The structure of the tricellular region of endothelial tight junctions of pulmonary capillaries analyzed by freeze-fracture.

This study provides the first description of tight junction organization in the tricellular regions of pulmonary capillary endothelial cells. Three important characteristics of tight junction organization at these corners were observed. First, endothelial tight junctions are discontinuous at these corners where an intramembranous gap which averages 27.4 +/- 2.3 (SE) nm in width and 1.1 + 0.17 (SE) micron in length was observed to cross the depth of the junctional complex and endothelium. Second, the depth of the tricellular region of endothelial tight junctions at these corners was made possible by an overlapping flap provided by one of the three cells, the flap cell, which covers the remaining two adjacent cells. Third, the observations in this study demonstrated that the tricellular regions of endothelial tight junctions are oriented parallel to the plane of the endothelium rather than perpendicular as in epithelium. A model of this organization, based upon freeze-fracture replicas of 16 tricellular regions, thin section data, and scanning electron microscopic data of perfusion fixed guinea pig lungs is provided.