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Tissues from adult Sprague-Dawley rats fixed by perfusion with buffered aldehydes for a combined study of the vascular system of the brain are described in light and electron microscopy. In these preparations lack of shrinkage prevents the formation of perineuronal and perivascular spaces. However, connective tissue stains indicate restricted tissue space along the course of small arteries and veins. In fine structure this space is found within the walls of the vessels. It consists of a tubular extension of tissue space bounded inwardly by the endothelial boundary (basement) membrane and outwardly by the neural boundary membrane. Between these boundaries the formed elements of the media and the adventitia are found. The media consists of a thin layer of smooth muscle cells; each cell being enclosed in its own boundary membrane. The adventitia consists of cells and fibrous elements of the connective tissues which are derived, near the surface of the brain, from the intermingling of pial and vascular leptomeninges. This "neural" portion of the tissue space extends from the depths of the capillary bed (where it is obliterated by the fusion of boundary membranes), along the course of the blood vessels, through the subarachnoid space and into the general tissue space of the body.

Young dogs six to 14 weeks old were perfused with aldehydes by standard techniques followed by MICROFIL to prevent blood vessel collapse. Tissue pieces possessing natural surfaces of the subarachnoid space at spinal cord levels were removed, post-osmicated, dehydrated, critical point dried, coated with palladium-gold and examined in a Cambridge Stereoscan S4 scanning electron microscope.The surfaces of the spinal pia mater and arachnoid are clearly expressions of connective tissues with fenestrations and underlying connective tissue fibrils. Fenestrations are especially common in the conus medullaris where connective tissue fibrils are more likely exposed to cerebrospinal fluid than covered by pial lining cells. Long and short microvillae and blebs are present in profusion and with distributions comparable to the same structures on other cells. The surfaces of traversing structures; blood vessels, nerve roots, arachnoid trabeculae and denticulate ligaments are covered with tissues comparable to and continuous with pia and arachnoid despite underlying differences in histological makeup. Free cells are present on all spinal leptomeningeal surfaces in surprising profusion. They exhibit extreme pleomorphism of surface contours which is comparable to that observed in the scanning electron microscopy of cells from other sources. It is highly probable that these free cells are macrophages.