Classification Of Epithelium

The traditional classification of epithelium is descriptive and based on two factors: the number of cell layers and the shape of the surface cells. The terminology, therefore, reflects only structure, not function. Thus, epithelium is described as

• Simple, when it is one cell layer thick

• Stratified, when it has two or more cell layers

The individual cells that compose an epithelium are described as

• Squamous, when the width of the cell is greater than its height

• Cuboidal, when the width, depth, and height are approximately the same

• Columnar, when the height of the cell appreciably exceeds the width (the term low columnar is often used when a cell's height only slightly exceeds its other dimensions)

Thus, by describing the number of cell layers (i.e., simple or stratified) and the surface cell shape, the various configurations of epithelia are easily classified. The cells in some exocrine glands are more or less pyramidal, with their apices directed toward a lumen. However, these cells are still classified as either cuboidal or columnar, depending on their height relative to their width at the base of the cell.

In a stratified epithelium, the shape and height of the cells usually vary from layer to layer, but only the shape of the cells that form the surface layer is used in classifying the epithelium. For example, stratified squamous epithe lium consists of more than one layer of cells, and the surface layer consists of flat or squamous cells.

In some instances, a third factor—specialization of the apical cell surface domain—can be added to this classification system. For example, some simple columnar epithelia are classified as simple columnar ciliated when the apical surface domain possesses cilia. The same principle applies to stratified squamous epithelium, in which the surface cells may be keratinized or nonkeratinized. Thus, epidermis would be designated as stratified squamous keratinized epithelium because of the keratinized cells at the surface.

Pseudostratified epithelium and transitional epithelium are special classifications of epithelium

Two special categories of epithelium are pseudostratified and transitional.

• Pseudostratified epithelium appears stratified, although some of the cells do not reach the free surface; all rest on the basement membrane. Thus, it is actually a simple epithelium. The distribution of pseudostratified epithelium is limited in the body. Also, it is often difficult to discern whether all of the cells contact the basement membrane. For these reasons, identification of pseudostratified epithelium usually depends on knowing where it is normally found.

• Transitional epithelium (urothelium) is a term applied to the epithelium lining the lower urinary tract, extending from the minor calyces of the kidney down to the proximal part of the urethra. Urothelium is a stratified epithelium with specific morphologic characteristics that allow it to distend. This epithelium is described in Chapter 19.

The cellular configurations of the various types of epithelia and their appropriate nomenclature are illustrated in Table 4.1.

Endothelium and mesothelium are the simple squamous epithelia lining the vascular system and body cavities

Specific names are given to epithelium in certain locations:

• Endothelium is the epithelial lining of the vascular system.

• Mesothelium is the epithelium that lines the walls and covers the contents of the closed cavities of the body, i.e., the abdominal, pericardial, and pleural cavities.

Both endothelium and mesothelium are almost always simple squamous epithelia. An exception is found in postcapillary venules of certain lymphatic tissues, where the endothelium is cuboidal. These venules are called high endothelial venules (HEV). Another exception is found in the spleen, in which endothelial cells of the venous si-

TABLE 4.1. Types of Epithelium

Classification

Some Typical Locations

Major Function

Simple squamous

Simple cuboidal

Simple columnar

Pseudostratified

Stratified squamous

Stratified cuboidal

Vascular system (endothelium)

Body cavities (mesothelium) Bowman's capsule (kidney) Respiratory spaces in lung

Small ducts of exocrine glands Surface of ovary (germinal epithelium) Kidney tubules

Small intestine and colon Stomach lining and gastric glands Gallbladder

Trachea and bronchial tree

Ductus deferens

Efferent ductules of epididymis

Epidermis

Oral cavity and esophagus Vagina

Sweat gland ducts

Large ducts of exocrine glands

Anorectal junction

Exchange, barrier in central nervous system

Exchange and lubrication

Barrier

Exchange

Absorption, conduit Barrier

Absorption and secretion

Absorption and secretion

Secretion

Absorption

} Secretion, conduit Absorption, conduit

Barrier, protection

Barrier, conduit

Stratified columnar

Transitional (urothelium)

Largest ducts of exocrine glands Anorectal junction

Renal calyces Ureters Bladder Urethra

Barrier, conduit

Barrier, distensible property nuses are rod-shaped and arranged like the staves of a barrel.

Diverse epithelial functions can be found in different organs of the body

A given epithelium may serve one or more functions, depending on the activity of the cell types that are present:

• Secretion, as in the columnar epithelium of the stomach and the gastric glands

• Absorption, as in the columnar epithelium of the intestines and proximal convoluted tubules in the kidney

• Transport, as in transport of materials or cells along the surface of an epithelium by motile cilia or in transport of materials across an epithelium to and from the connective tissue

• Protection, as in the stratified squamous epithelium of the skin (epidermis) and the transitional epithelium of the urinary bladder

• Receptor function, to receive and transduce external stimuli, as in the taste buds of the tongue, olfactory epithelium of the nasal mucosa, and the retina of the eye

Epithelia involved in secretion or absorption are typically simple or, in a few cases, pseudostratified. The height of the cells often reflects the level of secretory or absorptive activity. Simple squamous epithelia are compatible with a high rate of transepithelial transport. Stratification of the epithelium usually correlates with transepithelial impermeability. Finally, in some pseudostratified epithelia, basal cells are the stem cells that give rise to the mature functional cells of the epithelium, thus balancing cell turnover.

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