Ultrastructure of the orbital pathway for cerebrospinal fluid drainage in rabbits

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✓ An increasing number of physiological and morphological studies indicate that cerebrospinal fluid (CSF) drains via nonarachnoidal pathways in several mammalian species. Ultrastructural tracer studies were undertaken to examine the orbital route for CSF absorption in the rabbit. At the termination of the optic nerve subarachnoid space, an area of connective tissue containing numerous small tortuous channels is present. Ferritin (molecular weight 400,000) infused into the ventricles at normal and increased intraventricular pressure was present in these channels by 15 minutes postinfusion, and subsequently reached the intraorbital connective tissue. Elevating the intraventricular pressure did not noticeably alter the morphological appearance of this region or change the gross distribution pattern of the ferritin. Ferritin did not penetrate the scleral barrier to reach the choriocapillaris, nor did it breach the arachnoid barrier layer proximal to the transitional zone at the optic subarachnoid space to reach the dura mater. These results are very similar to those described for the hamster orbital region and the rabbit cribriform region. These experiments support the concept that macromolecules exit the subarachnoid space at the termination of the optic nerve via open channels, and that no significant barrier to drainage of macromolecules in CSF is present at this location.

Article Information

Address reprint requests to: J. Gordon McComb, M.D., 1300 North Vermont Avenue, Suite 906, Los Angeles, California 90027.

Address for Dr. Erlich: Department of Pathology (D-33), University of Miami School of Medicine, P.O. Box 016960, Miami, Florida 33101.

© AANS, except where prohibited by US copyright law.

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Figures

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    Photomicrograph showing the termination of the optic nerve subarachnoid space (SAS). A transitional zone (outlined) containing a network of small extracellular channels is found between the meninges and sclera. ON = optic nerve; F = episcleral fat. Toluidine blue, × 140.

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    Electron micrographs showing normal morphology. Left: Infoldings (arrows) of cytoplasmic processes lining the terminal subarachnoid space (SAS). The cells of the transitional zone form a weblike arrangement with intervening channels (C) of extracellular space. Bismuth subnitrate, × 1870. Right: Intracytoplasmic vacuoles (arrows) are common in the transitional zone cells, but are usually empty, even following ferritin administration. Bismuth subnitrate, × 3470.

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    Electron micrograph showing ferritin, which appears as fine black specks, within cytoplasmic vesicles (arrows) of transitional zone cell processes. Free ferritin is also present in the extracellular channels (C). Bismuth subnitrate, × 33,930.

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    Electron micrograph showing free ferritin within intercalations of the subarachnoid space (arrowheads) between cytoplasmic processes of the arachnoid barrier layer (A). D = dura. Bismuth subnitrate, × 6780.

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    Electron micrograph showing ferritin within a transitional zone channel, apparently leading into the opening (arrowheads) between the two adjacent cells. Typical intercellular junctions (arrow) are present. Bismuth subnitrate, × 38,000.

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    Photomicrograph showing blue-staining ferritin within the extracellular space (arrows) of the retro-orbital fat. Prussian blue, × 125.

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