EphB4 expression along adult rat microvascular networks: EphB4 is more than a venous specific marker.

Objective: EphrinB2 and EphB4 are considered to be markers of arterial/venous identity during embryonic development. However, the expression patterns of these arterial/venous-specific markers in adult microvascular networks remain unclear. The objective of this study was to characterize the cellular distribution of EphB4 expression along the hierarchy of unstimulated and remodeling adult rat mesenteric microvascular networks.

Methods: Mesenteric tissue was harvested from unstimulated and stimulated adult male Fisher rats, and stained for various combinations of EphB4, SM alpha -actin, NG2, and isolectin/CD-31. The distribution of EphB4 expression was compared to the cell-specific markers along arteriole, venule, and capillary segments for each mesenteric microvascular network (n = 32). Arterial/venous expression was also characterized in subcutaneous tissue and spinotrapezius muscle.

Results: Endothelial cells along both venules and arterioles stained positive for EphB4. EphB4 expression levels along capillaries correlated with capillary type, as the fluorescence staining intensity along capillary sprouts was significantly elevated in comparison to capillaries connecting arterioles and venules (mean intensity index = 25 for capillary sprouts; 6 for connected capillaries)

Conclusion: The results suggest that EphB4 is not an arterial/venous-specific marker in adult rat microvascular networks, and provide new data suggesting that its elevated expression in capillaries is indicative of capillary sprouting.