Structure of intercellular junctions in different segments of the intrapulmonary vasculature.

Freeze-fracture studies have shown that there is a segmental differentiation of the structure of intercellular junctions of intraacinar pulmonary vessels. Tight junctions vary in complexity, being composed in the arterial segment of the most highly interconnected and most numerous (two to seven) rows of tight junction particles, which are present in E face grooves. In capillaries, they consist of one to three rows of particles, which, in the venular segment, show some discontinuities. Tight junctions in intraacinar veins consist of one to five rows of particles in the E face. Large gap junctions are numerous and are intimately associated with arterial tight junctions. They are absent from capillaries, and are fewer in number and smaller in size in veins than in arteries. The above observations suggest that, as in the systemic circulation, the venular segment is likely to be the most permeable to small, water-soluble solutes. The presence of numerous gap junctions, particularly in the arteries, suggests that in this segment there is a high degree of electrotonic and/or metabolic coupling between endothelial cells.