Physical barrier Copper/Zinc Superoxide Dismutase (Cu/Zn SOD), shown in orange, is an important factor in the brain's immune response. Here it is seen in close association with the<unk> l fibrillary acidic protein (GFAP), an indicator of the presence of astrocytes, at the surface of the<unk>  The main role of the<unk>  is to act as a physical barrier against unwanted cells or molecules attempting to enter the CNS. The<unk>  compartmentalizes the brain to insulate the parenchyma from the vascular and subarachnoid compartments. Within the brain, the<unk> l limiting membrane is an important constituent of the blood–brain barrier. Experiments using electron-dense markers have discovered that functional components of the blood–brain barrier are the endothelial cells that compose the vessel itself. These endothelial cells contain highly impermeable tight junctions that cause the blood vessels of the brain to exhibit none of the “leakiness” found in arteries and veins elsewhere in the body. Through both in vivo and in vitro experiments the astrocytic foot processes of the<unk>  were shown to induce the formation of the tight junctions of the endothelial cells during brain development. The in vivo experiment involved harvested rat astrocytes that were placed into the anterior chamber of a chick-eye or on the chorioallantois. Permeable blood vessels from either the iris or chorioallantois became impermeable to blue-albumin once they had entered the transplanted bolus of astrocytes. In the in vitro experiment, endothelial cells were first cultured alone and the tight junctions were observed in freeze-fracture replicas to be discontinuous and riddled with gap junctions. Then, the brain endothelial cells were cultured with astroctytes resulting in enhanced tight junctions and a reduced frequency of gap junctions. The<unk>  also acts as a second line of defense against anything that passes the blood–brain barrier. However, because the astrocytes surrounding the vessels are connected by gap junctions, it is not considered part of the BBB and material can readily pass between the foot processes. Immunological barrier The astrocytes of the<unk>  are responsible for separating the brain into two primary compartments. The first compartment is the immune-privileged brain and spinal cord parenchyma. This compartment contains multiple immunosuppressive cell surface proteins such as CD200 and CD95L and it allows for the release of anti-inflammatory factors. The second compartment is that of the non-immune-privileged subarachnoid, subpial, and perivascular spaces. This area is filled with pro-inflammatory factors such as antibodies, complement proteins, cytokines, and chemokines. The astrocytes of the<unk>  are believed to be the component of the brain that secretes the pro- and anti-inflammatory factors.
Glia limitans
Tapetum lucidum
Livedo reticularis
Stratum spinosum
Geniculate fibers
External lamina
Chondrocranium
Locura