Functional Ultrastructure of the Arachnoid Villus

Examination of arachnoid villi with the electron microscope reveals that the cerebrospinal fluid spaces are always separated from the lumen of the sagittal sinus by a continuous layer of limiting endothelium. Observation of tissue after the intracisternal injection of a protein tracer reveals that protein cannot pass through intercellular junctions but is transported across the cytoplasm of endothelial cells by an active process. Cerebrospinal fluid absorption, therefore, requires metabolic activity of arachnoid villi endothelial cells instead of a passive pressure-dependent system as suggested in the past.