Surface-associated vesicles in retinal arterioles and venules.

The surface-associated vesicles in retinal arterioles and venules were studied after fixation in glutaraldehyde-tannic acid or after intravitreal injection of peroxidase or lactoperoxidase. The vesicles were concentrated along the abluminal (basal) surface of the endothelial cells and along the plasma membranes of smooth muscle cells in arterioles and of pericytes in post-capillary venules. They were rarely encountered in the deeper regions of these cells. In perpendicular sections through the cell surface the majority of vesicles were in continuity with the plasma membrane whereas in tangential sections, they appeared to lie "free" in the cytoplasm. All such vesicles were labeled after exposure to tannic acid or to the heme-proteins. Peroxidase-reaction product was never seen in the lumen of the vessels. These observations suggest that the surface vesicles in endothelial cells, smooth muscle cells and pericytes are invaginations of the plasma membrane and are thus not involved in the transcytosis or endocytosis of proteins. The vesicles in the latter two cell types may be involved in some aspect of contractility rather than pinocytosis.