Embryo at 18 Days

At this point, the fate of ectodermal cells, particularly that of the visibly thickened sheet of cells above the notochord called the neural plate, can be mapped fairly precisely. Stem cells found in local regions of the ectoderm and neuroectoderm from the front (nearer to the primitive knot/Hensen’s node) to the back will go on to form sensory specializations (lens and olfactory placode), endocrine tissue (the hypophysis), and, most notably, distinct regions of the central nervous system (CNS), including the forebrain (cerebral cortex, hippocampus, basal ganglia, basal forebrain regions such as the amygdala, olfactory bulb, and thalamus), midbrain (superior and inferior colliculi and tegmental areas), hindbrain (cerebellum and brainstem), and spinal cord. In addition, the neuroectodermal cells at the margin of the neural plate—farthest from the notochord and its instructive as well as protective signals—become a specialized population of neural stem cells called the neural crest. These neural crest cells eventually delaminate from the neuroectoderm and migrate throughout the embryo, where they make sensory ganglia as well as sympathetic and parasympathetic ganglia of the peripheral nervous system. In addition, neural crest cells contribute to the adrenal glands, and make pigment cells as well as well as cranial bones, teeth, and connective tissue. This geometric division of the neuroectoderm into a “fate map” for early populations of neural stem cells at distinct locations reflects a more fundamental molecular process. Because of variations in local signals exchanged between the notochord, the neuroectoderm, and some other early embryonic structures that arise during gastrulation, there are local changes in patterns of gene expression that distinguish the cells that will generate the forebrain, midbrain, hindbrain, and spinal cord. For the most part, these genes are transcription factors that then influence the subsequent expression of downstream genes that confer local identity in neuronal progeny. Thus the combination of cell movements and cell-cell signaling that occur during early embryogenesis establish a spatial and molecular template for the construction of the entire central and peripheral nervous system.

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