Because of its location in the terminal or apical portion of the cell and its barlike configuration, the stainable material visible in light microscopy was called the terminal bar. It is now evident that intercellular cement as such does not exist. The terminal bar, however, does represent a significant structural complex. Electron microscopy has shown that it includes a specialized site that joins epithelial cells (Fig. 5.14a). It is also the site of a considerable barrier to the passage (diffusion) of substances between adjacent epithelial cells. The specific structural components that make up the barrier and the attachment device are readily identified with the EM and are collectively referred to as a junctional complex (see Table 5.4, page 135). These complexes are responsible for joining individual cells together. There are three types of junctional complexes (Fig. 5.14b):  Occluding junctions are impermeable and allow epithelial cells to function as a barrier. Also called tight junctions, occluding junctions form the primary intercellular diffusion barrier between adjacent cells.
By limiting the movement of water and other molecules through the intercellular space, they maintain physico chemical separation of tissue compartments. Because they are located at the most apical point between adjoin ing epithelial cells, occluding junctions prevent the mi gration of lipids and specialized membrane proteins between the apical and lateral surfaces, thus maintaining the integrity of these two domains. In addition, occlud ing junctions recruit various signaling molecules to the cell surface and link them to the actin flaments of the cell cytoskeleton.
FIGURE 5.12 • Intrafagellar transport mechanism within the cilium. Assembly and maintenance of cilia depends on the intraflagellar transport mechanism (IFT ) that utilizes raftlike platforms. They move up and down between the outer doublets of microtubules and plasma membrane of the elongating cilium. Cargo molecules (including inactive cytoplasmic dynein) are loaded onto the IFT platform while it is docked near the base of the cilium. Using kinesin II as a motor protein, the fully loaded platform is moved upward toward the plus end of microtubules at the tip of the cilium (anterograde transport). The cargo is then unloaded at the tip of the cilium (the site of axoneme assembly). Here particles turn around, and the platform powered by cytoplasmic dynein heads back to the base of the cilium (retrograde transport) after picking up turnover products (including inactivated kinesin II). Inset. Electron micrograph of a longitudinal section of a Chlamydomonas flagellum with two groups of IFT platforms. 55,000. (Reprinted with permission from Pedersen LB, Veland IR, Schrer JM, and Christensen ST. Assembly of primary cilia. Dev Dyn. 2008; 237:1993–2006.)
FIGURE 5.13 • Terminal bars in pseudostratifed epithelium.
Photomicrograph of an H&E–stained specimen showing the terminal bars in a pseudostratified epithelium. The bar appears as a dot (arrowheads) when seen on its cut edge. When the bar is coursing parallel to the cut surface and lying within the thickness of the section, it is seen as a linear or barlike profile (arrows). 550.
FIGURE 5.14 • Junctional complex. a. Electron micrograph of the apical portion of two adjoining epithelial cells of the gastric mucosa, showing the junctional complex. It consists of the zonula occludens (ZO), zonula adherens (ZA), and macula adherens (MA). 30,000. b. Diagram showing the distribution of cell junctions in the three cellular domains of columnar epithelial cells. The apical domain with microvilli has been lifted to better illustrate spatial arrangements of junctional complexes within the cell.