The choroid plexus produces the majority of the CSF in addition to smaller amounts secreted by the ependyma and the perivascular spaces. The choroid plexus is derived embryologically from the neural epithelium and is composed of two layers including a specialized ependymal cell layer composed of the epithelial lining of the ventricular system and a highly vascularized pia mater layer. This bilayered structure, known as the choroidal epithelium, is folded into microvilli and forms a brush border on which cilia are present on the apical surfaces of some of these cells (Fig. 26-1 (Figure Not Available) ). The choroidal epithelium with accompanying blood vessels and interstitial connective tissue collectively form the choroid plexus.

The choroidal epithelium shares many of the same characteristics of the blood-brain barrier. The concept of a

Figure 26-1 (Figure Not Available) Diagram of choroidal epithelium demonstrating the ependymal microvilli and cilia forming a brush border into the ventricular h(From Page RB, Leure-dePree AE: Ependymal alterations in hydrocephalus. In Wood JH [ed]: Neurobiology of cerebrospinal fluid. New York, Plenum Press, 1983, p 802.)

blood-brain barrier was first considered in the 19th century when Paul Ehrlich injected dye into the blood of animals and noted that all the organs except the brain were stained. Later, in 1913, Edwin Goldmann injected dye into the CSF compartment and found that the brain was stained but no dye appeared in the blood. From these findings, the hypothesis that the brain was separated from blood and that the brain capillaries provide this barrier was proposed. When electron microscopy became available, it was revealed that capillary endothelial cells rest on a basement membrane and form a tube joined by continuous tight junctions with the merging of the outer leaflets of two adjoining cells. This barrier is involved in protecting the brain from substances in the blood, selectively transporting desired blood constituents, and metabolizing or modifying substances transported between compartments.

The properties of brain endothelial cells that differentiate them from systemic capillaries include complex tight junctions between individual capillary endothelial cells, which provide a high electrical resistance. These junctions are formed through the merging of the outer leaflets of two adjoining cells. Second, little bulk flow of molecules occurs through these cells because of the paucity of pinocytotic vesicles and fenestrae. These two features protect the brain from various neurotoxic agents that may be present in the blood. Only substances that can cross biological membranes owing to their lipophilic character may diffuse unrestricted across the blood-brain barrier. Specific carrier systems, however, do exist in the brain endothelial cells and allow for the controlled exchange of substances between the system blood and the nervous system. These include facilitated diffusion in the case of the glucose transporter and ATP-dependent active transport mechanisms. Third, in addition to the endothelial cells, pericytes, perivascular microglia, and astrocytes contribute to the barrier. Pericytes are contractile adventitial cells and serve as the counterpart to the smooth muscle of large systemic vessels. Whereas evidence exists to suggest that these cells are necessary for the growth and integrity of the capillary endothelial cells and cellular transport, their exact function is unknown. Perivascular microglial cells are derived from bone marrow and most likely migrate to the brain. In proximity to the capillary endothelial cells, these microglial cells most likely serve as phagocytes. Finally, astrocytic endfeet are also closely associated with capillary endothelial cells and are separated from the cell plasma membrane by only the basal lamina. The astrocytes and their associated processes may be involved in the induction of the blood-brain barrier.

The choroid plexus is found in the walls of the lateral ventricles, and this structure is continuous with choroid plexus in the roof of the third ventricle. Additionally, the fourth ventricle contains a T-shaped choroid plexus that projects into the cavity from the roof. Whereas the choroid plexus of the lateral ventricles receives its arterial supply from the anterior and posterior choroidal arteries, the third ventricular choroid plexus is supplied by branches of the posterior cerebral artery. The posterior inferior cerebellar artery supplies the choroid plexus of the fourth ventricle.

After the CSF is secreted by the choroid plexus it circulates throughout the ventricular system and the subarachnoid space that surrounds the brain and spinal cord ( Fig 26-2 ).

Figure 26-2 The ventricular system. This diagram demonstrates the flow of cerebrospinal fluid from the choroid plexus to the arachnoid (From Fishman RA: Cerebrospinal fluid in diseases of the nervous system. In Fishman RA [ed]: Cerebrospinal fluid in diseases of the nervous system, 2nd ed. Philadelphia, W.B. Saunders, 1992, p 8.)

The anatomy of the ventricular system allows for movement of CSF in and around all the major structures of the brain. The lateral ventricles are located within the cerebral hemispheres and communicate with the third ventricle via the foramina of Monroe. The third ventricle lies caudal to the lateral ventricles and is a midline structure within the diencephalon. At its caudal end, the third ventricle is connected by the aqueduct of Sylvius to the fourth ventricle, which is situated between the brain stem and cerebellum. Cerebrospinal fluid then flows into the basal cisterns and subarachnoid space by the lateral foramina of Luschka and the medial foramen of Magendie. From these cisterns, the CSF flows throughout the subarachnoid space and over the hemispheric convexities and around the spinal cord.

CSF is reabsorbed into the venous system by numerous microscopic arachnoid villi and larger but less common arachnoid granulations. The granulations have a collagenous trabecular core with associated channels and a cap of arachnoid cells on the apex. Villi and granulations represent outpouchings of the arachnoid membrane that penetrate gaps in the dura and protrude within the venous sinuses (Fig. 26-3 (Figure Not Available) ). Arachnoid villi are also present at various levels of the spinal cord and surrounding spinal nerve roots.

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