Epithelium

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.

Epithelial cells also are found lining the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands.

Endothelium (the inner lining of blood vessels) is a specialized form of epithelium.

Classification

Epithelial cells and the epithelium (collection of cells, or tissue) are classified by the following three factors:

• Shape
• Stratification
• Specializations

Shape

• 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.

Stratification

• 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.

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.

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	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	-
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	-
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	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	-

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.

References

ISBN links support NWE through referral fees

• 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.