Development Of Brain Vesicles

The closed lumen of the neural tube is altered in shape as a result of enlargement of the three brain vesicles, particularly the telencephalon. As the cerebral hemispheres of the telencephalon increase in size, two lateral expansions of the lumen form the first and second (lateral) ventricles, connected to each other and to the third ventricle by a thin, Y-shaped midline channel called the foramen of Monro. Continued posterior and inferior expansion of the hemispheres eventually forces the cortex and the underlying lateral ventricles into a C shape. Tissue surrounding the lateral ventricles develops into: (1) primary motor and sensory cortex, which provides conscious motor control and sensory perception; (2) limbic structures, which establish mood and emotion; and (3) basal ganglia, which contain relay nuclei for motor pathways. Two structures develop from the diencephalon and surround the third ventricle: the thalamus, which

C. 22 day

FIGURE 1 Formation of neural plate, neural crest, and neural tube. Human embryos at approximately (A) 18 days, (B) 20 days, (C) 22 days, and (D) 23 days. Left side, dorsal view; right side, cross section at the level indicated on the dorsal view. Note that the neural crest closes into a neural tube in the center first, and this closure moves rostrally and caudally, leaving a neuropore at each end that closes at about 4 weeks.

FIGURE 1 Formation of neural plate, neural crest, and neural tube. Human embryos at approximately (A) 18 days, (B) 20 days, (C) 22 days, and (D) 23 days. Left side, dorsal view; right side, cross section at the level indicated on the dorsal view. Note that the neural crest closes into a neural tube in the center first, and this closure moves rostrally and caudally, leaving a neuropore at each end that closes at about 4 weeks.

contains sensory relay nuclei, and the hypothalamus, which provides neurohormones controlling reproductive and other behaviors. A narrow cerebral aqueduct extends rostrally from the third ventricle through brain stem structures (midbrain, pons, and medulla), areas that contain ascending, descending, and reflex pathways for motor and sensory circuits, as well as nuclei controlling vegetative functions. The fourth ventricle emerges from the cerebral aqueduct and is flattened between the caudal portion of the brain stem and the overlying cerebellum, an area involved in fine motor control (Fig. 3).

Cerebrospinal fluid fills the ventricles, providing a cushion of support for delicate neuronal tissue. It is generated by the choroid plexus, which is composed of a capillary network surrounded by cuboidal or columnar epithelium lining the ventricular walls, in particular the roof of the lateral and fourth ventricles. Structurally similar to the distal and collecting tubules of the kidney, the choroid plexus maintains the chemical stability of the CSF. It has directional capabilities in that it continually produces CSF and actively transpors metabolites out of the ventricles. Cerebrospinal fluid circulates through the ventricles, exits through openings in the fourth ventricle to the subarachnoid space surrounding the outer surface of the brain, is reabsorbed by the arachnoid granulations, and eventually collects in the venous system of the meningeal covering of the cortex. This circulation serves an important function in maintaining the appropriate ionic milieu necessary for neuronal activity. Blockage of CSF flow by insufficient absorption, by obstructions such as brain tumors or genetic malformations, or by internal bleeding caused by trauma or stroke can cause increases in intracranial pressure, enlargement of ventricles, and development of hydrocephalus.

Samples of CSF are often removed from the spinal cord region for analysis in order to screen for possible infections or electrolyte imbalances. CSF bathes the full extent of the spinal cord within its surrounding dural sack, the most caudal region of which contains only groups of exiting spinal nerves called the cauda equina. This affords a reasonably safe region from which CSF samples can be removed with little danger of injury to brain or spinal cord. In a procedure called a spinal tap, a small amount of CSF is removed by inserting a needle through an intervertebral space in the lumbosacral region and piercing the underlying dura.

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