14.3 Skeletal musculature

• Origin of the skeletal musculature
  
• Differentiation of the somites
  
• Musculature of the limbs
  
• Autochtonous back musculature
  
• Muscles of the ventrolateral thorax and abdominal wall, diaphragm
  
• Muscles of the pharyngeal arches
  
• Musculature of the tongue
  
• External eye musculature
  

Origin of the skeletal musculature

Skeletal muscles stem from the dermatomyotomes of the paraxial mesoderm that extend from the primitive streak to the prechordal plate. In stage 9 the paraxial mesoderm - caudal to the otic vesicle - begins to segment and the insuing structures are called metameres . This is the start of the somite development 9 that progresses along a craniocaudal gradient. With 4½ weeks 13 ca. 30 somite pairs are present. In total, ca. 38 or 39 somite pairs (4) are formed (4 occipital, 8 cervical, 12 thoracic, 5 lumbal,5 sacral and 4 or 5 coccygeal somite pairs, whereby not all of them are present at the same time.

Fig. 9
The approximate extension of the supply areas of the individual myotomes is shown with various colors

The somites and the myotomal derivates that come from them – according to their level - form the somatic musculature. The occipital myotomes (4, light green) mainly form the pharynx (throat) and upper or anterior neck musculature, including the tongue muscles. They are also responsible for the musculature in the occipital head region. From the cervical myotomes (8, orange) the neck and shoulder musculature form. They are followed by the thoracic myotomes that form the musculature of the upper extremities and breast and abdominal wall musculature. The lumbal myotomes (5, light blue) are responsible for the formation of the musculature in the lower extremities. Finally follow the sacral and coccygeal myotomes (5 sacral and 4-5 coccygeal, green) that form the muscles in the lower pelvis and gluteal region.

Fig. 10
The somites with the somitocoel are formed out of the paraxial mesoderm. In this stage the intermediate mesoderm is also now visible. The embryo begins to fold laterally. The lateral mesoderm has divided into splanchno- and somatopleura. In this cross-section the neural crest cells are also shown schematically. From them come a large variety of cells and tissues.