Genesis of the middle germinal layer (mesoderm)
With only a few exceptions the musculature stems from mesoblast cells that – for their part – come originally from a population of epiblast cells (ectoderm cells), which immigrate via the primitive streak (stages 6-11) between the hypoblast and the remaining epiblast cells. They thus form a middle germinal layer, the mesoderm layer.
Differentiation of the mesoderm layer into muscle tissue
Except at two places, the cloacal and oropharyngeal membranes, the mesoderm layer separates the hypoblast completely from the ectoderm. The embryonic disk is now trilaminar (stage 8). Consequently, the cells of this mesoderm layer are called mesoblast cells until their specific mesenchymal or epithelial allocations are clear. Over the course of their further development, the mesoblast cells become clearer. A portion of these cells on both sides of the neural tube differentiate into somites and lateral plate (stage 9). The intermediary mesoderm (stage 10) becomes visible somewhat later.
Among other things, the skeletal musculature arises from the paraxial mesoderm. The intermediary mesoderm is involved in the formation of the urinary system. From the unsegmented lateral plate mesoderm, the somato- and splanchnopleura form with the coelom that lies in between them. Other mesoblast cells settle together to form the cardiogenic and the myocardial plates. This is a connected group of cuboid cells in the ventral mesoderm in front of the prechordal plate (see Fig. 1). From this ensemble of cells arises the cardiac musculature.