OVERVIEW OF EPITHELIAL STRUCTURE AND FUNCTION / 105 CLASSIFICATION OF EPITHELIUM / 106 CELL POLARITY / 107 THE APICAL DOMAIN AND ITS MODIFICATIONS / 109 Microvilli / 109 Stereocilia / 110 Cilia / 113 THE LATERAL DOMAIN AND ITS SPECIALIZATIONS IN CELL-TO-CELL ADHESION / 121 Occluding Junctions / 124 Anchoring Junctions / 127 Communicating Junctions / 131 Morphologic Specializations of the Lateral Cell Surface / 133 THE BASAL DOMAIN AND ITS SPECIALIZATIONS IN CELL-TO– EXTRACELLULAR MATRIX ADHESION / 134 Basement Membrane Structure and Function / 134 Cell-to–Extracellular Matrix Junctions / 144 Morphologic Modifications of the Basal Cell Surface / 146 GLANDS / 146 EPITHELIAL CELL RENEWAL / 150 Folder 5.1 Clinical Correlation: Epithelial Metaplasia / 109 Folder 5.2 Clinical Correlation: Primary Ciliary Dyskinesia / 120 Folder 5.3 Clinical Correlation: Junctional Complexes as a Target of Pathogenic Agents / 128 Folder 5.4 Functional Considerations: Basement Membrane and Basal Lamina Terminology / 138 Folder 5.5 Functional Considerations: Mucus and Serous Membranes / 150
Epithelium covers body surfaces, lines body cavities, and constitutes glands.
Epithelium is an avascular tissue composed of cells that cover the exterior body surfaces and line internal closed cavities (including the vascular system) and body tubes that communicate with the exterior (the alimentary, respiratory, and genitourinary tracts). Epithelium also forms the  secretory portion (parenchyma) of glands and their ducts. In addition, specialized epithelial cells function as  receptors for the special senses (smell, taste, hearing, and vision).
The cells that make up epithelium have three principal characteristics:  They are closely apposed and adhere to one another by means of specific cell-to-cell adhesion molecules that form specialized cell junctions (Fig. 5.1).
 They exhibit functional and morphologic polarity. In other words, different functions are associated with three distinct morphologic surface domains: a free surface or apical domain, a lateral domain, and a basal domain. The properties of each domain are determined by specific lipids and integral membrane proteins.
 Their basal surface is attached to an underlying basement membrane, a noncellular, protein–polysaccharide-rich layer demonstrable at the light microscopic level by histochemical methods (see Fig. 1.2, page 6).
FIGURE 5.1 • Diagram of small intestine absorptive epithelial cells. a. All three cellular domains of a typical epithelial cell are indicated on the diagram. The junctional complex provides adhesion between adjoining cells and separates the luminal space from the intercellular space, limiting the movement of fluid between the lumen and the underlying connective tissue. The intracellular pathway of fluid movement during absorption (arrows) is from the intestinal lumen into the cell, then across the lateral cell membrane into the intercellular space, and, finally, across the basement membrane to the connective tissue. b. This photomicrograph of a plastic-embedded, thin section of intestinal epithelium, stained with toluidine blue, shows cells actively engaged in fluid transport. Like the adjacent diagram, the intercellular spaces are prominent, reflecting fluid passing into this space before entering the underlying connective tissue. 1,250.
In special situations, epithelial cells lack a free surface (epithelioid tissues).