[Somitogenesis of the human embryo: appearance of myotomes]

In the human embryo, somitic segmentation takes place between stage 9 and 13, that is from 19-21 days to 28-32 days of age. Somites are initially formed of clusters of elongated epithelial cells. In embryos with 10-12 somites, a cavity (the myocele) appears in the central portion of the somites while the neural crest (with its migratory cells) begins to develop along the closed portion of the neural groove. Later on, the medial face of the somites gives rise to the sclerotomes in their ventral portion and to the myotomes in their dorsal portion. In embryos with 30 somites, the cells of the neural crest have developed and they push the myotomes in a ventral direction. The spinal ganglia that increase in volume form a relief on the surface of the embryo. The segmentation that is now visible is ganglionic and no more somitic.