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Development of the Brain
The closure of the neural groove into the neural tube begins at the level of the upper cervical cord. From here, further closure runs in the oral direction up to the rostral end of the brain (oral neuropore, later the terminal lamina) and in the caudal direction up to the end of the spinal cord. Further developmental events in the CNS proceed in the same directions. Thus, the brain’s divi-sions do not mature simultaneously but at intervals (heterochronous maturation).
The neural tube in the head region expands into several vesicles (p. 171, A). The rostral vesicle is the future forebrain, prosen- cephalon (yellow and red); the caudal ves- icles are the future brain stem, encephalic trunk (blue). Two curvatures of the neural tube appear at this time: the cephalic flexure (A1) and the cervical flexure (A2). Although the brain stem still shows a uni- form structure at this early stage, the future divisions can already be identified: medulla oblongata(elongatedcord) (A – D3), pons (bridge of Varolius)(A – D4), cerebellum (A – D5, dark blue), and mesencephalon (mid- brain) (A – C6, green). The brain stem is developmentally ahead of the prosen- cephalon; during the second month of human development, the telencephalon is still a thin-walled vesicle (A), whereas neu- rons have already differentiated in the brain stem (emergence of cranial nerves) (A7). The optic vesicle develops from the diencephalon (AB8, red) (p. 343, A) and forms the optic cup (A9). Anterior to it lies the telencephalic vesicle (telencephalon) (A – D10, yellow); ini- tially, its anlage is unpaired (impar telen-cephalon), but it soon expands on both sidesto form the two cerebral hemispheres.
During the third month, the prosen-cephalon enlarges (B). Telencephalon and diencephalon become separated by the telodiencephalic sulcus (B11). The anlage ofthe olfactory bulb (B – D12) has formed at the hemispheric vesicle, and the pituitaryanlage (B13) (p. 201 B) and the mamillary eminence (B14) have formed at the base of the diencephalon. A deep transverse sulcus (B15) is formed between the cerebellar an- lage and the medulla oblongata as a result of the pontine flexure; the underside of the cerebellum comes to lie in apposition to the membrane-thin dorsal wall of the medulla (p. 283, E).
During the fourth month, the cerebral hemispheres begin to overgrow the other parts of the brain (C). The telencephalon, which initially lagged behind all other brain divisions in its development, now exhibits the most intense growth (p. 170, A). The center of the lateral surface of each hemi- sphere lags behind in growth and later be- comes overlain with parts. This is the insula (CD16). During the sixth month, the insula still lies free (D). The first grooves and con- volutions appear on the previously smooth surfaces of the hemispheres. The initially thin walls of neural tube and brain vesicles have thickened during development. They contain the neurons and nerve tracts that make up the brain substance proper. (For development of cerebral hemispheres, see p. 208.)
Within the anterior wall of the impar telen- cephalon, nerve fibers run from one hemi- sphere to the other. The commissural sys- tems, which connect the two hemispheres, develop in this segment of the thickened wall, or commissural plate. The largest of them is the corpus callosum (E). The hemi- spheres grow mainly in the caudal direc- tion; in parallel with their increase in size, the corpus callosum also expands in the caudal direction during its development and finally overlies the diencephalon.
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