Fibrosis in senescent rat choroid plexus. Aging takes a toll on choroid plexus, functionally and structurally. This electron micrograph depicts massive collagen deposits in CP interstitium, i.e., between the vascular core and outer epithelial (E) ring. Fibrotic (F) bands in a 36-mo-old Brown-Norway/Fischer rat are 40–50 times thicker than corresponding collagenous layers in young adults. Excessive fibrosis likely impedes nutrient flow from plasma to ventricles and reabsorption of Aβ peptide fragments from CSF. Fibrosis in aging and AD also occurs in the arachnoid . Consequently, fibrosis interferes with CSF dynamics via multiple effects. Appreciation is extended to P. McMillan for electron microscopy. Scale bar = 2 μm.