The formation of fenestrations and channels by capillary endothelium in vitro

Microvascular endothelial cells isolated from fenestrated capillaries have been shown to form tubes in vitro, thereby demonstrating that they retain the ability to express some degree of their in vivo differentiated phenotype. However, some of their physiologically important structural features, such as transendothelial openings (i.e., diaphragmed fenestrations and transendothelial channels) are lost or are greatly reduced in number. In this study, cloned bovine adrenal cortex endothelial cells were cultured on plastic or on a basal lamina produced by Madin-Darby canine kidney (MDCK) cells for up to 17 days postconfluence. All cultures were then routinely fixed and processed for electron microscopic morphometry. For cells grown on plastic for 17 days postconfluence, the linear density of transendothelial openings in endothelial profiles <400 nm thick was found to be 0.007 openings per μm. On MDCK matric, however, the linear density of transendothelial openings in endothelial profiles <400 nm thick was found to be 0.157 per μm. Occasionally some cells formed 'tube-like' structures that also contained diaphragmed fenestrations and transendothelial channels on both sides of the tubes. These findings suggest that the substrate on which endothelial cells are grown can affect their differentiation.