Once induction has occurred, the elongated, slipper-shaped neural plate gradually expands toward the primitive streak (Fig. 5.2, B and C). By the end of the third week, the lateral edges of the neural plate become more elevated to form neural folds, and the depressed midregion forms the neural groove (Figs. 5.2, 5.3, A and B, and 5.4). Gradually, the neural folds approach each other in the midline, where they fuse (Fig. 5.3C). Fusion begins in the cervical region (fifth somite) and proceeds cranially and caudally (Figs. 5.5 and 5.6). As a result, the neural tube is formed. Until fusion is complete, the cephalic and caudal ends of the neural tube communicate with the amniotic cavity by way of the cranial and caudal neuropores, respectively (Figs. 5.5, 5.6A, and 5.7). Closure of the cranial neuropore occurs at approximately day 25 (18- to 20-somite stage), whereas the posterior neuropore closes at day 27 (25-somite stage). Neurulation is then complete, and the central nervous system is represented by a closed tubular structure with a narrow caudal portion, the spinal cord, and a much broader cephalic portion characterized by a number of dilations, the brain vesicles (see Chapter 19).
As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm begin to dissociate from their neighbors. This cell population, the neural crest (Figs. 5.3 and 5.4), will undergo an epithelial-to-mesenchymal transition as it leaves the neuroectoderm by active migration and displacement to enter the underlying mesoderm. (Mesoderm refers to cells derived from the epiblast and extraembryonic tissues. Mesenchyme refers to loosely organized embryonic connective tissue regardless of origin.) Crest cells from the trunk region leave the neural folds after closure of the neural tube and migrate along one of two pathways: 1) a dorsal pathway through the dermis, where they will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles; and 2) a ventral pathway through the anterior half of each somite to become sensory ganglia, sympathetic and enteric neurons,
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