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
  

Differentiation of the somites

The tissue that immigrates via the primitive streak and initially forms the third germinal layer, the mesoderm, transforms itself several times.The epiblast-epithelial (ectoderm cells) are transformed into segmented mesoderm cells. In contrast to the epithelial cells, these are loosely organized. A new transformation follows, i.e., an epithelialization to become somites, which represent an epithelialized mesenchyma portion with a central somitocoel.

These somites develop on both sides of the neural plate, caudal to the otic placode (post-otic), and are square-shaped, externally easily visible formations.
In their form as epithelialized somites, they do not remain very long, but rather very early exhibit a polarization in all directions.

Fig. 11
On the left is shown an embryo in stage 11, ca. 29 days.

In a renewed transformation of the somites into mesenchymal tissue, cells are given off into the cavity from the medial and ventral walls of the somites. The central cells as well as the cells of the medioventral wall form the sclerotome that as a result forms an important component of the axial skeleton.The dorsolateral portions of the somite wall is called a dermatomyotome 12. This group of cells also reproduces and forms a new layer of cells. The dermatomyotome consists now of two portions: myotome, which lie more medially and form the origin of the skeletal musculature (5) as well as the dorsolateral dermatome, out of which - among other things - the subcutaneous tissue proceeds. The tissue thus has again lost its epithelial character.
The myotome form into a dorsomedially lying epaxial portion 12, which represents the origin of the spinal extensor muscles (autochtonous back musculature or musculus erector trunci). These muscles retain their segmental ordering. A second ventrolaterally lying portion forms the pars hypaxialis of the myotome. Out of this arise, among other things, the spinal flexor muscles, the shoulder muscles and those of the hip region, as well as the trunk walls and musculature of the extremities.

Fig. 12
On the left is shown an embryo in stage 12, ca. 30 days.

On the right a transversal section through the embryo at level A is displayed. The somites have released themselves and form dermatomes, myotomes and sclerotomes. The aorta is no longer paired.

In the group of myoblasts that have a hypaxial origin and that have partially migrated into laterally lying regions of the embryo, the segmental ordering is no longer so clearly visible. It is interesting in this connection that over the course of the development the placement or definitive extension of a muscle can change. This happens, for example, in the muscles that affect the shoulders. The latissimus dorsi muscle and the major pectoral muscle obtain a secondary connection to the trunk part of the skeleton, but retain their original innervation from the brachial plexus.