CHOROID PLEXUSES AND CEREBROSPINAL FLUID
Along the medial part of the wall of each lateral ventricle, and on the roof of the third and fourth ventricles the pia mater contacts the ependyma. The tela choroidea and the capillaries of the pia mater form choroid plexuses that project inside the ventricles to form the cerebrospinal fluid (CSF).
During development, the roof of the diencephalic vesicle becomes very thin, formed by an ependymal cell layer covered by the pia mater. It has a triangular shape and extends between the choroid fissure laterally (this is the fissure between the roof of the third ventricle and the telencephalic vesicle), the columns of fornix, and the mesencephalic tectum. The choroid fissure invaginates inside the lateral ventricles forming the choroid plexuses of the lateral ventricles. The capillaries that remain on top of the third ventricle form two longitudinal rows, the choroid plexuses of the third ventricle. These are continuous with the plexuses of the lateral ventricles through the interventricular foramina. The choroid plexuses of the fourth ventricle are located on the roof of the fourth ventricle, caudally to caudal medullary velum.
The choroid plexuses produce most of the CSF that occupies the ventricles, the central canal of the spinal cord and subarachnoid space. These plexuses are formed by arterioles lined by modified ependymal cells that invaginate into the ventricular lumen, becoming highly convoluted. An additional amount of CSF is secreted at the level of the ependymal cells and at the blood vessels of pia-arachnoid membrane.The CSF is produced partly by an active secretory process and, partly, by passive diffusion. Circulation of CSF begins at the level of the choroid plexus of the lateral ventricles. From there, it flows to the third ventricle and then, through the mesencephalic aqueduct, to the fourth ventricle. A small quantity enters the central canal of the spinal cord while most passes through a pair of openings (lateral apertures of the fourth ventricle), named after the German anatomist Hubert von Luschska, to the subarachnoid space. Each opening is located in a recess on each lateral wall of the fourth ventricle, just caudal to the caudal cerebellar peduncules.
The drainage of the CSF takes place at different sites: the arachnoid villi (the main route), the leptomeningeal veins, the lymphatic vessels (located at the level where roots continue as cranial and spinal nerves), and at the level of the ependyma that lines the ventricles. The CSF drainage can also occur through the cribriform plate into the nasal submocosa, where it is absorbed by lymphatics (Mollanji et al) and between the intracranial and the subarachnoid space of the optic nerve (Killer et al.).
Arachnoid villi are evaginations of the arachnoid into the venous sinuses of the dura mater, especially the superior sagittal sinus. They function as valves, allowing the flow into the venous sinuses when the CSF pressure exceeds the pressure in the sinus.
 R. Mollanji, R. Bozanovic, I. Silver, B. Li, C. Kim, R. Midha and M. Johnston. Intracranial pressure accommodation is impaired by blocking pathways leading to extracranial lymphatics. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 280: R1573-R1581, 2001.
 H. E. Killer, G. P. Jaggi, J. Flammer, N. R. Miller, A. R. Huber and A. Mironov. Cerebroespinal fluid dynamics between the intracranial and the subarachnoid space of the optic nerve. Is it always bidirectional. Brain, 130, 514-520, 2007.