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

Evolution of the mesoblast

The extraembryonic mesoblast 5 is formed already during the 2nd week. It is involved in the construction of the placenta and the other appendage organs.

The intraembryonic mesoblast is the third germinal layer. It arises in the 3rd week via the immigration of cells at the primitive streak. Out of this develop the various tissues and organs of the embryo.

In the beginning, the cells of the mesoblast (mesodermal cells) build a thin, widely meshed layer on both sides of the median line, between the ectoderm and the endoderm.
While the notochord is forming - it grows to the same extent that the primitive streak recedes - the intraembryonic mesoderm cells multiply on both sides of the median line and so form 3 structures in the shape of longitudinal columns.
This process begins at the cranial pole and continues up to the 4th week in the caudal direction. In the following section we shall see which specific structures arise from these three formations.

Fig. 13
Schematic 3D-diagram of an embryo at around the 25th day. The red part shows the intraembryonic mesoderm, which consists of three parts. The paraxial mesoderm forms on both sides of the median line 9.
A bit more laterally one recognizes the intermediate mesoderm 10(2nd pouch) that gets thinner laterally into the lateral plate mesoderm. This last divides in order to participate in the formation of the somatopleural and splanchnopleural mesoderm which enwrap the intraembryonic coelom.

The intraembryonic mesoderm differentiates itself into three subdivisions on both sides of the primitive streak as it recedes:

• Paraxial mesoderm
• Intermediate mesoderm
• Lateral plate mesoderm