The structure of tight junctions in the ciliary epithelium.

The tight junctions of the ciliary epithelium act as a barrier preventing the passage of blood borne macromolecules into the posterior chamber. The use of the freeze-fracture technique has led to a good knowledge of their morphological pattern in various species. However, in order to attempt a correlation of the morphology of the tight junctions with their physiological properties, their intimate substructure must be considered. As in other glutaraldehyde-fixed epithelia, the tight junctions appear as networks (variable in their apico-basal thickness) of more or less discontinuous P-face ridges and as complementary E-face furrows in which some particles or short bars are found. The significance of the discontinuities of ridges has been analysed. The continuity of the junctional fibrils was evident as assessed both by quantitative measurements as well as morphological examination of complementary fracture faces. In addition, the absence of loss of junctional material showed that the integrity of the junction was preserved during the freeze-fracture process, even in conditions where an increase in "transfer" of junctional elements was experimentally induced. Most of all, a "pore system" due to visualizable gaps in the fibrils is not tenable for the ciliary epithelium. Furthermore, the analysis of transition steps at the level of membrane "fusion" showed that the tight junctions of the ciliary epithelium must now be considered as formed by two slightly offset fibrils, one per adjacent plasma membrane.

Quantitative analysis of tight junctions during ciliary epithelium development.

The tight junctions of the ciliary epithelium constitute the main morphological counterpart of the blood-aqueous barrier. Although well-known in the adult, they have only been poorly studied during the morphogenesis of the ciliary processes. We thus analysed them during this period in order to detect whether any changes appear in their general pattern. In the rat, results show that during ciliary body development, the junctional depth and the number of superimposed junctional fibrils decrease, the P-face intramembranous particle density within and outside the junctional domains increase, and particular structures, namely the 'complex strands', are numerous at the early stages and become rare in the adult. Thus, the tight junctions appear as non-stable structures during ciliary-body development and these modifications in their morphology may correspond to changes in barrier properties during the development of this tissue.

Selective and reversible breakdown of the tight junctional barrier in the rabbit ciliary body induced by arachidonic acid. A tracer and freeze-fracture study.

Arachidonic acid induced a reversible breakdown of the blood-aqueous barrier in the iridial processes of the rabbit ciliary body, whereas the ciliary processes were not affected under the conditions of this study. The focal passage of horseradish peroxidase through the tight junctions was demonstrated. In freeze-fracture images, this breakdown of the barrier function was associated with focal changes: an alteration in the geometry of the tight junctional network; an increase in the number of discontinuities of the P-face ridges and particles and short bars in the E-face furrows; an increase in the density of intramembranous particles within the junctional area. The significance of the ultrastructural alterations of the tight junctions as a structural counterpart of the physiological changes is discussed.