Can cerebrospinal fluid pass through the pia mater or ependymal cells of the ventricles?
Both the blood brain barrier and the blood CSF barrier tightly regulate the CNS fluid interstitial fluid environment of the CNS. You, however, are talking about a different barrier, the barrier formed by the pia mater, sometimes called the brain/CSF barrier. This barrier is less well studied. It does appear to prevent transport of particulate matter and red blood cells, but allow the passage of immune cells and small molecules. In addition to your professor's statement that the composition of CSF and brain interstitial fluid is similar, intrathecal injection directly into the CSF in the subarachnoid space provides an effective way to deliver drugs with CNS action, suggesting the pia mater does not effectively restrict small molecules (see Goodman and Gilman's The Pharmacological Basis of Therapeutics Ch 2).