An electron microscope study of human arachnoid villi.

The fine structure of human arachnoid villi obtained by surgical biopsy during operations for intracranial diseases was investigated by electron microscopy. The observation of the cerebrospinal fluid-blood interface, represented by the endothelial covering of arachnoid villi, provided ultrastructural evidence for both "closed" and "open" systems for CSF reabsorption. Ultrastructural findings such as micropinocytotic vesicles, giant intracellular vacuoles, gaps between endothelial cells and tubul-like endothelium-lined structures, previously identified in animals by other workers, were for the first time demonstrated in human specimens.