During a lecture, a professor commented that the cerebrospinal fluid and interstitial fluid of the brain parenchyma have similar composition because they can exchange with one another. This struck me as surprising because I was always under the following impressions:
pia mater is essentially impermeable to fluid (therefore CSF from the subarachnoid space could not penetrate it to be in a passive exchange with the underlying brain tissue)
the ependymal cells of the ventricles (besides the choroid plexus where blood is 'pushed' through) is impermeable to fluid...i.e. CSF cannot leave the ventricles to be in passive exchange with the surrounding brain tissue.
Could someone please confirm if these two impressions are correct, and, further, if they are correct, could someone confirm the accuracy of the professor's claim and by what means it is true?