A combined physiological and morphological study of the secretory process in the rabbit choroid plexus.

The ultrastructure of the secreting and non-secreting rabbit choroid plexus has been examined with the electron microscope. Secretion was inhibited by perfusion with an artificial cerebrospinal fluid containing one of the following drugs that have been shown to inhibit CSF formation and also the rate of penetration of sodium ions into the fluid: Ouabain, Diamox, Spirolactone and Amiloride. Perfusion with Karnovsky's fixative enabled good preservation of the ultrastructure to be obtained in situ. In the inhibited state, large intercellular spaces, some of them reaching 5 /tm diameter, were observed along the length of the intercellular cleft, whilst other regions of the cleft retained normal dimensions, apparently limited by desmosomes. In the normally secreting plexus subject to identical conditions of fixation, no such dilations were seen. This 'reverse' system contrasts with the ' forward' transporting system found in the gall bladder. These findings are in accordance with the Diamond concept of water transfer for 'reverse' systems since, if fluid is moving out of the cleft under the influence of active sodium transport, it could be predicted that no dilation would occur jn the transporting condition. A possible explanation of the mechanism is offered in terms of the high hydrostatic pressure which may exist in the choroidal capillaries which, unlike the rest of the cerebral vasculature, are of the fenestrated type and are thus most likely to be highly permeable.