Primary and secondary neurulation
Primary neurulation comprises the formation of the primitive neural tube from that region of the superficial ectoderm that is located dorsal to the notochord. This process is induced by the axial mesoblasts, the notochord and the prechordal plate (chorda-mesoderm complex). Indeed, the inductive effect of these structures in the transformation of ectoblasts into neuroectoderm has been reliably proven since the principle became known already at the beginning of the 20th century (Spemann, 1924). Besides neuroblastic induction the determination of the central nervous system also includes the histologic differentiation (histogenesis) of neuronal tissue (molecular mechanisms during the early embryonic development of the CNS).
In contrast to primary neurulation, secondary neurulation pertains to the development of the caudal part of the neural tube at the level of the 31st somite (from the 4th to the 7th week). In this recall that, before it disappears on the 29th day, the primitive streak generates a permanent mesodermal structure termed the caudal eminence. From this is derived the caudal part of the neural tube and thus the extension of the spinal cord. In the originally compact cord, a hollow space arises that connects with the central canal and that finally becomes coated with neural epithelium.
The induction of the neural tube leads to differentiation into three ectodermal regions, each of which having its own purpose:
- The dorsomedian ectoderm (future neural plate), out of which the neural tube arises
- The ectoderm in the border region of the neural epithelium, out of which the neural crest arises
- The lateral ectoderm, out of which the epidermis and the epiblastic placodes arise