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In histology, fenestræ are small pores in endothelial cells that allow for rapid exchange of molecules between fenestrated blood vessels, sinusoid blood vessels and surrounding tissue. These pores may enlarge and contract at the action of various stimuli such as the hormone noradrenaline. Fenestræ (singular: fenestra) is a Latin word that means "windows".
Depiction of the major types of capillaries, showing fenestrations as well as intercellular gaps.