Figure 14-13.

Organization of the Meninges.

The meninges, from outside to inside, are the dura mater, arachnoid mater, and pia mater as illustrated in the diagram. The arachnoid mater and the pia mater form the leptomeninges. These two layers of the leptomeninges also enclose the subarachnoid space, which contains the arteries, veins, and nerves and is filled with cerebrospinal fluid. The pia mater is attached to the surface of the central nervous system (CNS). Astrocytes and their foot processes underlie the pia mater and form the glia limitans (inset 1) and surround the endothelial cells that form the blood-brain barrier. As arterioles penetrate the cortex to supply the tissue with blood, they carry the pia and glia limitans with them for 1 to 3 mm until the arteriole structurally becomes a capillary. At this transition site within the cortex, the capillary penetrates the pia and is surrounded by the glia limitans, and the end feet of the astrocytes become part of the blood-brain barrier (inset 2). Components of the blood-brain barrier are capillary endothelial cells, basement membrane, and astrocytic foot processes, but the barrier is formed structurally by tight junctions between endothelial cells and functionally by specialized transport systems in these cells.

(Courtesy Dr. J.F. Zachary, College of Veterinary Medicine, University of Illinois.)