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

The paraxial mesoblast and the differentiation of the somites

The paraxial mesoblast comes from the epiblast cells that migrated into the region of the primitive node or the cranial portion of the primitive streak. It forms a pair of cylinder-shaped epithelially-organized mesenchyma segments that are in the immediate vicinity of the neural tube and the notochord.
After the beginning of the 3rd week, these cylinders become segmented from the cranial to the caudal end into so-called somitomeres (process of metamerization). Originally, each consists of a pseudostratified epithelium that is arranged around a central cavity, the somitocoel.

Fig. 14
Transversal section at the 25th day.
To be seen is the differentiation of the mesoblast into the paraxial, intermediate and lateral plate mesoderm

Fig. 15
Transversal section through a 23-day-old embryo. One can recognize the first space of the future intraembryonic coelom.

Fig. 16
Transverse section with a dorsal view at around the 25th day. The cylindrical cell collections of the paraxial mesoderm segment form the somitomeres (arrows). The intermediate mesoblast is involved in the urogenital development.