Ultrastructure and formation of diaphragmed fenestrae in cultured endothelial cells of bovine adrenal medulla.

The ultrastructure of diaphragmed fenestrae and the process of their de novo formation were examined in cultured endothelial cells cloned from fenestrated capillaries of bovine adrenal medulla. One clone frequently formed many diaphragmed fenestrae in highly attenuated regions of endothelium during 1-1.5 months of culture on reconstituted collagen gel. Stereo views of thick sections showed round or oval clusters of geometrically arranged fenestrae, each with a central knob. The number of diaphragmed fenestra per cluster was 50 +/- 13/microns 2. The inner diameter of the fenestrae was 50-60 nm. In the cells having small clusters of fenestrae, plasmalemmal vesicles, each having a thin diaphragm with a central knob, were accumulated (56 +/- 18/microns 2) and arranged geometrically, mostly on the basal plasmalemma. At the border between the cytoplasm and the cluster of fenestrae, plasmalemmal vesicles of the basal plasmalemma fused with the opposing apical plasmalemma. A model for the process of fenestrae formation in vitro is proposed.