Ventricles and Cisterns Overview

Ventricles and Choroid Plexus

Basic embryology : Early in embryonic development, the forebrain cavity divides into 2 lateral ventricles, which develop as outpouchings from the rostral 3rd ventricle and are connected to it by the interventricular foramen (a.k.a. foramen of Monro). In the coronal plane, these form a central H-shaped “monoventricle.” The cerebral aqueduct develops from the midbrain vesicle. The 4th ventricle develops from a cavity within the hindbrain and merges caudally with the central canal of the spinal cord.

Anatomic overview : The brain cerebrospinal fluid (CSF) spaces include both the ventricular system and subarachnoid spaces (SASs). The ventricular system is comprised of 4 interconnected CSF-filled, ependymal-lined cavities that lie deep within the brain. The paired lateral ventricles communicate with the 3rd ventricle via the Y-shaped foramen of Monro . The 3 rd ventricle communicates with the 4th ventricle via the cerebral aqueduct (of Sylvius). In turn, the 4 th ventricle communicates with the SAS via its outlet foramina (the midline foramen of Magendie and the 2 lateral foramina of Luschka ).

Lateral ventricles : Each lateral ventricle has a body, atrium, and 3 projections (“horns”). The roof of the frontal horn is formed by the corpus callosum genu. It is bordered laterally and inferiorly by the head of the caudate nucleus. The septi pellucidi is a thin, bilayered membrane that extends from the corpus callosum genu anteriorly to the foramen of Monro posteriorly and forms the medial borders of both frontal horns.

The body of the lateral ventricle passes posteriorly under the corpus callosum. Its floor is formed by the dorsal thalamus, and its medial wall is bordered by the fornix. Laterally, it curves around the body and tail of the caudate nucleus.

The atrium contains the choroid plexus glomus and is formed by the confluence of the body with the temporal and occipital horns. The temporal horn extends anteroinferiorly from the atrium and is bordered on its floor and medial wall by the hippocampus. Its roof is formed by the tail of the caudate nucleus. The occipital horn is surrounded entirely by white matter fiber tracts, principally the geniculocalcarine tract and the forceps major of the corpus callosum.

Foramen of Monro is a Y-shaped structure with 2 long arms extending toward each lateral ventricle and a short inferior common stem that connects with the roof of the 3rd ventricle.

3rd ventricle : The 3rd ventricle is a single, slit-like, midline, vertically oriented cavity that lies between the thalami. Its roof is formed by the tela choroidea, a double layer of invaginated pia. The lamina terminalis and anterior commissure lie along the anterior border of the 3rd ventricle.

The floor of the 3rd ventricle is formed by several critical anatomic structures. From front to back these include the optic chiasm, hypothalamus with the tuber cinereum and infundibular stalk, mammillary bodies, and roof of the midbrain tegmentum.

The 3rd ventricle has 2 inferiorly located CSF-filled projections: The slightly rounded optic recess and the more pointed infundibular recess . Two small recesses, the suprapineal and pineal recesses , form the posterior border of the 3rd ventricle. A variably sized interthalamic adhesion (also called the massa intermedia ) lies between the lateral walls of the 3rd ventricle. The massa intermedia is not a true commissure.

Cerebral aqueduct is an elongated tubular conduit that lies between the midbrain tegmentum and the quadrigeminal plate. It connects the 3rd ventricle with the 4th ventricle.

4th ventricle : The 4th ventricle is a roughly diamond-shaped cavity that lies between the pons anteriorly and the cerebellar vermis posteriorly. Its roof is covered by the anterior (superior) medullary velum above and the inferior medullary velum below.

The 4th ventricle has 5 distinctly shaped recesses. The posterior superior recesses are paired, thin, flat, CSF-filled pouches that cap the cerebellar tonsils. The lateral recesses curve anterolaterally from the 4th ventricle, extending under the brachium pontis (major cerebellar peduncle) into the lower cerebellopontine angle (CPA) cisterns. The lateral recesses transmit choroid plexus through the foramina of Luschka into the adjacent SAS. The fastigium is a triangular, blind-ending, dorsal midline outpouching that points toward the cerebellar vermis. The 4th ventricle gradually narrows as it courses inferiorly, forming the obex . Near the cervicomedullary junction, the obex becomes continuous with the central canal of the spinal cord.

Choroid plexus and the production of CSF : The choroid plexus is comprised of highly vascular papillary excrescences with a central connective tissue core coated by an ependyma-derived secretory epithelium. The embryonic choroid plexus forms where the infolded tela choroidea contacts the ependymal lining of the ventricles, thus developing along the entire choroidal fissure.

The largest mass of choroid plexus, the glomus , is located in the atrium of the lateral ventricles. The choroid plexus extends anteriorly along the floor of the lateral ventricle, lying between the fornix and thalamus. It then dives through the interventricular foramen (of Monro) and curves posteriorly along the roof of the 3rd ventricle. The choroid plexus, in the body of the lateral ventricle, curls around the thalamus into the temporal horn, where it fills the choroidal fissure and lies superomedial to the hippocampus.

CSF is predominantly, but not exclusively, secreted by the choroid plexuses. Brain interstitial fluid, ependyma, and capillaries may also play a poorly defined role in CSF secretion. The choroid plexus epithelium secretes CSF at the rate of about 0.2-0.7 mL/min or 600-700 mL/day. The mean CSF volume is 150 mL, with 25 mL in the ventricles and 125 mL in SASs. CSF flows through the ventricular system and passes through the exit foramina of the 4th ventricle into the SASs. The bulk of CSF resorption is through the arachnoid villi along the superior sagittal sinus. CSF also drains into lymphatic vessels around the cranial cavity and spinal canal.

Not all CSF is produced in the choroid plexus. Drainage of brain interstitial fluid is a significant extrachoroidal source.

CSF plays an essential role in the maintenance of brain interstitial fluid homeostasis and regulation of neuronal functioning.