Sinusoid (blood vessel)

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Scanning electron micrograph of a liver sinusoid with fenestrated endothelial cells. Fenestræ are approx 100 nm in diameter.

A sinusoid is a small blood vessel similar to a capillary but with a fenestrated endothelium. Fenestrations are pores in the endothelial cells that greatly increase their permeability. In addition, permeability is increased by large inter-cellular clefts and fewer tight junctions. The level of permeability is such as to allow small and medium-sized proteins such as albumin to readily enter and leave the blood stream.

Sinusoids are found in the liver, lymphoid tissue, endocrine organs, and hematopoietic organs such as the bone marrow and the spleen. Sinusoids found within terminal villi of the placenta are not comparable to these because they possess a continuous endothelium and complete basal lamina.

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