 Pseudostratifed epithelium appears stratified, although some of the cells do not reach the free surface; all rest on the basement membrane (Plate 2, page 154). 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 stratifed epithelium with specific morphologic characteristics that allow it to distend (Plate 3, page 156). This epithelium is described in Chapter 20.
The cellular configurations of the various types of epithelia and their appropriate nomenclature are illustrated in Table 5.1.
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 blood and lymphatic vessels.
 Endocardium is the epithelial lining of ventricles and atria of the heart.
 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; Plate 1, page 152).
Both endothelium and endocardium, as well as mesothelium, are almost always simple squamous epithelia. An exception is found in postcapillary venules of certain lymphatic tissues in which 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 sinuses 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;  transportation, as in the transport of materials or cells along the surface of an epithelium by motile cilia or in the 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; and  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.
Epithelial cells exhibit distinct polarity. They have an apical domain, a lateral domain, and a basal domain. Specific biochemical characteristics are associated with each cell surface. These characteristics and the geometric arrangements of the cells in the epithelium determine the functional polarity of all three cell domains.