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

Genesis of the germ layers

From the 17th day 6 the primitive streak forms an entry location for the epiblast cells to invade the underlying mesoblast and to proliferate there.
Histological methods have shown that during their migration along the primitive groove, the epiblast cells form pseudopodia. They lose contact with each other thereby. This phenomenon of the inflow of cells to form the third embryonic layer is termed epithelio-mesenchymal transition 6-7 (gastrulation in the lower vertebrates) .

Fig. 3
Oval-shaped embryonic disk (dorsal view).

The red arrows show schematically the migration directions of the epiblast cells to their points of final destination.

Depending on their origin and the moment of their flowing in, the epiblast cells migrate away from the primitive streak in various directions.

The first cells that enter through the node and the primitive groove replace the hypoblast layer and form the definitive endoblast (origin of the future intestine and its derivates). At the same time, due to the migration of cells through the primitive node in the cranial direction, two further structures are formed:

• the prechordal plate 6, which is located cranial to the primitive node
• the notochordal process8the development of which will be treated in the next section.

Fig. 4
Transverse section at the level of the primitive groove with the immigration of epiblast cells, which form the future mesoblast, as well as the endoblast, which replaces the hypoblast.

The largest proportion of these immigrated cells form a third germinal layer, the intraembryonic mesoblast 6. The mesoblast cells wander in all directions: laterally, cranially and caudally. This middle germinal layer lies between the definitive endoblast and epiblast. Exceptions are the cloacal membrane as well as the pharyngeal membrane, where the ectoderm and endoderm lie directly opposite each other 6.

Cranial to the prechordal plate, mesenchyma cells of the embryonic disk will form the pericardium as well as the septum transversum 9 . At the caudal extremity the cloacal membrane forms the primordium of the future opening of the uro-genital tract and the rectum.