AQP4 and neuromyelitis optica: Our observation of AQP4 expression on the basolateral surface of ependymal cells in normal tissue and the loss of ependymal AQP4 expression in NMO tissue, as well as evidence of ependymal discontinuity and C9neo deposition on ependymal cells, suggests that CSF NMO IgG gained access to this CSF–brain barrier and elicited damage.