- The tubular nature of the developing nervous system persists as theembryonic brain matures. The large cavities within the cerebral vesicles develop into the ventricular system of the brain, and the remaining caudal cavity becomes the central canal of the spinal cord. as the five vesicles develop, the cavity in the forebrain differentiates into the two lateral ventricles (formerly called the first and second ventricles) and the third ventricle. The rostral wall of the third ventricle is formed by the lamina terminalis, which, at an earlier developmental stage, represented the rostral end of the neural tube. An interventricular foramen (of Monro) interconnects each lateral ventricle and the third ventricle. The ventricles contain cerebrospinal fluid, which is produced by the choroid plexus present in each of them. Because the entire central nervous system is bathed by cerebrospinal fluid, the ventricles provide an important route for chemical communication between different brain regions.
- The cavity within the midbrain, somewhat dilated in the embryo, narrows to become the cerebral aqueduct (of Sylvius) as the dorsal region (or tectum) of the midbrain develops. The cerebral aqueduct is a narrow conduit for the flow of cerebrospinal fluid to the fourth ventricle, which is located dorsal to the pons and medulla. During development a small amount of cerebrospinal fluid produced in the fourth ventricle as well as that produced in the lateral and third ventricles flows into the spinal cord through the central canal. Late in fetal life, however, the central canal closes. Thus, all cerebrospinal fluid passes through three apertures in the roof of the fourth ventricle (the two foramina of Lushka and the foramen of Magendie) to the subarachnoid space, from which it bathes the surface of the entire central nervous system. As with the subarachnoid space overlying the spinal cord, cerebrospinal fluid accumulates in cisterns in the subrarchnoid space overlying the brain. The cerebral cisterns will be discussed in the next chapter because their locations can be mapped precisely using modern brain imaging techniques. The hydrostatic pressure within the subarachnoid space is maintained at a low level beacuse cerebrospinal fluid is passively reabsorbed into the blood through small unidirectional valves, called arachnoid granulations, located along the major venous sinus, the superior sagittal sinus, and other sites.
- Obstruction of the cerebral aqueduct during development results in hydrocephalus. In this condition, cerebrospinal fluid produced by the choroid plexuses of the lateral and third ventricles cannot pass freely to more caudal parts of the ventricular system and subarachnoid space. As a consequence, pressure increases within the lateral and third ventricles, eventually cumpressing the cerebral hemispheres and enlarging the cranium, which in the infant is still free to enlarge, since the bones of the skull have not yet fused. Llft untreated, this discrder can result in mental retardation. Recently an in utero surgical thchnique has been developed to insert a tube that bypasses the obstruction and prevents damage to the developing forebrain.