Mitosis occurs on the luminal side of the cell layer. DNA synthesis (S phase) occurs while the nucleus is at the outside edge of the neural tube, and the nucleus migrates luminally as the cell cycle proceeds ( Figure 12.15). The nuclei move within their cells as they go through the cell cycle. Sauer (1935) and others have shown that all the cells of the germinal epithelium are continuous from the luminal surface of the neural tube to the outside surface, but that the nuclei of these cells are at different heights, thereby giving the superficial impression that the neural tube has numerous cell layers. This is a layer of rapidly dividing neural stem cells. The original neural tube is composed of a germinal neuroepithelium that is one cell layer thick. The neurons of the brain are organized into layers ( cortices) and clusters ( nuclei), each having different functions and connections.
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