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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.

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The metameres are based on the division of the original embryo into several segments.

Except for the somitomeres (1 to 7) that form no somites, but are involved in the formation of the pharyngeal arch mesoblast, the others form somites in the cranio-caudal direction. After the 25th day 3-4 somites per day are formed thereby (stage 9).
In humans 42-44 somite pairs (stage 9-13) are formed along the neural tube. These range from the cranial region up to the embryo’s tail. Several caudal somites disappear again, which is why only 35-37 somite pairs can be counted in the end.
The number of the somites that are found is used to determine the embryo's age in this developmental stage.

Fig. 14 - Evolution of the mesoblast

  1. Paraxial mesoderm
  2. Intermediate mesoderm
  3. Lateral plate mesoderm
  4. Neural groove
  5. Ectoblast
  6. Endoblast

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 - Genesis of the intraembryonic
coelom, at roughly the 23rd day

  1. Paraxial mesoblast
  2. Intermediate mesoblast
  3. Lateral plate mesoblast
  4. Chordal process
  5. Sectional edge of the amnion
  6. Intraembryonic coelom

Fig. 16 - Appearance of the somitomeres
at roughly the 25th

Somatopleure with ectoblast
Splanchnopleure with endoblast

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.

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The somites are embryonic transitional organs that are formed through the segmentation of the paraxial mesenchyma. They organize themselves without cell differentiation (primary organs).

They are responsible for the segmental organization of the embryo and contribute to its restructuring.They contain the precursor cells for the axial skeleton (sclerotome), the striated musculature of the neck, the trunk and the extremities (myotome), as well as those of the subcutaneous tissue and skin (dermatome).

The somites are the prerequisites for metamerism. The segmental (metamere) partitioning of the spine, the neural tube, the trunk wall and the thorax (ribs) depends on the ordered arrangement of the somites.