Scanning electron microscopic observations of the arachnoid granulations in monkeys with cerebrospinal fluid hypotension.

The changes in arachnoid granulations following the depletion of cerebrospinal fluid (CSF) were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the normal tissue, the arachnoid granulations located at the inner walls of the superior sagittal sinus and the straight sinus had bulging protrusions of various sizes, as viewed with the SEM. With TEM, the outermost cells covering the surface of the arachnoid granulations had "giant vacuoles" in the cytoplasm. With CSF hypotension, the arachnoid granulations were smaller in size and the outermost layer of cells were thinner. The vacuoles in the outer layer were not developed. In the apical region of the individual arachnoid granulations with CSF hypotension, the arachnoid cells were densely clustered under the endothelial cells. With recovery to normal CSF pressure, the arachnoid cells appeared to protrude between the endothelial cells covering the apical portion of the arachnoid granulation.