8.2 From the blastoderm layer to the embryo



From the second to the fourth week


After the end of the 2nd week to the 4th week the cell mass from which the embryo arises, experiences numerous modifications. Cell proliferation and migration lead to the process of the mesoblast immigration. This leads to the genesis of a trilaminar germ disk whose structure already exhibits the precursors (anlage) of the most important organs.
The processes of cellular proliferation and migration constitute the so-called epithelio-mesenchymal transition (gastrulation in lower vertebrates).
At the end of this period various systems are already differentiated. In particular, a large part of the central (neural tube) and the peripheral nervous systems (neural crest) are developed. The anlage of the muscle system, the skeletal axis and the dermis appear at the level of the somites. The somites consist of mesoblastic, metamerized cell collections.

Fig. 4 - Transversal section and dorsal view of a 21 day-old trilaminar embryo  Legend

1
2
3
4
5
6
7
8
9
10
11
12		 Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
Notochordal process
Amnion
Intraembryonic coelom
Endoderm
Ectoderm
Somatopleura (mesoderm and ectoderm)
Splanchnopleura (mesoderm and endoderm)
Neural groove
Neural plate

Fig. 4
Schematic diagram of a transversal section and dorsal view of a trilaminar embryo towards the end of the third week.


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