The central nervous system (CNS) appears at the beginning of the third week as a slipper-shaped plate of thickened ectoderm, the neural plate, in the middorsal region in front of the primitive node. Its lateral edges soon elevate to form the neural folds (Fig. 19.1). With further development, the neural folds continue to elevate, approach each other in the midline, and finally fuse, forming the neural tube (Figs. 19.2 and 19.3). Fusion begins in the cervical region and proceeds in cephalic and caudal directions (Fig. 19.3A). Once fusion is initiated, the open ends of the neural tube form the cranial and caudal neuropores that communicate with the overlying amniotic cavity (Fig. 19.35). Closure of the cranial neuropore proceeds cranially from the initial closure site in the cervical region (19.3A) and from a site in the forebrain that forms later. This later site proceeds cranially, to close the rostralmost region of the neural tube, and caudally to meet advancing closure from the cervical site (19.3B). Final closure of the cranial neuropore occurs at the 18- to 20-somite stage (25th day); closure of the caudal neuropore occurs approximately 2 days later.
The cephalic end of the neural tube shows three dilations, the primary brain vesicles: (a) the prosencephalon, or forebrain; (b) the mesencephalon, or midbrain; and (c) the rhombencephalon, or hindbrain (Fig. 19.4). Simultaneously it forms two flexures: (a) the cervical flexure
Figure 19.1 A. Dorsal view of a late presomite embryo at approximately 18 days. The amnion has been removed, and the neural plate is clearly visible. B. Dorsal view at approximately 20 days. Note the somites and the neural groove and neural folds. C. Scanning electron micrograph of a mouse embryo at a stage similar to that in B. F, forebrain; M, midbrain; H, hindbrain.
at the junction of the hindbrain and the spinal cord and (b) the cephalic flexure in the midbrain region (Fig. 19.4).
When the embryo is 5 weeks old, the prosencephalon consists of two parts: (a) the telencephalon, formed by a midportion and two lateral out-pocketings, the primitive cerebral hemispheres, and (b) the diencephalon, characterized by outgrowth of the optic vesicles (Fig. 19.5). A deep furrow, the rhombencephalic isthmus, separates the mesencephalon from the rhombencephalon.
The rhombencephalon also consists of two parts: (a) the metencephalon, which later forms the pons and cerebellum, and (b) the myelencephalon. The boundary between these two portions is marked by the pontine flexure (Fig.19.5).
The lumen of the spinal cord, the central canal, is continuous with that of the brain vesicles. The cavity of the rhombencephalon is the fourth ventricle, that of the diencephalon is the third ventricle, and those of the cerebral hemispheres are the lateral ventricles (Fig. 19.5). The lumen of the mesencephalon connects the third and fourth ventricles. This lumen becomes very narrow and is then known as the aqueduct of Sylvius. The lateral ventricles communicate with the third ventricle through the interventricular foramina of Monro (Fig. 19.5).
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