MITOCHONDRIAL CONTENT OF CHOROID-PLEXUS EPITHELIUM

The objective of the present study was to examine the apparent work ca pacity of one of the two separate membrane systems (the blood-cerebros pinal fluid barrier) that isolate the mammalian brain extracellular fl uid (and cerebrospinal fluid, CSF) from plasma. Digitized analyses of electron-microscopic images provided estimates of mitochondrial volume s, which were expressed as a percentage of the cell cytoplasm. We reco rded a high mitochondrial content of 12-15% in the cuboidal epithelium of primate choroid plexus, which was consistent in vervet, rhesus, an d squirrel monkeys, as well as in baboons. Similarly high mitochondria l contents were observed in the rabbit, rat, and mouse choroid plexus. It has been postulated that the high mitochondrial content of brain e ndothelium is associated with maintaining the ionic gradients within t he central nervous system. We observed that the mitochondrial content of the choroid plexus (where CSF is produced) was slightly higher than in (prior measurements of) the blood-brain barrier (BBB). In addition , surface areas at the apical borders of the choroid plexus epithelia (where the Na(+)K(+)ATPase activity has been localized) were increased 7- to 13-fold over the basal borders, in the primate species examined . The observation of high mitochondrial volumes in choroid plexus cell s is consistent with the suggestion that increased mitochondrial densi ties seen in choroidal epithelia and BBB capillaries provide a metabol ic work capability for both secretory activities and maintaining ionic gradients across blood-CSF barriers.