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.
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.