[Structure of the arachnoid layer of the human spinal meninges: a barrier that regulates dural sac permeability]

Drugs injected into the epidural space are known to penetrate the subarachnoid space by simple diffusion through the dural sac. We aimed to study the cellular ultrastructure of the arachnoid membrane and the type of intercellular junctions responsible for creating the barrier that regulates the passage of drugs through the dural sac in humans.

Fourteen tissue samples of arachnoid membrane were taken from 2 patients during procedures that required opening the lumbar dural sac. The samples were treated with glutaraldehyde, osmium tetroxide, ferrocyanide and acetone, and then embedded in resin. Ultrathin sections were stained with lead citrate for examination by transmission electron microscopy.

The arachnoid membrane was 35 to 40 microm thick. The outer surface contained neurothelial cells (dural border cells) along the subdural compartment, while the internal portion was made up of a plane 5 to 8 microm thick with 4 to 5 arachnoid cells overlapping to form a barrier layer. The intercellular spaces on this plane were 0.02 to 0.03 microm wide; the arachnoid cells were bridged by specialized junctions (desmosomes and other tight junctions).

Structural features of the arachnoid cells provide a barrier within the human dural sac. They occupy only the internal portion of the arachnoid membrane. Specialized intercellular junctions explain the selective permeability of this membrane.