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The formation of capillary‐like tubes by calf aortic endothelial cells grown in vitro

Cloned, large vessel endothelial cells derived from fetal bovine and bovine calf aortas formed three‐dimensional structures in vitro without tumor‐conditioned medium or special substrata. Transmission electron microscopy showed the structures to be hollow tubes composed of typical endothelial cells with overlapping and interdigitating cytoplasmic processes typical of those seen in in vivo capillaries. The putative lumen of these tubes generally contained abundant electron‐dense fibrous material, which by ruthenium red and indirect immunofluorescent staining appeared to be extracellular matrix. This suggests that the endothelial cell orientation in the tubes is the reverse of that normally found in in vivo vessels.