The neural tube is the embryonic structure that ultimately forms the brain and spinal cord. It is formed in a process called neurulation, in primary and secondary neurulation processes. In mice, primary neurulation prevails in the rostral sections of the embryo, while secondary neurulation occurs in the caudal section. During primary neurulation, the ectoderm is divided into the internally positioned neural plate cells, the externally positioned epidermis and the neural crest cells.
Primary neurulation begins when the underlying dorsal mesoderm (and pharyngeal endoderm in the head region) signals the ectodermal cells above it to elongate into columnar neural plate cells. The edges of the neural plate thicken and migrate toward the midline of the embryo and their eventual fusion at the dorsal midline creates a hollow neural tube. The closed cylinder separates from the surface ectoderm in a process thought to be mediated by the expression of different cell adhesion molecules. In the mouse, neurulation begins on E8.5 (corresponding to week 3 of human embryo) and ends on day E10.5, when closure is completed at the upper sacral level. The neural tube closure propagation front spreads rapidly, and the open, elevating neural folds, just caudal to it, represent the next region to undergo closure. The two open ends of the neural tube are called the anterior neuropore and the posterior neuropore. The process of primary neurulation is conserved across species such as amphibians, reptiles, birds and mammals.
Secondary neurulation involves the formation of a medullary cord and its subsequent hollowing into a neural tube. In frogs and chicks, secondary neurulation is usually seen in the neural tube of the lumbar (abdominal) and tail vertebrae. Neurulation plays a role in the timing of additional processes in the embryo, such as the beginning of neural crest migration. In mammals, the cranial neural crest cells (which form facial and neck structures) migrate while the neural folds are elevating (i.e., prior to neural tube closure), whereas in the spinal cord region, the crest cells wait until closure has been completed. Neural tube neurons proliferate in the ventricular zone, a layer adjacent to the tube cavity. Newly born neurons migrate outward to form two new layers: an internal densely packed layer called the mantle layer which becomes the gray matter, and an external marginal zone composed of axons, which becomes the white matter.