Are the arachnoid villi really the main drainage route for the cerebrospinal fluid into the blood stream? An electron microscopic study.

The arachnoid granulations or villi are not always present in lower mammals and even in human late prenatal fetus. They grow in number and size with advanced age, and thus it seems appropriate to regard them as safety-valves for the cerebrospinal fluid (CSF) to be prevented from its hypertension accompanied by the age and by the increasing volume of the brain. Using 2 macaque monkeys and 12 rats, and by means of scanning and transmission elecron microscopies, tracer experiments with horseradish peroxidase, elastic staining of thin sections, and freeze-substitution of rapidly frozen brains, evidences are presented to show that the drainage sites of the CSF in the brain are the sites being devoid of the bloodbrain barrier, that is, the subfornical organ at the root of the choroid plexus of lateral ventricle, the pineal body at the root of the choroid plexus of third ventricle, and the area postrema at the root of the choroid plexus of fourth ventricle, and also the hypophysis and its vicinity, and the choroid plexus itself.