Microvascular consequences of thrombosis in small venules: an in vivo microscopic study using a novel model in the ear of the hairless mouse.

Little is known of the development of chronic microvascular alterations following small vessel thrombosis, which is probably due to the lack of appropriate experimental models. Herein we report the first results on thrombosis-associated long-term changes of microvascular permeability and vessel tortuosity and diameter and blood perfusion using the ear of the hairless mouse, and intravital fluorescence microscopy. Thrombosis was induced photochemically in small venules (diameter: 75 to 100 microm) using light/dye exposure (fluorescein isothiocyanate-dextran 150,000), and the microcirculation compromised by the blockade of blood drainage was analyzed before and 30 minutes after induction of thrombosis as well as repeatedly over a 28-day observation period. Thrombus formation resulted in a marked increase (p<0.05) of microvascular permeability (0.85+/-0.11) when compared with baseline values (0.46+/-0.04). Permeability remained elevated (p<0.05) at days 1 (0.67+/-0.07), 3 (0.58+/-0.02), and 7 (0.60+/-0.06), but returned to normal after 28 days (0.43+/-0.03). Tortuosity, diameter, and red blood cell velocity of venular segments, located upstream of thrombus formation, were found unchanged during the entire 28-day observation period. This was probably due to the fact that blood flow from the thrombosis-affected tissue was frequently drained into nonaffected tissue via preexisting "through-fare" channels, serving as venulo-venular collaterals. In accordance, in 10 to 20% of these venular segments the direction of blood perfusion was found changed, while those changes were only rarely observed in venular vessel segments of the nonthrombotic contralateral ears. We conclude that thrombosis in small cutaneous venules is primarily characterized by an increased vascular permeability, reflecting an inflammatory response, similar to what is known from thrombophlebitis in patients. The model presented herein may be a versatile tool to study pathogenesis of chronic microcirculatory derangements in microthrombosis and their prevention by novel therapeutic strategies.