Columnar epithelial cells single layer

Airway cross-sections have the nominal anatomy shown in Fig. 5.16. Airway surface liquid (AST), primarily composed of mucus gel and water, surrounds the airway lumen with a thickness thought to vary from 5 to 10 mm. AST lies on the apical surface of airway epithelial cells (mostly columnar ciliated epithelium). This layer of cells, roughly two to three cells thick in proximal airways and eventually thinning to a single cell thickness in distal airways, rests along a basement membrane on its basal surface. Connective tissue (collagen fibers, basement membranes, elastin, and water) lies between the basement membrane and airway smooth muscle. Edema occurs when the volume of water within the connective tissue increases considerably. Interspersed within the smooth muscle are respiratory supply vessels (capillaries, arteriovenous anastomoses), nerves, and lymphatic vessels. 

The stomach has a surface epithelium composed of a single layer of columnar cells with few apical microvilli. The epithelial lining of the small intestine consists of a single layer of columnar cells with densely packed microvilli to promote absorption. The epithelium in the large intestine is similar to that in the small intestine except for the absence of villi in the large intestine. 

A single layer of epithelial cells is termed simple epithelium, whereas those composed of more than one layer are termed stratified epithelia. Stratified epithelia are found in areas which have to withstand large amounts of wear and tear, for example the inside of the mouth, orthe skin. Epithelial cells may be, for example, squamous (flattened), columnar (tall), cuboidal (intermediate between squamous and columnar) and may contain surface specializations, such as cilia in the nasal epithelium and keratin in the skin. 

While epithelia are basically simple arrangements made up of single (in simple epithelia) or several (in stratified epithelia) layers of characteristically shaped, e.g. cuboidal, columnar or squamous (i.e. flattened) cells, they are capable of modification to serve a variety of functions in different organs. Thus the epithelial cells in the small intestine have numerous finger-like microvilli projecting from their surfaces. These microvilli greatly increase the surface area and are correlated with the absorptive function of the tissue. 

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