7.2 The trilaminar germ disk (3rd week)

• Introduction
  
• Formation of the primitive streak
• Genesis of the germ layers
• Genesis of the notochord
• Location of the epiblast cell target and the development of the primitive streak
• The bilaminar membranes
• Evolution of the mesoblast
  • The paraxial mesoblast and the differentiation of the somites
  • The intermediate mesoblast
  • The lateral mesoblast
• The intraembryonic coelom
• Induction of the neural plate - neurulation

Induction of the neural plate - neurulation

With the primary neurulation begins the genesis of the nervous system. The notochord exercises an inductive effect on the ectoblast that lies above it. This causes the ectoblast cells to transform themselves into neuroectoblast cells. This was already known at the beginning of the 20th century and has now been proven to be true.

On the 19th day the neural plate 7appears. It represents the first step in the genesis of the nervous system. The neural plate is identifiable as the medio-sagittal thickening of the ectoblast rostral to the primitive streak. At the cranial end the neural plate is wider and consists of the region where the brain will arise. At the caudal end it is narrower and gives rise to the spinal cord.
Approximately 50% of the ectoblast becomes the neural plate and the remainder forms the epidermis.

The neural plate develops in step with the genesis of the notochord, i.e. under the inducing influence of the axial mesoderm lying below it (prechordal plate and cranial portion of the chordal plate). The induction process is very complex, but has its origin in the secretion of inducing factors by axial mesoderm cells. These factors diffuse in the direction of the ectoderm cells that lie above them where they activate genes that are responsible for the differentiation of epithelium that has come from the ectoderm into several rows of prismatic epithelium: the neuroectoblast. In the course of the 3rd week the edges of the neural plate rise up and become neural folds 9, enclosing the neural groove8.

Fig. 22
With the appearance of the neural plate on the 19th day, the development of the future nervous system begins.

Fig. 23
The cranial end the neural plate is wider and encloses the region where the brain will arise.
The caudal end is narrower; there the spinal cord will form.

The neural folds approach each other after the 25th day and merge to form the neural tube (delimitating the future central canal, which is coated with ependymal cells). The closure of the neural tube begins in the cervical area (in the middle of the embryo) and extends from there in both the cranial and caudal directions.
The anterior neuropore (cranial) closes itself on the 29th day 11.
The posterior neuropore (caudal) closes a day later 12. The top of the anterior neuropore corresponds to the terminal lamina of the adult brain and the posterior neuropore to the terminal filum at the end of the spinal cord.

If the posterior neuropore does not close, a spina bifida occurs. If, on the other hand, the closure of the anterior neuropore fails to take place, an anencephaly results.
While the neural tube is in the process of closing, cells on the lateral side of the neural plate detach themselves and form the neural crest.

Fig. 24
In the course of the 3rd week the edges of the neural plate rise and form two folds that bound the neural groove.

Fig. 25
The closure of the neural tube begins in the cervical area (in the middle of the embryo) and spreads from there in the cranial and caudal directions.

The neural crest cells 9 form, so to speak, a 4th embryonic germinal layer. This contains a partial segmentation that contributes to the formation of the peripheral nervous system (neurons and glia cells of the sympathetic, parasympathetic and sensory nervous systems).
The neural crest cells are distinguished by a great migrating ability and phenotypic heterogeneity, since numerous and various differentiated cell types will arise from them. Not only do the nerve and glia cells mentioned above arise from the neural crest, the epidermal pigment cells (melanocytes), the calcitonin cells of the thyroid gland, the cells of the adrenal medulla and some components of the skeletal and connective tissue in the head area are also generated from them.

Fig. 26
The beginning of neurulation in the cervical region.
The neural groove forms. The cells of the future neural crest are in orange. The arrows show the direction of the lateral folding.

Fig. 27, Fig. 28
On the neural plate the formation of the neural groove 9 follows and afterwards the neural tube 11.
Masses of cells detach themselves from the lateral side of the neural plate and form the neural crest. As soon as the neural crest cells leave the neuroepithelium they lose their cohesion (8 - 9).


NB: Note that in the neural tube formation stage the neuralepithelium is multilayered which, in this diagram, for the sake of simplicity, could not be shown.