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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:

  1. 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)

  2. 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?

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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).

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  • $\begingroup$ Well then, an even better question is, "Okay, so it makes it past the pia mater. How does fluid then make it past the glia limitans?" $\endgroup$ – S.Cramer Feb 26 at 15:52
  • $\begingroup$ @S.Cramer there's definitely a good question in there. I'd recommend writing it up and asking it. You would, of course, want to phrase it more specifically than "How does fluid make it past the glia limitans?" $\endgroup$ – De Novo Feb 26 at 16:41

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