Ultrastructural differentiation of the junctional complex of the avian choroidal epithelium

The junctional complex of the choroidal epithelium of the adult domestic fowl consists of three characteristic components—the zonula occludens, zonula adherens and macula adherens. The quintuple‐layered zonula occludens (tight junction) occupies a juxtaluminal position and extends in depth from 0.2–0.4 μ. As a consequence of the intimate fusion of the outer leaflets of the apposed cell membranes, the tight junction is void of any discernible intercellular space. The zonula adherens normally varies in depth from 0.35–0.55 μ. Moderate to heavy accumulations of fibrillar and granular cytoplasmic material occur adjacent to the inner leaflets of the apposed cell membranes. These dense, cytoplasmic bands mesh with fibrillar elements of the general cytoplasm. The intercellular space of the intermediate junction is frequently attenuated by constrictions in the cell membranes. The tight and intermediate junctions appear to form continuous belts encircling each epithelial cell. The macula adherens (desmosome) is not especially well developed, usually lacking well developed cytoplasmic plaques. The early development of the zonula occludens is marked by the convergence of the apposed cell membranes, starting first at a localized point at or near the basal margin of the presumptive tight junction. Subsequently, convergence occurs at one or a few additional points, often near the middle of the developing tight junction. Finally, preparatory to membrane fusion, the adjacent cell membranes are brought into close apposition throughout the linear extent of the tight junction. Membrane fusion usually commences at or near the lower margin of the zonula occludens, in the area of initial membrane convergence, and then proceeds toward the luminal border. With the onset of membrane convergence, a network of fine fibrillar and granular cytoplasmic material accumulates adjacent to the inner leaflets of the apposed cell membranes. This cytoplasmic development subsides in the tight junction subsequent to membrane fusion but persists in moderate to dense accumulations in the other areas of the junctional complex. Numerous other associated events occurring during the differentiation of the junctional complex are discussed in detail.