The Ventricles, Choroid Plexus, and Cerebrospinal Fluid

Figure 6-1. The early development of the brain and ventricular system, showing how brain growth and the configuration of the ventricles interrelate. Diagrammatic dorsal views (A-C) correlate in general with lateral views (D-F) at about 5 weeks (D), 6 weeks (E), and 8.5 weeks (F) of gestation. The outlines of the ventricles are shown in D to F as dashed lines.

The choroid plexus, which secretes the CSF that fills the ventricles and the subarachnoid space, arises from tufts of cells that appear in the wall of each ventricle during the first trimester. These cells are specialized for a secretory function. The production of CSF is an active process that requires an expenditure of energy by the choroidal cells.

Any condition that causes CSF to accumulate, such as overproduction or an obstruction of its movement through the ventricle system, produces serious neurologic deficits. Perhaps the most widely recognized example is hydrocephalus as seen in a fetus or in a newborn. This condition is usually caused by an obstruction of CSF flow with resultant enlargement of the ventricular spaces upstream to the blockage. The bones of the developing skull move apart, and the head may enlarge significantly. In most of these cases, some type of surgical diversion of CSF flow (a shunting procedure) is necessary.

DEVELOPMENT

The anterior (rostral) and posterior (caudal) neuropores close at about 24 and 26 days, respectively. At this point, the neural tube is lined by the differentiating neuroepithelial cells of the ventricular zone, which are undergoing waves of cell division. Some of these precursor cells give rise to the ependymal cells that line the developing (and mature) ventricular system and the central canal.

The brain is initially composed of three primary brain vesicles—rhombencephalon, mesencephalon, and prosencephalon—each containing a portion of the cavity of the neural tube (Fig. 6-1A, D). The appearance of the pontine flexure in the rhombencephalon and a progressively deepening groove that separates the diencephalon from the telencephalon (the diencephalic-telencephalic sulcus, shortened here to telencephalic flexure) divide these three vesicles into the five brain vesicles (myelencephalon, metencephalon, mesencephalon, diencephalon, telencephalon) characteristic of the adult brain (Fig. 6-1B, E). With subsequent development, the telencephalon enlarges significantly (Fig. 6-1C, F). As the brain enlarges from three to five vesicles, each part pulls along a portion of the cavity of the primitive neural tube. These spaces in each brain vesicle form the ventricle of that part of the brain in the adult. Consequently, the shape of the ventricular system conforms, in general, to the changes in configuration of the surrounding parts of the brain.

The lateral ventricles follow the enlarging cerebral hemispheres, and the third ventricle remains a single midline space (Fig. 6-1A-C). The communications between the lateral ventricles and the third ventricle, the interventricular foramina (of Monro), are initially large but become small, in proportion to the enlarging brain, as development progresses (Fig. 6-1C, F). A colloid cyst is a type of glioma that, although comprising only about 1% of all intracranial tumors, has a predilection for being found at the position of the interventricular foramen. In this location, it may block the egress of CSF from the lateral ventricles and result in enlarged ventricles and thinning of the corpus callosum. These patients usually present with headache, nausea, and vomiting (features of increased intracranial pressure) and with mental changes and gait disturbances. Proliferation of the neural elements of the mesencephalon results in a reduction in the size of the cavity of this vesicle to form the cerebral aqueduct of the adult brain (Fig. 6-1C, F). This creates a constricted region in the ventricular system and thus a point at which the flow of CSF may be easily blocked. Occlusion of the cerebral aqueduct during development may be the result of glial scarring (gliosis) due to infection or a consequence of developmental defects of the forebrain, a rupture of the amnionic sac in utero, or forking of the aqueduct. The last entity is a genetic sex-linked condition in which the aqueduct is reduced to two or more very small channels that do not properly meet. In addition, the cerebral aqueduct may be reduced to such a small channel that CSF flow is reduced or essentially blocked. Whatever the cause, occlusion of the cerebral aqueduct results in a lack of communication between the third and fourth ventricles and blocks the egress of CSF from the third ventricle. Caudally, the cerebral aqueduct flares open into the fourth ventricle (Fig. 6-1B, C, E, F).

Foramina of the Fourth Ventricle

The ventricles and central canal of the spinal cord initially form a closed system. However, in the second and third months of development, three openings form in the roof of the fourth ventricle, rendering the ventricular system continuous with the subarachnoid space surrounding the brain and spinal cord. The caudal part of the roof of the fourth ventricle consists of a layer of ependymal cells internally and a delicate layer of connective tissue externally (Fig. 6-2). The future apertures first appear in the form of small bulges in the roof of the fourth ventricle. The membrane forming the roof at these points becomes thinned and breaks down. The resultant openings are the medial foramen of Magendie and the lateral foramina of Luschka (Fig. 6-2).

Figure 6-2. Development of the foramina of Luschka and Magendie in the fourth ventricle.

Formation of the Choroid Plexus

The caudal roof of the fourth ventricle is composed of ependymal cells on the luminal surface and a delicate layer of connective tissue, the pia mater, on its external surface. These collectively form the tela choroidea (Fig. 6-3). Developing arteries in the immediate vicinity invaginate the roof of the ventricle to form a narrow groove, the choroid fissure, in the tela choroidea. These small developing arteries are branches of what will become the posterior inferior cerebellar artery in the adult. The involuted ependymal cells, along with vessels and a small amount of connective tissue, represent the primordial choroid plexus inside the ventricular space. As development progresses, the choroid plexus enlarges, forms many small elevations called villi, and begins to secrete CSF (Fig. 6-3; see also Fig. 6-18). By about the end of the first trimester, the choroid plexus is functional, the openings in the fourth ventricle are patent, and there is circulation of CSF through the ventricular system and into the subarachnoid space.

Figure 6-3. Development of the choroid plexus.

The choroid plexuses of the third and lateral ventricles develop much in the same manner (Fig. 6-4). A choroid fissure appears in the roof of the third ventricle and in the medial wall of the lateral ventricle. The choroid plexus develops along these lines, bulges into the respective space, and is continuous from lateral to third ventricles through the interventricular foramen. The small arteries serving the choroid plexus of the third ventricle are branches of the medial posterior choroidal artery, and those serving the choroid plexus of the lateral ventricles are branches of the lateral posterior choroidal artery and the anterior choroidal artery (see Chapter 8).

Figure 6-4. Lateral (A) and posterior (dorsal) (B) views of the lateral, third, and fourth ventricles and the cerebral aqueduct. Structures that border on the various parts of the ventricular system are shown in green; the choroid plexus is shown in red.

In the adult, the choroid plexus is found in both lateral ventricles and in the third and fourth ventricles (Fig. 6-4). The development of this structure is essentially the same in all of these spaces and is described here for the fourth ventricle.

VENTRICLES

Lateral Ventricles

The cavities of the telencephalon are the lateral ventricles, of which there is one in each hemisphere (Fig. 6-4). As the development of the hemispheres creates the frontal, temporal, and occipital lobes, the lateral ventricles are pulled along and thus acquire their definitive adult shape of a flattened C with a short tail (Fig. 6-4). This shape is present by birth. The lateral ventricle consists of an anterior horn, a body, and posterior and inferior horns (Fig. 6-4). The junction of the body with the posterior and inferior horns constitutes the atrium of the lateral ventricle. An especially large clump of choroid plexus, the glomus (or glomus choroideum), is found in the atrium (Fig. 6-4). In adults and especially in elderly persons, the glomus may contain calcifications that are visible (as white spots) on radiographs or computed tomography (CT) scans (Fig. 6-5). Shifts in the position of the glomus, usually accompanied by alterations in the volume or shape of the surrounding ventricle, may indicate some type of ongoing pathologic process or space-occupying lesion.

Figure 6-5. CT scan showing a calcified glomus in a 66-year-old man. Note that calcifications are also present in the pineal in this patient. These calcifications form a triangle, the shape of which may be altered by changes in brain shape or by midline shift secondary to some pathologic process.

The elaborate shape of the lateral ventricle means that different structures border on different parts of this space. The anterior horn and body of the lateral ventricle are bordered medially by the septum pellucidum (at rostral levels) and by a bundle of fibers called the fornix (at caudal levels) and posteriorly (superiorly) by the corpus callosum (Figs. 6-4 and 6-6). The floor of the body of the lateral ventricle is made up of the thalamus, and the caudate nucleus is characteristically found in the lateral wall of the lateral ventricle throughout its extent (Figs. 6-4 and 6-6). In the temporal lobe, the inferior horn of the lateral ventricle contains the tail of the caudate nucleus in its lateral wall, the hippocampal formation in its medial wall, and a large group of cells (the amygdaloid complex) in its rostral end (Figs. 6-4 and 6-6).

Figure 6-6. Lateral views of the ventricles (A and B) and representative cross sections (C and D, details from B) showing the lateral and third ventricles and the major structures that border on these spaces.

The openings between the lateral and third ventricles, the interventricular foramina, are located between the column of the fornix and the rostral and medial ends of the thalamus. There are two interventricular foramina, one opening from each lateral ventricle into the single midline third ventricle (Fig. 6-4B).

Third Ventricle

The third ventricle, the cavity of the diencephalon, is a narrow, vertically oriented midline space that communicates rostrally with the lateral ventricles and caudally with the cerebral aqueduct (Figs. 6-4 and 6-8). The third ventricle has an elaborate profile on a sagittal view (Fig. 6-4A), but it is narrow in the coronal and axial planes (Fig. 6-7).

Figure 6-7. Magnetic resonance images of the third ventricle in coronal (A) and axial (B) views.

Figure 6-8. Midsagittal view of the brain showing the third ventricle, cerebral aqueduct, and fourth ventricle and structures closely related to these spaces.

The boundaries of the third ventricle are formed by a variety of structures, the most important being the dorsal thalamus and hypothalamus, and by structures that form small outpocketings called recesses (Figs. 6-4A and 6-8). These are the supraoptic recess (above the optic chiasm), the infundibular recess (in the infundibulum, the stalk of the pituitary), the pineal recess (in the stalk of the pineal), and the suprapineal recess (above the pineal). The rostral wall of the third ventricle is formed by a short segment of the anterior commissure and a thin membrane, the lamina terminalis, that extends from the anterior commissure anteriorly (ventrally) to the rostral edge of the optic chiasm (Figs. 6-4A and 6-8). The floor of the third ventricle is formed by the optic chiasm and infundibulum and their corresponding recesses plus a line extending caudally along the rostral aspect of the midbrain to the cerebral aqueduct. The caudal wall is formed by the posterior commissure and the recesses related to the pineal, whereas the roof is the tela choroidea, from which the choroid plexus is suspended (Figs. 6-4A and 6-8).

Cerebral Aqueduct

The cerebral aqueduct, the extension of the ventricle through the mesencephalon, communicates rostrally with the third ventricle and caudally with the fourth ventricle (Figs. 6-4A and 6-8). This midline channel is about 1.5 mm in diameter in adults and contains no choroid plexus. Its narrow diameter makes it especially susceptible to occlusion. For example, cellular debris in the ventricular system (from infections or hemorrhage) may clog the aqueduct. Tumors in the area of the midbrain (such as pinealoma) may compress the midbrain and occlude the aqueduct. The result is a blockage of CSF flow and enlargement of the third and lateral ventricles at the expense of the surrounding brain tissue. The cerebral aqueduct is surrounded on all sides by a sleeve of gray matter that contains primarily small neurons; this is the periaqueductal gray or central gray.