The musculature of the human body stems from the middle germinal layer, the mesoderm. One distinguishes three varieties of muscles:
- skeletal musculature
- cardiac musculature
- smooth musculature
The middle germinal layer forms through an inflow of cells with an ectodermal origin via the primitive streak and nodes. The mesoderm on both sides of the neural tube is called paraxial mesoderm. Out of it develops the musculature, among other things. Mesodermal cells more lateral form the intermediary mesoderm and completely lateral, at the transition to the extraembryonic structure, the lateral plate mesoderm forms. In the most cranial section of the embryo, in front of the prechordal plate, an accumulation of mesodermal cells form the cardiogenic plate, out of which the material for the cardiac musculature derives.
As an in between step, the somites form from the paraxial mesoderm. These represent pairs of epithelialized mesodermal segments to the left and right of the neural tube. They do not last long in this form and differentiate further into skeletal musculature (myotome), the skin (dermatome) and the skeletal axis (sclerotome).
The myotome cells go through various stages during their development to become skeletal musculature. The premyoblast is the first differentiated preliminary stage of the muscle cells, in that the cell bodies and the nucleus are lengthened. Responsible for this are several genes on chromosome 11. They belong in the family of myogenic regulatory factors (MRF) and activate the transcription of muscle-specific genes. The premyoblast then transforms itself into the myoblast that synthesizes the muscle-specific proteins actin and myosin. Subsequently, several myoblasts move together and form a syncytium, the myotubes. Initially, their nuclei are centrally located but with the beginning of neural activity they move to the periphery.
The smooth musculature forms in the surroundings of organs such as the trachea, the digestive tract, blood vessels, etc. Like with skeletal musculature the differentiation occurs through induction of muscle-specific genes in the corresponding myoblasts.