epithelium, 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.

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.

Classification

Epithelial cells are classified by the following three factors:

Shape (of most superficial cells)

Squamous

All Squamous cells are soft cells with an irregular shape. A one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane, and the endothelium of capillaries, and is a minimal barrier to diffusion. Squamous cells can be found included in 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 the same size as its height. The nuclei of these cells are usually located in the center. The cuboidal epithelium forms the smallest.

Columnar

These cells are taller than they are wide. 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 hairlike projections called microvilli. They 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.

Stratification

There is clearly more than one layer of cells.

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 upper edge of 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).

Specializations

• 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 fussion. absorbtion is also a main key in this .

Examples

System	Tissue	Epithelium	Subtype
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	-
nervous	ependyma	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	renal pelvis	Transitional	urothelium
urinary	ureter	Transitional	urothelium
urinary	urinary bladder	Transitional	urothelium
urinary	prostatic urethra	Transitional	urothelium
urinary	membranous urethra	Pseudostratified columnar, non-ciliated	-
urinary	penile urethra	Pseudostratified columnar, non-ciliated	-
urinary	external urethral orifice	Stratified squamous	-

Cell junctions

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.

Secretory epithelia

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

Embryology

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.

Additional images

References

• Molecular Biology of the Cell, 4th edition, Alberts et al., 2002