Right after the egg is fecundated by the sperm, a rapid succession of divisions occur, and within hours distinct areas of the embryo can be distinguished. From very early on, three distinct areas of the embryo are classified: the endoderm, which is the area closest to the uterine wall, the mesoderm, and the ectoderm, which is the area that faces the uterine cavity. These divisions are important not only in an anatomical sense, but because they each contain the specific cells that will eventually differentiate into the different organs and body parts. The nervous system is derived exclusively from cells of the ectoderm.

By day 17 of development, the embryo is formed in a elongated and flattened state, with its dorsal area (the area that faces away from the uterus wall) displaying a bulge in the middle. This bulging is called the neural plate and constitutes what will become the nervous system. When this structure reaches its maximum thickness, it begins to invaginate in the longitudinal axis, eventually coming together at the middle, and closing towards the extremes, thus forming a tube, called the neural tube.

The neural plate in its unvaginate state has a series of cell linings along its periphery, that remain during the formation of the neural tube, thus forming two longitudinal structures on the dorsal side of the neural tube, called the neural crests. The cells that constitute the neural tube (formerly the neural plate) are going to give rise to the neurons of the central nervous system, astrocytes and oligodendrocytes, whereas the cells of the neural crest will give rise to ganglionar neurons, meningeal cells and Shwan cells.

Once the neural tube is closed on both extremes, which happens after the first month after conception, the cavity that is formed inside the tube is surrounded by ependymal cells, that are the cells that will give rise to the different types of neurons of the CNS, whereas the cells of the outer portion of the tube will give rise to all other types of cells. This cavity will eventually form the ventricles of the encephalon and the central canal of the spinal cord.

By 3-4 weeks the anterior portion of the neural tube is divided in three distinct, but connected rounded cavities, that distinguishes the portion that will become the encephalon, whereas the middle and caudal portions remain as a tube and will give rise to the spinal cord. At this stage of development the embryo is curled and it is possible to notice where the head is located, the heart is pumping and stumps can be seen where the extremities will be. This stage is thus called the three-vesicle stage.

The most anterior vesicle or cavity is called the forebrain or prosencephalon, the second is the midbrain or mesencephalon, and finally the caudal one is the hindbrain or rhombencephalon. By five weeks the development of the CNS is said to be in a five vesicle stage, since the forebrain and hindbrain have subdivided each in two. These divisions are considered the last mayor subdivision of the nervous system during development, and therefore the nomenclature for the different sections is carried even for describing the mature brain.

The forebrain divides in an anterior and a posterior section. The anterior is now called the telencephalon, and thru a series of flexions and invaginations, additionally to the constant but uneven growth, is divided laterally into two cavities, each of which will give rise to the cerebral hemispheres, and the cavities will become the respective lateral ventricles. The second division of the forebrain is the diencephalon, which will give rise to the central structures of the brain, and its cavity will become the third ventricle. From the lateral sides of this section, two cup shaped structures arise, but with no continuity with the inner cavity. These structures will eventually grow into the retina and the optic nerve.

The division of the hindbrain is not as evident, and is only divided thru the middle. The anterior portion is the metencephalon and the posterior the myelencephalon. The midbrain and caudal portions (spinal cord) remain undivided, but continue to grow and differentiate, and in the case of the midbrain its cavity will derive in the cerebellar (Sylvian) aqueduct.

At the five-vesicle stage begins a process called segmentation. Segmentation is crucial for the development of each of the wide variety of structures and nuclei of the nervous system. In the three anterior portions of the brain (telencephalon, diencephalon and mesencephalon), segmentation is carried out mostly by growth from the ventricles outwards, such that the cortex is the first to grow (but not to differentiate) followed by limbic areas. The growth of these areas is mostly radial from the mesencephalon, which by now is folded into an inverted U shape. Therefore the growing structures of the telencephalon and diencephalon encase completely the mesencephalon and the anterior portions of the metencephalon, thus resulting in the cortex being the only outer brain structure in the anterior of the head.

Between three to seven months all brain structures and nuclei will be differentiated. From the telencephalon derive the cerebral cortex, basal ganglia, hippocampal formation, amygdala and olfactory bulb. From the diencephalon the thalamus and surrounding nuclei, hypothalamus, retina and optic nerve.

The mesencephalon gives rise to the midbrain structures, and the metencephalon the pons and cerebellum. The myelencephalon derives in the medulla. The caudal part of the neural tube develops and differentiates into the spinal cord.

Segmentation in the hindbrain structures and spinal cord occurs not from the ventricles, as in the other sections, but from multiple specialized structures adjascent to the neural tube. In the case of the hindbrain (metencephalon and myelencephalon) cells derived from the neural crest aggregate into eight distinct segments that surround this structure.

These segments are called rhombomeres and are labeled r1 to r8 from anterior to caudal. To each segment corresponds the entrance of a cranial nerve and location of its nucleus. It is thought that these segments determine the organization of the cranial nerves' innervation and nuclei in the hindbrain, and possibly of surrounding nonneural tissue. In the case of the spinal cord, it is nonneural tissues that determine its segmentation. Very early in development, when the neural plate is curling into the neural tube (19 days of gestation), mesodermic tissue conforms a grid on each side of the forming tube and is divided into segments, which will eventually form the ribs and vertebrae. These segments are known as somites, and determine the formation and aggregation of neurons to form the motor ganglions of the spinal cord.

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