Electron microscopic observations on subarachnoid and perivascular spaces of the Syrian hamster brain

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Abstract
Normal and experimental pathologic specimens from the brains of Syrian hamsters were studied with the electron microscope with reference to the organization and relationships of the subarachnoid and perivascular spaces. The brain surface is composed of astrocytic processes which are completely covered by a continuous basement membrane. Pial cells make up a unicellular sheet and, together with similar processes from the multilayered arachnoid, bound the subarachnoid space. Arteries and veins possess a basement membrane enclosing elements of the tunica media and continuous with that of the endothelium. The marginal glia and their basement membrane follow and surround the penetrating vessels. The space lying between the glial basement membrane and the basement membrane of the vessel wall contains adventitial elements and is considered the "true" perivascular space. Plasma exudation within this space was seen in locally X-irradiated animals. The glial and vascular basement membranes also serve to confine inflammatory cells and are the structural basis for the circumscribed perivascular "cuffing" in these animals with experimental cerebral infections. The true perivascular space terminates when the two basement membranes become confluent. At the capillary level, where adventitia is absent, the perivascular space is also nonexistent. In occasional preparations, the marginal glia appeared torn, resulting in an artificial space similar to the artifactitious space of His or Held.