Zonula occludens. a. Electron micrograph of the zonula occludens, showing the close approximation of the outer lamellae of adjoining plasma membranes. The extracellular domains of proteins involved in the formation of this junction (occludins) appear as a single electron-dense line (arrows), x 100,000. b. Diagram showing the organization and pattern of distribution of the transmembrane protein

The arrangement of the various junctional proteins in forming the zona occludens seal is best visualized by the freeze fracture technique (Fig. 4.10). When the plasma membrane is fractured at the site of the zonula occludens, the junctional proteins are observed on the P-face of the

occludin within the occluding junction. Compare the linear pattern of grooves with the ridges detected in the freeze-fracture preparation in Figure 4.10. c. Diagram showing the occludin molecule and the major associated proteins of the occluding junction. Note that one of the associated proteins, ZO-1, interacts with the cell cytoskeleton binding actin filaments.

membrane, where they appear as ridge-like structures. The opposing surface of the fractured membrane, the E-face, reveals complementary grooves that result from detachment of the protein particles from the opposing surface. The ridges and grooves are arranged as a network of anastomosing strands, thus creating a functional seal within the intercellular space. They correspond to the location of the rows of transmembrane proteins.

Observations of different kinds of epithelia reveal that the complexity and number of strands forming the zonulae occludentes varies. In epithelia in which anastomosing strands or fusion sites are sparse, such as certain kidney tubules, the intercellular pathway is partially permeable to water and solutes. In contrast, in epithelia in which the strands are numerous and extensively intertwined, e.g., in-

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