Freeze-fracture and lanthanum studies of the retinal microvasculature in diabetic rats

To see whether or not blood-retinal barrier breakdown during diabetes was associated with breakdown of the endothelial cell tight junctions or with other membrane alterations in the cells comprising the wall of the retinal microvasculature, streptozotocin-induced diabetic rat retinas were studied using lanthanum tracer and freeze-fracture electron microscopic morphometry. This study showed that endothelial cell tight junction permeability to lanthanum and luminal surface area were normal in these diabetic rats. However, freeze-fracture morphometry showed several alterations in the diabetic retinal microvessels. First, the endothelial cell membranes had abnormally large (80-120 nm) plasmalemmal vesicles not evident in the control retinas, suggesting that membrane turnover was abnormal. Second, endothelial cell P-face membranes at the blood front contained more larger particles than those in the control rats (P < 0.05), implying an alteration in endothelial cell luminal membrane composition. Third, endothelial cell P-face membranes in areas of close apposition with pericyte membranes showed abnormal areas of particle clearing not seen in the control animals, suggesting a change in pericyte-endothelial cell interactions. Finally, pericyte membranes facing the neural retina contained increased numbers of plasmalemmal vesicles compared with control membranes (P < 0.05). Moreover, the association of these vesicles with collagen fibrils in the extracellular space suggested an alteration in extracellular matrix turnover.