Because of their location, this set of precursors is called the neural crest ( Figure 22.2). At the most dorsal limit of the neural tube, a third population of cells emerges in the region where the edges of the folded neural plate join together.
Precursor cells farther away from the ventral midline give rise to sensory neurons within the spinal cord and hindbrain. Inductive signals from the floorplate lead to the differentiation of cells in the ventral portion of the neural tube that eventually give rise to spinal and hindbrain motor neurons (which are thus closest to the ventral midline). The position of the floorplate at the ventral midline determines the dorso-ventral polarity of the neural tube and further influences the differentiation of neural precursor cells. As a result of their proximity to the notochord, the cells at the ventral midline of the neural tube differentiate into a special strip of epithelial-like cells called the floorplate. These precursors are dividing stem cells that produce more precursors and, eventually, nondividing neuroblasts that differentiate into neurons.
The progenitor cells of the neural tube are known as neural precursor cells. This structure, the neural tube, subsequently gives rise to the brain and spinal cord. The lateral margins of the neural plate then fold inward, eventually transforming the neural plate into a tube. During this process, called neurulation, the midline ectoderm that contains these cells thickens into a distinct columnar epithelium called the neural plate. Thus the notochord (along with the primitive pit) sends inductive signals to the overlying ectoderm that cause a subset of neuroectodermal cells to differentiate into neural precursor cells. In addition to specifying the basic topography of the embryo and determining the position of the nervous system, the notochord is required for subsequent neural differentiation (see Figure 22.1). The ectoderm that lies immediately above the notochord is called the neuroectoderm, and gives rise to the entire nervous system. As a result of these cell movements during gastrulation, the notochord comes to define the embryonic midline.
The notochord forms from an aggregation of mesoderm that invaginates and extends inward from a surface indentation called the primitive pit, which subsequently elongates to form the primitive streak. One key consequence of gastrulation is the formation of the notochord, a distinct cylinder of mesodermal cells that extends along the midline of the embryo from anterior to posterior. On the left are dorsal views of the embryo at several different stages of early development each boxed view on the right is a midline cross section through the embryo at the same stage.
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