The main transmembrane proteins forming these strands are the claudins, which are essential for tight-junction formation and function. Mice that lack the claudin-1 gene, for example, fail to make tight junctions between the cells in the epidermal layer of the skin; as a result, the baby mice lose water rapidly by evaporation through the skin and die within a day after birth. Conversely, if nonepithelial cells such as fibroblasts are artificially caused to express claudin genes, they (A) (B) 0.5 µm 0.5 µm
Figure 19–18 The role of tight junctions in transcellular transport. For clarity, only the tight junctions are shown. transport proteins are confined to different regions of the plasma membrane in epithelial cells of the small intestine. this segregation permits a vectorial transfer of nutrients across the epithelium from the gut lumen to the blood. In the example shown, glucose is actively transported into the cell by Na+-driven glucose transporters at its apical surface, and it leaves the cell through passive glucose transporters in its basolateral membrane. tight junctions are thought to confine the transport proteins to their appropriate membrane domains by  acting as diffusion barriers, or “fences,” within the lipid bilayer of the plasma membrane; these junctions also block the backflow of glucose from the basal  side of the epithelium into the gut lumen (see Movie 11.2).
Figure 19–19 The role of tight junctions in allowing epithelia to serve as barriers to solute diffusion. (a) the drawing shows how a small extracellular tracer molecule added on one side of an epithelium is prevented from crossing the epithelium by the tight junctions that seal adjacent cells together. adherens junctions and other cell junctions are not shown for clarity.
(B) Electron micrographs of cells in an epithelium in which a small, extracellular, electron-dense tracer molecule has been added to either the apical side (on the left) or the basolateral side (on the right). the tight junction blocks passage of the tracer in both directions. (B, courtesy of Daniel Friend.) (A) lateral plasmaridges of transmembrane focal connection focal connection plasma membrane 50 nm0.5 µm intestinal lumenmicrovilli (C)(B) tightjunctioncell 1 cell 2
Figure 19–20 The structure of a tight junction between epithelial cells of the small intestine. the junctions are shown (a) schematically, (B) in a freeze-fracture electron micrograph, and (C) in a conventional electron micrograph. In (B), the plane of the micrograph is parallel to the plane of the membrane, and the tight junction appears as a band of branching sealing strands that encircle each cell in the epithelium (see Figure 19–21a). In (C), the junction is seen in cross section as a series of focal connections between the outer leaflets of the two interacting plasma membranes, each connection corresponding to a sealing strand in cross section. (B and C, from N.B. Gilula, in Cell Communication [r.p. Cox, ed.], pp. 1–29. New York: Wiley, 1974.) will form tight-junctional connections with one another. Normal tight junctions also contain a second major transmembrane protein called occludin, which is not essential for the assembly or structure of the tight junction but is important for limiting junctional permeability. A third transmembrane protein, tricellulin, is required to seal cell membranes together and prevent transepithelial leakage at the points where three cells meet.