[Anatomical description of a natural perforation present in the human lumbar pia mater]

The pia mater has always been considered more permeable than other meningeal membranes. Natural pia mater perforations found in some animals at-test to this membrane's permeability. Such perforations, however, have never been demonstrated in human tissue. Our objective was to study human pia mater from the dorsal lumbar region, looking for perforations that facilitate the diffusion of substances to the spinal cord following subarachnoid administration.

The specimens were removed from four human cadavers aged 70, 72, 77 and 78 years between 8 and 12 hours after death. The specimens were fixed in a phosphate glutaraldehyde buffer solution, followed by desiccation in acetone and critical point elimination of acetone, treatment with carbon and metallization with gold.

The pia mater was composed of a smooth surfaced, thin layer of cells and underlying connective tissue formed mainly of collagen fibers and fundamental amorphous matter. The collagen fibers were oriented in various directions. Throughout the surface of the pia mater, natural circular, elliptical and ovoid perforations were distributed irregularly. Size varied. Most measured over 10 to 15 micrometers in diameter or less than 5 to 8 micrometers. Inside the openings, fibers similar to collagen fibers could be seen at the point where they would normally be found beneath the cell layer.

The total thickness of the pia mater varies in different zones of the spine, as a result of variations in the thickness of the cell layer and in the underlying extracellular layer. The existence of natural fenestrations in all the analyzed specimens of human dorsal lumbar pia mater accounts for the high permeability of this membrane, which permits substances administered in spinal fluid to reach the spinal cord. These natural fenestrations are areas where the cell surface in absent, with underlying collagen fibers usually visible. The pia mater is generally believed to be composed of a complete cell layer that forms a barrier between the central nervous system and the subarachnoid space; however, the presence of fenestrations would indicate that such a barrier does not exist, the base membrane being placed under the connective fibers, the only intact structure prior to medullary glial cells.