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.
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