In some locations, cells are closely apposed to one another but lack a free surface. Although the close apposition of these cells and the presence of a basement membrane would classify them as epithelium, the absence of a free surface more appropriately classifies such cell aggregates as epithelioid tissues. The epithelioid cells are derived from progenitor mesenchymal cells (nondifferentiated cells of embryonic origin found in connective tissue). Although the progenitor cells of these epithelioid tissues may have arisen from a free surface or the immature cells may have had a free surface at some time during development, the mature cells lack a surface location or surface connection. Epithelioid organization is typical of most endocrine glands; examples of such tissue include the interstitial cells of Leydig in the testis (Plate 3, page 156), the lutein cells of the ovary, the islets of Langerhans in the pancreas, the parenchyma of the adrenal gland, and the anterior lobe of the pituitary gland. Epithelioreticular cells of the thymus also may be included in this category. Epithelioid patterns are also formed by accumulations of connective tissue macrophages in response to certain types of injury and infections, as well as by many tumors derived from epithelium.
Epithelium creates a selective barrier between the external environment and the underlying connective tissue.
Covering and lining epithelium forms a sheetlike cellular investment that separates underlying or adjacent connective tissue from the external environment, internal cavities, or fluid connective tissue such as the blood and lymph. Among other roles, this epithelial investment functions as a selective barrier that facilitates or inhibits the passage of specific substances between the exterior (including the body cavities) environment and the underlying connective tissue compartment.
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 and  stratifed 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  cuboidal when the width, depth, and height are approximately the same; and  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 stratifed 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 epithelium 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.