Scanning electron microscopy of the subarachnoid space in the dog. II. Spinal nerve exits

Young dogs were perfused with buffered aldehydes. For scanning electron microscopy MICROFIL was injected to help prevent vascular collapse. The dura‐arachnoid was exposed leaving the nerve exits and rootlets intact. These were identified according to level and dissected free. Selected specimens were photographed at low magnification. Others were embedded in Epon 812 and sectioned for light and transmission electron microscopy. Similar specimens were prepared by critical point drying and palladium‐gold coating for scanning electron microscopy. Nerve roots pierce the dura through single exits (upper cervical, lower lumbar, sacral), or by exits separate for dorsal and ventral roots (lower cervical, thoracic, lumbar), or by multiple exits (cervical). Scanning electron microscopy reveals complicated tissue relationships chiefly involving arachnoid trabeculae. At the point of nerve root exits the arachnoid gives rise to numerous arachnoid trabeculae which form highly fenestrated sheets. These attach to blood vessels and nerve roots. A lateral recess occurs between dorsal and ventral roots (cervical, lumbar). Numerous macrophages are present on the various surfaces studied.