Pericyte changes in branch retinal vein occlusion

The model of experimental branch vein occlusion (BVO) in the monkey offers the opportunity to examine retinal capillaries under stress. Electron microscopic morphometry was done on 812 capillaries of 13 eyes of cynomolgus monkeys, comparing 579 capillary collaterals of 9 BVO eyes with 233 normal capillaries of 4 control eyes. The tissue underwent the myosin subfragment-1 technique to decorate and quantify bundles of actin filaments in capillary pericytes. The duration of BVO was 2-48 months. Capillary collaterals of BVO eyes had an enlarged caliber, endothelial hyperplasia, and pericyte hypertrophy, but no proportional increase in basement membrane area. Collaterals near the inner plexiform layer (IPL) had a greater wall thickness, pericyte coverage, and actin coverage than collaterals near the outer plexiform layer (OPL). Pericyte hypertrophy was proportionate to caliber increase in OPL vessels and exceeded caliber increase only in IPL vessels. Actin coverage was proportional with the vessel dilation and size of pericyte cytoplasm in all vessels. These findings indicate that capillary collaterals in BVO are not equipped morphologically for an increased regulatory role in microvascular flow beyond their normal function.