Formation of cerebrospinal fluid. Relation of studies of isolated choroid plexus to the standing gradient hypothesis.

After a brief summary of current views on the origin of cerebrospinal fluid (CSF) and the processes underlying its elaboration, the author discusses studies of isolated choroid plexus in extracorporeal perfusion systems and flux chambers. The results suggest that transependymal water flow is secondary to the electrically silent pumping of sodium. The author presents evidence in support of the standing gradient hypothesis as the structural basis of CSF secretion.