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Scanning electron micrograph of a liver sinusoid with fenestrated endothelial cells. Fenestræ are approx 100 nm in diameter, with a sinusoidal width of 5 micrometres. Magnification 30,000X
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 noradrenaline. Fenestræ (singular: fenestra) is a Latin word that means "windows".