Formation of the neural tube
The appearance of the neural plate on the 19th day (stage 7) represents the initial event in the development the future nervous system. The neural plate develops cranially to the primitive streak (stages 6-7) in the form of a medio-sagittal thickening of the ectoderm that follows a cranio-caudal gradient. At the cranial end the neural plate develops more quickly so it is distinctly broader at the location of the future brain while the caudal portion, from which the spinal cord arises, remains narrow. Thereby the Neural plate takes on the form of a club. These transformation processes take place simultaneously with gastrulation
The neural plate develops above the notochord and due to the inductive effects of the axial mesoderm (prechordal plate and notochord). The mechanism of the induction is complex and should be guided by inductive substances that are secreted by the cells of the axial mesoderm. These guiding factors diffuse to the overlying ectodermal cells in which they activate genes that are responsible for the differentiation of the ectodermal epithelium in a multi-row highly prismatic epithelium, the neuroectoderm.
Thus, the neural plate appears first at the cranial end of the embryo and expands further at its caudal end by recruiting additional neuroectodermal cells. During the 3rd week the edges of the neural plate curl up as neural folds and thereby delimit the neural groove (stage 8). By means of the C-shaped longitudinal bending of the embryo the anterior and posterior portions of the neural plate are rolled up ventrally.
The edges of the neural groove approach each other and finally fuse after the 28th day through the formation of the neural tube (stage 10). At the same time collections of cells isolate themselves in the transition region between the ectoderm and the neuroectoderm. These lateral cellular cords form the neural crest (stage 9).
The closure of the neural tube begins in the neck area (at the level of the 4th somites) and, at the same time, proceeds cranially (closure of the neuroporus rostralis on the 29th day (stage 11)) and caudally (closure of the neuroporus caudalis on the 30th day (stage 12).
In the adult brain the location of the neuroporus rostralis corresponds to the lamina terminalis, the neuroporus caudalis to the filum terminale. In defective closures of the rear neuroporus the malformation of a deformity arises known as spina bifida. If the closure of the anterior neuroporus fails to occur an anencephaly results.
The closure of the neural tube is closely linked with the fact that die neuroectodermal cells recognize each other and, thanks to an increased expression of N-cadherins and N-CAMs, their adhesive properties are improved.
The superficial ectoderm closes again over the neural tube and neural crest. Roughly 50% of the ectoderm develops into the neural plate while, from the other half, the future epidermis arises.
Neurulation and Carnegie stages:
- Stage 7, ca. 19th day : neural plate
- Stage 8, ca. 23th day: neural folds and neural groove
- Stage 9, ca. 25th day: neural groove still open, formation of the neural crest, rhombomeres
- Stage 10, ca. 28th day: fusing of the neural folds, formation of the neural tube
- Stage 11, ca. 29th day: closure of the neuroporus rostralis
- Stage 12, ca. 30th day: closure of the neuroporus caudalis, secondary neurulation
- Stage 13, ca. 32th day: complete closure of the neural tube