Metazoans, which are thought to have evolved in an ocean-like, saline environment, had to solve a fundamental problem: separating the inside of the organism from the outside. The external surfaces of all metazoan animals, as well as the surfaces of their internal organs, are covered by a sheet-like layer of tissue called an epithelium. Epithelia commonly serve as barriers and protective surfaces, as exemplified by the sheets of epidermal cells that form the skin (see Figure 1-4). Other epithelia are one cell layer thick and line internal organs such as the small intestine, where they are crucial for transport of the products of digestion (e.g., glucose and amino acids) into the blood (see Chapter 11). As discussed in Chapter 20, epithelia in different body locations have characteristic morphologies and functions.

Cells that form epithelial tissues are said to be polarized because their plasma membranes are organized into at least two discrete regions. Typically, the distinct surfaces of a polarized epithelial cell are the apical surface—the “top” of the cell facing the external world—and the basal and lateral (collectively, basolateral) surfaces that face the organism’s interior. As shown in Figure 1-4, the basal surface usually contacts an underlying extracellular matrix, the basal lamina. Specialized junction proteins in the basolateral plasma membrane link adjacent cells together and also bind the cells to the basal lamina.