CELL-JUNCTIONS IN THE ENDOTHELIA AND CONNECTIVE-TISSUE OF THE RAT CHOROID-PLEXUS

Cell junctions between endothelial cells and between connective tissue cells (leptomeninges) of the rat choroid plexus were studied by freeze‐fracture. Special attention was paid to the problems of identifying fractured cell membranes and fracture faces. Tight junctions in arterioles are comprised of continuous small‐meshed networks of strands appearing as shallow grooves with aligned particles on E‐faces, and low ridges with some particles on P‐faces. In the fenestrated capillaries the tight junctions are represented by discontinuous, isolated slightly elevated ridges with some particles on P‐faces, and corresponding shallow grooves devoid of particles on E‐faces. Junctions in venules appeared as a less developed version of the junctions in the fenestrated capillaries. Extended areas of the meningeal cell membranes exhibit cell junctions. Tight junctions in this tissue are comprised of a loose discontinuous arrangement of isolated, often branched junctional elements. On P‐faces the elements appear as elongated crests with a distinct furrow running along the edge of the crest, occupied by a few particles. On E‐faces corresponding elongated impressions are present and occupied by aligned particles and a few short bars. The arrangement of these tight junction elements are not supportive of any significant barrier function. Small gap junctions occur “freely” in the membrane, or they are sometimes surrounded by tight junction elements. Very large gap junctions (up to about 10,000 particles) are mostly surrounded and interconnected by tight junctional elements. The possible significance of the variability of gap junction size, and the relation to tight junction elements is discussed. Copyright © 1979 Wiley‐Liss, Inc.