudostratified (male urethra), sometimes ciliated (respiratory passages)—all cells reach to basement membrane but some extend toward the surface only part of the way, while others reach the surface.
IV. Transitional (urinary bladder)—like stratified squamous in the fully distended bladder; in the empty bladder, superficial cells rounded, almost spherical.

An important property of epithelium is the ability of its cells to glide over surfaces. This allows replacement of dead cells to take place in the normal state, while presenting a closed surface to the external environment; replacement is especially important in wound repair. Gliding ability is also manifested normally in the movement of cells which slide over each other in transitional epithelium, for example, when the urinary bladder is being distended or contracted. See Gland

Epithelium

(1) A tissue of multicellular animals. The epithelium, found on the body surface, also lines all cavities of the body as a layer of cells. It is the prevalent component of glands. The epithelium has a high capacity for regeneration. The two main types of epithelium are the stratified (laminated) epithelium and the glandular epithelium. The epithelium is underlain by a basal membrane. It contains no blood vessels but receives nourishment from the underlying connective tissue.

The epithelium performs various functions: demarcation, protection, absorption, excretion, and secretion. The structure of the cells reflects functional specialization. For example, cells specializing in absorption have a brush border of microvilli that greatly increases the area of contact with the environment. Others are equipped with cilia. The protective epithelium has the capacity to form horny scales, and the glandular epithelium is distinguished by its well-developed granular endoplasmic reticulum and Golgi apparatus.

(2) In plants, a layer of thin-walled parenchymatous excretory cells lining the inner surface of certain plant organs, for example, resin ducts in conifers.

epithelium

[‚ep·ə′thē·lē·əm] (histology) A primary animal tissue, distinguished by cells being close together with little intercellular substance; covers free surfaces and lines body cavities and ducts.

epithelium

an animal tissue consisting of one or more layers of closely packed cells covering the external and internal surfaces of the body. The cells vary in structure according to their function, which may be protective, secretory, or absorptive

epithelium

[ep″ĭ-the´le-um] (pl. epithe´lia) (Gr.) the cellular covering of internal and external surfaces of the body, including 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.ciliated epithelium epithelium bearing vibratile, hairlike processes (cilia) on its free surface.columnar epithelium epithelium whose cells are of much greater height than width.cuboidal epithelium epithelium whose cells are of approximately the same height and width, and appear square in transverse section.germinal epithelium thickened peritoneal epithelium covering the gonad from earliest development; formerly thought to give rise to germ cells.glandular epithelium that composed of secreting cells.pigmentary epithelium (pigmented epithelium) that made of cells containing granules of pigment.sense epithelium (sensory epithelium) neuroepithelium (def. 1).simple epithelium that composed of a single layer of cells.squamous epithelium that composed of flattened platelike cells.stratified epithelium epithelium made up of cells arranged in layers.transitional epithelium a type characteristically found lining hollow organs, such as the urinary bladder, that are subject to great mechanical change due to contraction and distention; originally thought to represent a transition between stratified squamous and columnar epithelium.

ep·i·the·li·um

, pl.

ep·i·the·li·a

(ep'i-thē'lē-ŭm, -ă), [TA] The purely cellular avascular layer covering all free surfaces, cutaneous, mucous, and serous, including the glands and other structures derived therefrom. [G. epi, upon, + thēlē, nipple, a term applied originally to the thin skin covering the nipples and the papillary layer of the border of the lips]

epithelium

(ĕp′ə-thē′lē-əm)n. pl. epithe·lia (-lē-ə) or epithe·liums Membranous tissue composed of one or more layers of cells separated by very little intercellular substance and forming the covering of most internal and external surfaces of the body and its organs.

ep′i·the′li·al adj.

ep·i·the·li·um

, pl. epithelia (ep'i-thē'lē-ŭm, -ă) [TA] The purely cellular avascular layer covering all the free surfaces, cutaneous, mucous, and serous, including the glands and other structures derived therefrom. [G. epi, upon, + thēlē, nipple, a term applied originally to the thin skin covering the nipples and the papillary layer of the border of the lips]

epithelium

(ep?i-the'le-um) ('le-a) plural.epithelia [ epi- + Gr. thele, nipple, teat + -ium (2)]

GLANDULAR EPITHELIUM: (Orig. mag. ×430)The layer of cells forming the epidermis of the skin and the surface layer of mucous and serous membranes. The cells rest on a basement membrane and lie in close approximation with little intercellular material between them. They are devoid of blood vessels. The epithelium may be simple, consisting of a single layer, or stratified, consisting of several layers. Cells making up the epithelium may be flat (squamous), cube-shaped (cuboidal), or cylindrical (columnar). Modified forms of epithelium include ciliated, pseudostratified, glandular, and neuroepithelium. The epithelium may include goblet cells, which secrete mucus. Stratified squamous epithelium may be keratinized for a protective function or abnormally keratinized in pathological response. Squamous epithelium is classified as endothelium, which lines the blood vessels and the heart, and mesothelium, which lines the serous cavities. Epithelium serves the general functions of protection, absorption, and secretion, and specialized functions such as movement of substances through ducts, production of germ cells, and reception of stimuli. Its ability to regenerate is excellent; it may replace itself as frequently as every 24 hr. See: illustration; skinepithelial (-al), adjective

ciliated epithelium

Epithelium with hairlike processes on the surface that wave actively only in one direction. This type is present in the respiratory tract and fallopian tubes.

columnar epithelium

Epithelium composed of cylindrical cells.

cuboidal epithelium

Epithelium consisting of cube-shaped or prismatic cells with height about equal to their width.

germinal epithelium

1. Epithelium that covers the surface of the genital ridge of the urogenital folds of an embryo. It gives rise to the seminiferous tubules of the testes and the surface layer of the ovary. It was once thought to produce the germ cells (spermatozoa and ova). 2. The epithelium that covers the surface of a mature mammalian ovary.

glandular epithelium

Epithelium consisting of secretory cells.

junctional epithelium

A band of nonkeratinized stratified squamous epithelium that attaches both to the gingiva (on one side) and the crown of the tooth (on the other). Synonym: epithelial attachment; gingival cuff

laminated epithelium

Stratified epithelium.

mesenchymal epithelium

Squamous epithelium that lines the subarachnoid and subdural cavities, the chambers of the eye, and the perilymphatic spaces of the ear.

pavement epithelium

Epithelium consisting of flat, platelike cells in a single layer.

pigmented epithelium

Epithelium containing pigment granules.

pseudostratified epithelium

Epithelium in which the bases of cells rest on the basement membrane but the distal ends of some do not reach the surface. Their nuclei lie at different levels, giving the appearance of stratification.

reduced enamel epithelium

Combined epithelial layers of the enamel organ, which form a protective layer over the enamel crown as it erupts and then become the primary epithelial attachment surrounding the tooth.

squamous epithelium

The flat form of epithelial cells.

stratified epithelium

Epithelium with the cells in layers; mitosis takes place in the lowest layer. Synonym: laminated epithelium

sulcular epithelium

The nonkeratinized epithelium that lines the gingival sulcus.

transitional epithelium

A form of stratified epithelium in which the cuboidal cells adjust to mechanical changes such as stretching and recoiling. This type of tissue is found only in the urinary system (renal pelvis, ureter, bladder, and a part of the urethra).

epithelium

The non-stick coating cell layer for all surfaces of the body except the insides of blood and lymph vessels. Epithelium may be single-layered, or ‘stratified’ and in several layers, with the cells becoming flatter and more scaly towards the surface, as in the skin. It may be covered with fine wafting hair-like structures (cilia), as in the respiratory tract, and it may contain mucus-secreting ‘goblet’ cells. See also ENDOTHELIUM and EPIDERMIS.

Fig. 153 Epithelium (1) . Types of epithelium.

epithelium

(in animals) a layer of covering cells lying on a basement membrane that is called simple epithelium when one cell thick and compound epithelium when several cells thick, and usually covers connective tissue embryologically derived from the ECTODERM. The cells sometimes have a secretory function and are held together by a cementing substance to form a sheet. Their shape gives rise to names descriptive of the cells, e.g. columnar, cubical, squamous (see Fig. 153 ). Where the epithelium is more than one cell thick it is described as stratified. Similar cells can be derived from MESODERM and are referred to as mesothelium when lining the COELOM, and as ENDOTHELIUM when lining blood vessels.
(in plants) a layer of cells lining cavities and secretory canals, for example, resin canals.

Epithelium

Cells composing the lining of an organ.Mentioned in: Colon Cancer, Rectal Cancer

ep·i·the·li·um

, pl. epithelia (ep'i-thē'lē-ŭm, -ă) [TA] The purely cellular avascular layer covering all free surfaces, cutaneous, mucous, and serous, including the glands and other structures derived therefrom. [G. epi, upon, + thēlē, nipple, a term applied originally to the thin skin covering the nipples and the papillary layer of the border of the lips]

epithelium

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ep·i·the·li·um

ep·i·the·li·a

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epithelium

ciliated epithelium

columnar epithelium

cuboidal epithelium

germinal epithelium

glandular epithelium

junctional epithelium

laminated epithelium

mesenchymal epithelium

pavement epithelium

pigmented epithelium

pseudostratified epithelium

reduced enamel epithelium

squamous epithelium

stratified epithelium

sulcular epithelium

transitional epithelium

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epithelium

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ep·i·the·li·um

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Synonyms for epithelium

noun membranous tissue covering internal organs and other internal surfaces of the body

Synonyms

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