Ultrastructure of the microvessels of the iris in mammals with special reference to their permeability.

The ultrastructure of the microvessels of the guinea pig, rat, rabbit and cat was studied in an effort to discover the morphological basis of the permeability of the iris vessels. The microvessels of the iris revealed a non-fenestrated endothelium. A relatively wide intercellular space and macula occludens were seen at the intercellular junctions of the endothelial cells in the capillaries and postcapillary venules. It is concluded that these intercellular spaces may form the small pore system which makes the diffusion through the iris vessels possible. Transcapillary exchange of high molecular substances is made possible by the vesicular transport system of the micropinocytosis vesicles.