Ventricles The lateral ventricles are crescentic in shape, situated on the medial side of the hemispheres, with a dorsal extension rostrally and a ventro-lateral extension caudally. The interventricular foramina are narrow, and communicate with the vertical narrow third ventricle, which has a rostral dorsal expansion containing the choroid plexus and paraphysis, and chiasmatic and infundibular recesses on the ventral side. The infundibular recess is connected to the ventricle by a narrow isthmus and expands as it extends ventrally and caudally, to continue as a trifurcated lumen to the point where the neurohypophysis fuses with the adenohypophysis in the pituitary fossa. The midbrain ventricle is situated towards the dorsal side, and has dorsi-lateral extensions into the optic lobes. At the caudal end, where the roof is thin ependyma, it is continuous with the rostral part of the fourth ventricle. There is a dorsal extension of the fourth ventricle into the cerebellum, and lateral recesses extending into the cerebello-pontine angle.
Hindbrain roof The roof of the middle portion of the fourth ventricle is formed by dense choroid plexus, and caudally the roof bulges out as an ependymal pouch extending for some 300um. The ependymal pouch is fragile but intact and beyond this the shallow fourth ventricle extends into the medulla, where it is somewhat ventrally placed, and is continuous with the prominent central canal of the spinal cord. The ventricular system of the pigeon is, therefore, closed by a thin ependymal sac.
Meninges The pia invests the neuraxis closely, but is not visible as a separate layer over the roof of the fourth ventricle. Outside the pia is arachnoid consisting of regular interspersed nuclei in a non-collagenous membrane covered by flat cells on its inner aspect; it is rather irregular in thickness and forms folds in and around the neuraxis. The arachnoid is mostly adherent to the endosteal dura, and it is also adherent to the ventral side of the hindbrain; it reinforces the ependymal pouch of the roof of the fourth ventricle, and it dips down between the cerebral hemispheres to lie close to the tela choroidea of the third ventricle. Ventrally the infundibular recess extends through the arachnoid and the pituitary is in an extra-arachnoid position. The subarachnoid space contains aggregations of meningeal blood vessels, particularly around the infundibulum and in the tela choroidea of the ventricles. Between the arachnoid and the fibrous dura, where they are separate, there are venous sinuses running longitudinally, and a few points at which the arachnoid is in contact with the venous sinus forming simple granulations.
Ependyma The ependyma of the ventricular system is mainly columnar or cuboidal ciliated, and the sub-commissural organ is present at the junction between the third ventricle and the midbrain. Just rostral to this is a complex pineal gland consisting of glia and islands of epithelium with central chromatin and large vacuoles mixed with blood vessels; the pineal lies against the skull vault in an extra-arachnoid position. The paraphysis is a simple cystic structure lying over the dorsum of the roof of the third ventricle at the point of origin of the diencephalic choroid plexus, and within the arachnoid. In the floor of the fourth ventricle the ciliated epithelium is thicker in the midline and in the lateral recesses it is very thin ependyma covered by pia and arachnoid, and containing loose folds of choroid plexus.
Choroid plexuses The choroid plexuses of the lateral ventricles fill the ventral horns as delicate villi of high columnar epithelium around a stroma of loose areolar fibrous tissue and blood vessels. Through the interventricular foramina the lateral and third ventricle choroid plexues are continuous, and caudal to this the main body of the third ventricle choroid plexus is suspended in the dorsally enlarged part of the ventricle as a collection of delicate villi covered by high columnar densely ciliated epithelium. The choroid plexus of the fourth ventricle has long villi covered with regular columnar ciliated epithelium and with a delicate stroma of blood vessels and areolar tissue. Further caudally the choroid plexus is compressed between the rhombic lip and the caudal end of the cerebellar vermis.
CSF-vascular relationships The CSF of the ventricular system is separated by the thin ependyma of the third ventricle roof and of the infundibular recess from the meningeal vascular network and across the choroid plexus epithelium from the tela choroida. The internal CSF is separated from the external CSF by the thin membranous pouch of the posterior tela of the fourth ventricle, and the external CSF is separated from the venous system by endothelium and cap cells at the points of the primitive arachnoid granulations.
The CSF system of the chick is the same as that of the pigeon.
Figure 1 TS Forebrain - Haemotoxylin and Eosin Ventrally the optic chiasm can be seen and, dorsal to this the third ventricle, with a narrow ventral portion, and a wider dorsal portion containing choroid plexus. The cerebral hemispheres lie dorsal to this and contain crescentic-shaped lateral ventricles within which can be seen the choroid plexuses ventrally, closely adjacent to, but not continuous with, the third ventricle choroid plexus. The interhemispheric fissure is narrow, and bridged over by arachnoid. Dorsal to this is the skull vault with its contained air spaces. The hemispheres have fragmented in this preparation.
Figure 2 TS Midbrain - Haemotoxylin and Eosin The midbrain ventricle at this level, caudal to the sub-commissural organ, has a thin dorsal roof composed of ependyma and glia, and extends laterally; at a more caudal level, it communicates with the optic ventricles, which are visible here, well lateral in the optic lobes. Dorsal to the midbrain is the rostral part of the cerebellum, surrounded by arachnoid and dura, outside which is the skull, filled with air cells. Ventral to the tegmentum of the midbrain is the caudal end of the pituitary fossa containing venous sinuses. The ventral subarachnoid space, just dorsal to the pituitary fossa, contains the basilar artery.
Figure 3 TS Rostral hindbrain - Van Gieson's The pons is ventral and the fourth ventricle, at this level, is a shallow horizontal structure the roof of which is formed by choroid plexus which is more densely-staining, and extends laterally to the lateral angles where it is continuous with an extra-arachnoid tela of blood vessels. Dorsal to the choroid plexus is the cerebellum, in transverse folds, surrounded by arachnoid. The subarachnoid space is visible ventrally, but not dorsally, where the space is subdural, and the arachnoid is closely adherent to the cerebellum. The auditory-vestibular nerve is visible on both sides as it proceeds from the cerebello-pontine angle to the inner ear.
Figure 4 TS Caudal hindbrain - Haemotoxylin and Eosin The medulla has the shallow caudal end of the fourth ventricle just visible in the midline, and the interval between this and the dorsally-placed caudal cerebellum is enclosed by a very thin-walled sac of ependyma which can just be seen laterally, at both sides. Lateral to the ependymal sac are folds of arachnoid and the subarachnoid space. The arachnoid is closely adherent to the dorsal aspect of the cerebellum.
Figure 5 TS Cervical spinal cord - Haemotoxylin and Eosin The cord is closely adjacent to the vertebral body ventrally, but dorsally the arachnoid can be seen as a separate layer, and the blood vessels of the pia surrounding the cord are just visible. The central canal is minute. The thicker layer outside the arachnoid is separated endosteal dura.
Figure 6 LS Whole brain - Masson's Trichrome To the left is the cerebral hemisphere in paramedian section showing the slits of the curved lateral ventricle. Just caudal to this is the third ventricle, the wall of which has been obliquely cut in the section. The roof of the third ventricle, just below the cerebellum, consists of the pineal gland and sub-commissural organ caudally, and in front of this is the bunched choroid plexus of the third ventricle. Ventrally, the hypothalamus has been sectioned and the infundibular recess can be traced to lie dorsal to the adenohypophysis, which is visible as an oval structure within the base of the skull. The midbrain ventricle is apparently enlarged because of the partial section of the tegmentum. The fourth ventricle is closed by the ependymal sac which is faintly visible. Caudal to this can be seen arachnoid membrane over the cerebello-medullary cistern and dorsal aspect of the spinal cord.
© Badgerwood 2002
| Click here to return to Evolution of the CSF System - Synopsis |