A new occludens-like junction linking endothelial cells of small capillaries (probably venules) of rat jejunum.

Freeze-cleave replicas of small capillaries of rat jejunum have revealed the presence of a new type of junction linking endothelial cells. This new junction reveals tight junctions (zonulae occludentes) in that the adjacent plasma membranes are held closely together along lines of attachment organized in the form of a loose, but frequently discontinuous network. In contrast to tight junctions, the A-face ridges possess a very low profile, and only at low shadowing angles can a repeating, particulate substructure occasionally be resolved. The shallow B-face furrows lack any particulate components. Images of cross-fractured focal points of attachment suggest that the external leaflets of adjacent membranes are closely apposed but not actually fused, as is the case with zonulae occludentes. It appears that this new type of endothelial junction is characteristic of small venules. Thus we propose that it be termed small venule endothelial junction.